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llvm-mirror/lib/Transforms/Scalar/LICM.cpp
Nikita Popov ea8b19209c [LICM] Remove MaybePromotable set (PR50367) 
The MaybePromotable set keeps track of loads/stores for which
promotion was not attempted yet. Normally, any load/stores that
are promoted in the current iteration will be removed from this
set, because they naturally MustAlias with the promoted value.
However, if the source program has UB with metadata claiming that
a store is NoAlias, while it is actually MustAlias, and multiple
different pointers are promoted in the same iteration, it can
happen that a store is removed that is still in the MaybePromotable
set, causing a use-after-free.

While this could be fixed by explicitly invalidating values in
MaybePromotable in the LoopPromoter, I'm going with the more
radical option of dropping the set entirely here and check all
load/stores on each promotion iteration. As promotion, and especially
repeated promotion, are quite rare, this doesn't seem to have any
impact on compile-time.

Fixes https://bugs.llvm.org/show_bug.cgi?id=50367.
2021-05-18 20:26:01 +02:00

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//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass performs loop invariant code motion, attempting to remove as much
// code from the body of a loop as possible. It does this by either hoisting
// code into the preheader block, or by sinking code to the exit blocks if it is
// safe. This pass also promotes must-aliased memory locations in the loop to
// live in registers, thus hoisting and sinking "invariant" loads and stores.
//
// Hoisting operations out of loops is a canonicalization transform. It
// enables and simplifies subsequent optimizations in the middle-end.
// Rematerialization of hoisted instructions to reduce register pressure is the
// responsibility of the back-end, which has more accurate information about
// register pressure and also handles other optimizations than LICM that
// increase live-ranges.
//
// This pass uses alias analysis for two purposes:
//
// 1. Moving loop invariant loads and calls out of loops. If we can determine
// that a load or call inside of a loop never aliases anything stored to,
// we can hoist it or sink it like any other instruction.
// 2. Scalar Promotion of Memory - If there is a store instruction inside of
// the loop, we try to move the store to happen AFTER the loop instead of
// inside of the loop. This can only happen if a few conditions are true:
// A. The pointer stored through is loop invariant
// B. There are no stores or loads in the loop which _may_ alias the
// pointer. There are no calls in the loop which mod/ref the pointer.
// If these conditions are true, we can promote the loads and stores in the
// loop of the pointer to use a temporary alloca'd variable. We then use
// the SSAUpdater to construct the appropriate SSA form for the value.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LICM.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/LazyBlockFrequencyInfo.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/PredIteratorCache.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include <algorithm>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "licm"
STATISTIC(NumCreatedBlocks, "Number of blocks created");
STATISTIC(NumClonedBranches, "Number of branches cloned");
STATISTIC(NumSunk, "Number of instructions sunk out of loop");
STATISTIC(NumHoisted, "Number of instructions hoisted out of loop");
STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
STATISTIC(NumPromoted, "Number of memory locations promoted to registers");
/// Memory promotion is enabled by default.
static cl::opt<bool>
DisablePromotion("disable-licm-promotion", cl::Hidden, cl::init(false),
cl::desc("Disable memory promotion in LICM pass"));
static cl::opt<bool> ControlFlowHoisting(
"licm-control-flow-hoisting", cl::Hidden, cl::init(false),
cl::desc("Enable control flow (and PHI) hoisting in LICM"));
static cl::opt<unsigned> HoistSinkColdnessThreshold(
"licm-coldness-threshold", cl::Hidden, cl::init(4),
cl::desc("Relative coldness Threshold of hoisting/sinking destination "
"block for LICM to be considered beneficial"));
static cl::opt<uint32_t> MaxNumUsesTraversed(
"licm-max-num-uses-traversed", cl::Hidden, cl::init(8),
cl::desc("Max num uses visited for identifying load "
"invariance in loop using invariant start (default = 8)"));
// Default value of zero implies we use the regular alias set tracker mechanism
// instead of the cross product using AA to identify aliasing of the memory
// location we are interested in.
static cl::opt<int>
LICMN2Theshold("licm-n2-threshold", cl::Hidden, cl::init(0),
cl::desc("How many instruction to cross product using AA"));
// Experimental option to allow imprecision in LICM in pathological cases, in
// exchange for faster compile. This is to be removed if MemorySSA starts to
// address the same issue. This flag applies only when LICM uses MemorySSA
// instead on AliasSetTracker. LICM calls MemorySSAWalker's
// getClobberingMemoryAccess, up to the value of the Cap, getting perfect
// accuracy. Afterwards, LICM will call into MemorySSA's getDefiningAccess,
// which may not be precise, since optimizeUses is capped. The result is
// correct, but we may not get as "far up" as possible to get which access is
// clobbering the one queried.
cl::opt<unsigned> llvm::SetLicmMssaOptCap(
"licm-mssa-optimization-cap", cl::init(100), cl::Hidden,
cl::desc("Enable imprecision in LICM in pathological cases, in exchange "
"for faster compile. Caps the MemorySSA clobbering calls."));
// Experimentally, memory promotion carries less importance than sinking and
// hoisting. Limit when we do promotion when using MemorySSA, in order to save
// compile time.
cl::opt<unsigned> llvm::SetLicmMssaNoAccForPromotionCap(
"licm-mssa-max-acc-promotion", cl::init(250), cl::Hidden,
cl::desc("[LICM & MemorySSA] When MSSA in LICM is disabled, this has no "
"effect. When MSSA in LICM is enabled, then this is the maximum "
"number of accesses allowed to be present in a loop in order to "
"enable memory promotion."));
static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI);
static bool isNotUsedOrFreeInLoop(const Instruction &I, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
TargetTransformInfo *TTI, bool &FreeInLoop);
static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
BasicBlock *Dest, ICFLoopSafetyInfo *SafetyInfo,
MemorySSAUpdater *MSSAU, ScalarEvolution *SE,
OptimizationRemarkEmitter *ORE);
static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
BlockFrequencyInfo *BFI, const Loop *CurLoop,
ICFLoopSafetyInfo *SafetyInfo, MemorySSAUpdater *MSSAU,
OptimizationRemarkEmitter *ORE);
static bool isSafeToExecuteUnconditionally(Instruction &Inst,
const DominatorTree *DT,
const TargetLibraryInfo *TLI,
const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE,
const Instruction *CtxI = nullptr);
static bool pointerInvalidatedByLoop(MemoryLocation MemLoc,
AliasSetTracker *CurAST, Loop *CurLoop,
AAResults *AA);
static bool pointerInvalidatedByLoopWithMSSA(MemorySSA *MSSA, MemoryUse *MU,
Loop *CurLoop, Instruction &I,
SinkAndHoistLICMFlags &Flags);
static bool pointerInvalidatedByBlockWithMSSA(BasicBlock &BB, MemorySSA &MSSA,
MemoryUse &MU);
static Instruction *cloneInstructionInExitBlock(
Instruction &I, BasicBlock &ExitBlock, PHINode &PN, const LoopInfo *LI,
const LoopSafetyInfo *SafetyInfo, MemorySSAUpdater *MSSAU);
static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
AliasSetTracker *AST, MemorySSAUpdater *MSSAU);
static void moveInstructionBefore(Instruction &I, Instruction &Dest,
ICFLoopSafetyInfo &SafetyInfo,
MemorySSAUpdater *MSSAU, ScalarEvolution *SE);
static void foreachMemoryAccess(MemorySSA *MSSA, Loop *L,
function_ref<void(Instruction *)> Fn);
static SmallVector<SmallSetVector<Value *, 8>, 0>
collectPromotionCandidates(MemorySSA *MSSA, AliasAnalysis *AA, Loop *L);
namespace {
struct LoopInvariantCodeMotion {
bool runOnLoop(Loop *L, AAResults *AA, LoopInfo *LI, DominatorTree *DT,
BlockFrequencyInfo *BFI, TargetLibraryInfo *TLI,
TargetTransformInfo *TTI, ScalarEvolution *SE, MemorySSA *MSSA,
OptimizationRemarkEmitter *ORE);
LoopInvariantCodeMotion(unsigned LicmMssaOptCap,
unsigned LicmMssaNoAccForPromotionCap)
: LicmMssaOptCap(LicmMssaOptCap),
LicmMssaNoAccForPromotionCap(LicmMssaNoAccForPromotionCap) {}
private:
unsigned LicmMssaOptCap;
unsigned LicmMssaNoAccForPromotionCap;
std::unique_ptr<AliasSetTracker>
collectAliasInfoForLoop(Loop *L, LoopInfo *LI, AAResults *AA);
};
struct LegacyLICMPass : public LoopPass {
static char ID; // Pass identification, replacement for typeid
LegacyLICMPass(
unsigned LicmMssaOptCap = SetLicmMssaOptCap,
unsigned LicmMssaNoAccForPromotionCap = SetLicmMssaNoAccForPromotionCap)
: LoopPass(ID), LICM(LicmMssaOptCap, LicmMssaNoAccForPromotionCap) {
initializeLegacyLICMPassPass(*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L))
return false;
LLVM_DEBUG(dbgs() << "Perform LICM on Loop with header at block "
<< L->getHeader()->getNameOrAsOperand() << "\n");
auto *SE = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
MemorySSA *MSSA = EnableMSSALoopDependency
? (&getAnalysis<MemorySSAWrapperPass>().getMSSA())
: nullptr;
bool hasProfileData = L->getHeader()->getParent()->hasProfileData();
BlockFrequencyInfo *BFI =
hasProfileData ? &getAnalysis<LazyBlockFrequencyInfoPass>().getBFI()
: nullptr;
// For the old PM, we can't use OptimizationRemarkEmitter as an analysis
// pass. Function analyses need to be preserved across loop transformations
// but ORE cannot be preserved (see comment before the pass definition).
OptimizationRemarkEmitter ORE(L->getHeader()->getParent());
return LICM.runOnLoop(
L, &getAnalysis<AAResultsWrapperPass>().getAAResults(),
&getAnalysis<LoopInfoWrapperPass>().getLoopInfo(),
&getAnalysis<DominatorTreeWrapperPass>().getDomTree(), BFI,
&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(
*L->getHeader()->getParent()),
&getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
*L->getHeader()->getParent()),
SE ? &SE->getSE() : nullptr, MSSA, &ORE);
}
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
if (EnableMSSALoopDependency) {
AU.addRequired<MemorySSAWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
}
AU.addRequired<TargetTransformInfoWrapperPass>();
getLoopAnalysisUsage(AU);
LazyBlockFrequencyInfoPass::getLazyBFIAnalysisUsage(AU);
AU.addPreserved<LazyBlockFrequencyInfoPass>();
AU.addPreserved<LazyBranchProbabilityInfoPass>();
}
private:
LoopInvariantCodeMotion LICM;
};
} // namespace
PreservedAnalyses LICMPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR, LPMUpdater &) {
// For the new PM, we also can't use OptimizationRemarkEmitter as an analysis
// pass. Function analyses need to be preserved across loop transformations
// but ORE cannot be preserved (see comment before the pass definition).
OptimizationRemarkEmitter ORE(L.getHeader()->getParent());
LoopInvariantCodeMotion LICM(LicmMssaOptCap, LicmMssaNoAccForPromotionCap);
if (!LICM.runOnLoop(&L, &AR.AA, &AR.LI, &AR.DT, AR.BFI, &AR.TLI, &AR.TTI,
&AR.SE, AR.MSSA, &ORE))
return PreservedAnalyses::all();
auto PA = getLoopPassPreservedAnalyses();
PA.preserve<DominatorTreeAnalysis>();
PA.preserve<LoopAnalysis>();
if (AR.MSSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
char LegacyLICMPass::ID = 0;
INITIALIZE_PASS_BEGIN(LegacyLICMPass, "licm", "Loop Invariant Code Motion",
false, false)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LazyBFIPass)
INITIALIZE_PASS_END(LegacyLICMPass, "licm", "Loop Invariant Code Motion", false,
false)
Pass *llvm::createLICMPass() { return new LegacyLICMPass(); }
Pass *llvm::createLICMPass(unsigned LicmMssaOptCap,
unsigned LicmMssaNoAccForPromotionCap) {
return new LegacyLICMPass(LicmMssaOptCap, LicmMssaNoAccForPromotionCap);
}
llvm::SinkAndHoistLICMFlags::SinkAndHoistLICMFlags(bool IsSink, Loop *L,
MemorySSA *MSSA)
: SinkAndHoistLICMFlags(SetLicmMssaOptCap, SetLicmMssaNoAccForPromotionCap,
IsSink, L, MSSA) {}
llvm::SinkAndHoistLICMFlags::SinkAndHoistLICMFlags(
unsigned LicmMssaOptCap, unsigned LicmMssaNoAccForPromotionCap, bool IsSink,
Loop *L, MemorySSA *MSSA)
: LicmMssaOptCap(LicmMssaOptCap),
LicmMssaNoAccForPromotionCap(LicmMssaNoAccForPromotionCap),
IsSink(IsSink) {
assert(((L != nullptr) == (MSSA != nullptr)) &&
"Unexpected values for SinkAndHoistLICMFlags");
if (!MSSA)
return;
unsigned AccessCapCount = 0;
for (auto *BB : L->getBlocks())
if (const auto *Accesses = MSSA->getBlockAccesses(BB))
for (const auto &MA : *Accesses) {
(void)MA;
++AccessCapCount;
if (AccessCapCount > LicmMssaNoAccForPromotionCap) {
NoOfMemAccTooLarge = true;
return;
}
}
}
/// Hoist expressions out of the specified loop. Note, alias info for inner
/// loop is not preserved so it is not a good idea to run LICM multiple
/// times on one loop.
bool LoopInvariantCodeMotion::runOnLoop(
Loop *L, AAResults *AA, LoopInfo *LI, DominatorTree *DT,
BlockFrequencyInfo *BFI, TargetLibraryInfo *TLI, TargetTransformInfo *TTI,
ScalarEvolution *SE, MemorySSA *MSSA, OptimizationRemarkEmitter *ORE) {
bool Changed = false;
assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
// If this loop has metadata indicating that LICM is not to be performed then
// just exit.
if (hasDisableLICMTransformsHint(L)) {
return false;
}
std::unique_ptr<AliasSetTracker> CurAST;
std::unique_ptr<MemorySSAUpdater> MSSAU;
std::unique_ptr<SinkAndHoistLICMFlags> Flags;
// Don't sink stores from loops with coroutine suspend instructions.
// LICM would sink instructions into the default destination of
// the coroutine switch. The default destination of the switch is to
// handle the case where the coroutine is suspended, by which point the
// coroutine frame may have been destroyed. No instruction can be sunk there.
// FIXME: This would unfortunately hurt the performance of coroutines, however
// there is currently no general solution for this. Similar issues could also
// potentially happen in other passes where instructions are being moved
// across that edge.
bool HasCoroSuspendInst = llvm::any_of(L->getBlocks(), [](BasicBlock *BB) {
return llvm::any_of(*BB, [](Instruction &I) {
IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I);
return II && II->getIntrinsicID() == Intrinsic::coro_suspend;
});
});
if (!MSSA) {
LLVM_DEBUG(dbgs() << "LICM: Using Alias Set Tracker.\n");
CurAST = collectAliasInfoForLoop(L, LI, AA);
Flags = std::make_unique<SinkAndHoistLICMFlags>(
LicmMssaOptCap, LicmMssaNoAccForPromotionCap, /*IsSink=*/true);
} else {
LLVM_DEBUG(dbgs() << "LICM: Using MemorySSA.\n");
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
Flags = std::make_unique<SinkAndHoistLICMFlags>(
LicmMssaOptCap, LicmMssaNoAccForPromotionCap, /*IsSink=*/true, L, MSSA);
}
// Get the preheader block to move instructions into...
BasicBlock *Preheader = L->getLoopPreheader();
// Compute loop safety information.
ICFLoopSafetyInfo SafetyInfo;
SafetyInfo.computeLoopSafetyInfo(L);
// We want to visit all of the instructions in this loop... that are not parts
// of our subloops (they have already had their invariants hoisted out of
// their loop, into this loop, so there is no need to process the BODIES of
// the subloops).
//
// Traverse the body of the loop in depth first order on the dominator tree so
// that we are guaranteed to see definitions before we see uses. This allows
// us to sink instructions in one pass, without iteration. After sinking
// instructions, we perform another pass to hoist them out of the loop.
if (L->hasDedicatedExits())
Changed |=
sinkRegion(DT->getNode(L->getHeader()), AA, LI, DT, BFI, TLI, TTI, L,
CurAST.get(), MSSAU.get(), &SafetyInfo, *Flags.get(), ORE);
Flags->setIsSink(false);
if (Preheader)
Changed |= hoistRegion(DT->getNode(L->getHeader()), AA, LI, DT, BFI, TLI, L,
CurAST.get(), MSSAU.get(), SE, &SafetyInfo,
*Flags.get(), ORE);
// Now that all loop invariants have been removed from the loop, promote any
// memory references to scalars that we can.
// Don't sink stores from loops without dedicated block exits. Exits
// containing indirect branches are not transformed by loop simplify,
// make sure we catch that. An additional load may be generated in the
// preheader for SSA updater, so also avoid sinking when no preheader
// is available.
if (!DisablePromotion && Preheader && L->hasDedicatedExits() &&
!Flags->tooManyMemoryAccesses() && !HasCoroSuspendInst) {
// Figure out the loop exits and their insertion points
SmallVector<BasicBlock *, 8> ExitBlocks;
L->getUniqueExitBlocks(ExitBlocks);
// We can't insert into a catchswitch.
bool HasCatchSwitch = llvm::any_of(ExitBlocks, [](BasicBlock *Exit) {
return isa<CatchSwitchInst>(Exit->getTerminator());
});
if (!HasCatchSwitch) {
SmallVector<Instruction *, 8> InsertPts;
SmallVector<MemoryAccess *, 8> MSSAInsertPts;
InsertPts.reserve(ExitBlocks.size());
if (MSSAU)
MSSAInsertPts.reserve(ExitBlocks.size());
for (BasicBlock *ExitBlock : ExitBlocks) {
InsertPts.push_back(&*ExitBlock->getFirstInsertionPt());
if (MSSAU)
MSSAInsertPts.push_back(nullptr);
}
PredIteratorCache PIC;
bool Promoted = false;
if (CurAST.get()) {
// Loop over all of the alias sets in the tracker object.
for (AliasSet &AS : *CurAST) {
// We can promote this alias set if it has a store, if it is a "Must"
// alias set, if the pointer is loop invariant, and if we are not
// eliminating any volatile loads or stores.
if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
!L->isLoopInvariant(AS.begin()->getValue()))
continue;
assert(
!AS.empty() &&
"Must alias set should have at least one pointer element in it!");
SmallSetVector<Value *, 8> PointerMustAliases;
for (const auto &ASI : AS)
PointerMustAliases.insert(ASI.getValue());
Promoted |= promoteLoopAccessesToScalars(
PointerMustAliases, ExitBlocks, InsertPts, MSSAInsertPts, PIC, LI,
DT, TLI, L, CurAST.get(), MSSAU.get(), &SafetyInfo, ORE);
}
} else {
// Promoting one set of accesses may make the pointers for another set
// loop invariant, so run this in a loop (with the MaybePromotable set
// decreasing in size over time).
bool LocalPromoted;
do {
LocalPromoted = false;
for (const SmallSetVector<Value *, 8> &PointerMustAliases :
collectPromotionCandidates(MSSA, AA, L)) {
LocalPromoted |= promoteLoopAccessesToScalars(
PointerMustAliases, ExitBlocks, InsertPts, MSSAInsertPts, PIC,
LI, DT, TLI, L, /*AST*/nullptr, MSSAU.get(), &SafetyInfo, ORE);
}
Promoted |= LocalPromoted;
} while (LocalPromoted);
}
// Once we have promoted values across the loop body we have to
// recursively reform LCSSA as any nested loop may now have values defined
// within the loop used in the outer loop.
// FIXME: This is really heavy handed. It would be a bit better to use an
// SSAUpdater strategy during promotion that was LCSSA aware and reformed
// it as it went.
if (Promoted)
formLCSSARecursively(*L, *DT, LI, SE);
Changed |= Promoted;
}
}
// Check that neither this loop nor its parent have had LCSSA broken. LICM is
// specifically moving instructions across the loop boundary and so it is
// especially in need of sanity checking here.
assert(L->isLCSSAForm(*DT) && "Loop not left in LCSSA form after LICM!");
assert((L->isOutermost() || L->getParentLoop()->isLCSSAForm(*DT)) &&
"Parent loop not left in LCSSA form after LICM!");
if (MSSAU.get() && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
if (Changed && SE)
SE->forgetLoopDispositions(L);
return Changed;
}
/// Walk the specified region of the CFG (defined by all blocks dominated by
/// the specified block, and that are in the current loop) in reverse depth
/// first order w.r.t the DominatorTree. This allows us to visit uses before
/// definitions, allowing us to sink a loop body in one pass without iteration.
///
bool llvm::sinkRegion(DomTreeNode *N, AAResults *AA, LoopInfo *LI,
DominatorTree *DT, BlockFrequencyInfo *BFI,
TargetLibraryInfo *TLI, TargetTransformInfo *TTI,
Loop *CurLoop, AliasSetTracker *CurAST,
MemorySSAUpdater *MSSAU, ICFLoopSafetyInfo *SafetyInfo,
SinkAndHoistLICMFlags &Flags,
OptimizationRemarkEmitter *ORE) {
// Verify inputs.
assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
CurLoop != nullptr && SafetyInfo != nullptr &&
"Unexpected input to sinkRegion.");
assert(((CurAST != nullptr) ^ (MSSAU != nullptr)) &&
"Either AliasSetTracker or MemorySSA should be initialized.");
// We want to visit children before parents. We will enque all the parents
// before their children in the worklist and process the worklist in reverse
// order.
SmallVector<DomTreeNode *, 16> Worklist = collectChildrenInLoop(N, CurLoop);
bool Changed = false;
for (DomTreeNode *DTN : reverse(Worklist)) {
BasicBlock *BB = DTN->getBlock();
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (inSubLoop(BB, CurLoop, LI))
continue;
for (BasicBlock::iterator II = BB->end(); II != BB->begin();) {
Instruction &I = *--II;
// The instruction is not used in the loop if it is dead. In this case,
// we just delete it instead of sinking it.
if (isInstructionTriviallyDead(&I, TLI)) {
LLVM_DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
salvageKnowledge(&I);
salvageDebugInfo(I);
++II;
eraseInstruction(I, *SafetyInfo, CurAST, MSSAU);
Changed = true;
continue;
}
// Check to see if we can sink this instruction to the exit blocks
// of the loop. We can do this if the all users of the instruction are
// outside of the loop. In this case, it doesn't even matter if the
// operands of the instruction are loop invariant.
//
bool FreeInLoop = false;
if (!I.mayHaveSideEffects() &&
isNotUsedOrFreeInLoop(I, CurLoop, SafetyInfo, TTI, FreeInLoop) &&
canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, MSSAU, true, &Flags,
ORE)) {
if (sink(I, LI, DT, BFI, CurLoop, SafetyInfo, MSSAU, ORE)) {
if (!FreeInLoop) {
++II;
salvageDebugInfo(I);
eraseInstruction(I, *SafetyInfo, CurAST, MSSAU);
}
Changed = true;
}
}
}
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
return Changed;
}
namespace {
// This is a helper class for hoistRegion to make it able to hoist control flow
// in order to be able to hoist phis. The way this works is that we initially
// start hoisting to the loop preheader, and when we see a loop invariant branch
// we make note of this. When we then come to hoist an instruction that's
// conditional on such a branch we duplicate the branch and the relevant control
// flow, then hoist the instruction into the block corresponding to its original
// block in the duplicated control flow.
class ControlFlowHoister {
private:
// Information about the loop we are hoisting from
LoopInfo *LI;
DominatorTree *DT;
Loop *CurLoop;
MemorySSAUpdater *MSSAU;
// A map of blocks in the loop to the block their instructions will be hoisted
// to.
DenseMap<BasicBlock *, BasicBlock *> HoistDestinationMap;
// The branches that we can hoist, mapped to the block that marks a
// convergence point of their control flow.
DenseMap<BranchInst *, BasicBlock *> HoistableBranches;
public:
ControlFlowHoister(LoopInfo *LI, DominatorTree *DT, Loop *CurLoop,
MemorySSAUpdater *MSSAU)
: LI(LI), DT(DT), CurLoop(CurLoop), MSSAU(MSSAU) {}
void registerPossiblyHoistableBranch(BranchInst *BI) {
// We can only hoist conditional branches with loop invariant operands.
if (!ControlFlowHoisting || !BI->isConditional() ||
!CurLoop->hasLoopInvariantOperands(BI))
return;
// The branch destinations need to be in the loop, and we don't gain
// anything by duplicating conditional branches with duplicate successors,
// as it's essentially the same as an unconditional branch.
BasicBlock *TrueDest = BI->getSuccessor(0);
BasicBlock *FalseDest = BI->getSuccessor(1);
if (!CurLoop->contains(TrueDest) || !CurLoop->contains(FalseDest) ||
TrueDest == FalseDest)
return;
// We can hoist BI if one branch destination is the successor of the other,
// or both have common successor which we check by seeing if the
// intersection of their successors is non-empty.
// TODO: This could be expanded to allowing branches where both ends
// eventually converge to a single block.
SmallPtrSet<BasicBlock *, 4> TrueDestSucc, FalseDestSucc;
TrueDestSucc.insert(succ_begin(TrueDest), succ_end(TrueDest));
FalseDestSucc.insert(succ_begin(FalseDest), succ_end(FalseDest));
BasicBlock *CommonSucc = nullptr;
if (TrueDestSucc.count(FalseDest)) {
CommonSucc = FalseDest;
} else if (FalseDestSucc.count(TrueDest)) {
CommonSucc = TrueDest;
} else {
set_intersect(TrueDestSucc, FalseDestSucc);
// If there's one common successor use that.
if (TrueDestSucc.size() == 1)
CommonSucc = *TrueDestSucc.begin();
// If there's more than one pick whichever appears first in the block list
// (we can't use the value returned by TrueDestSucc.begin() as it's
// unpredicatable which element gets returned).
else if (!TrueDestSucc.empty()) {
Function *F = TrueDest->getParent();
auto IsSucc = [&](BasicBlock &BB) { return TrueDestSucc.count(&BB); };
auto It = llvm::find_if(*F, IsSucc);
assert(It != F->end() && "Could not find successor in function");
CommonSucc = &*It;
}
}
// The common successor has to be dominated by the branch, as otherwise
// there will be some other path to the successor that will not be
// controlled by this branch so any phi we hoist would be controlled by the
// wrong condition. This also takes care of avoiding hoisting of loop back
// edges.
// TODO: In some cases this could be relaxed if the successor is dominated
// by another block that's been hoisted and we can guarantee that the
// control flow has been replicated exactly.
if (CommonSucc && DT->dominates(BI, CommonSucc))
HoistableBranches[BI] = CommonSucc;
}
bool canHoistPHI(PHINode *PN) {
// The phi must have loop invariant operands.
if (!ControlFlowHoisting || !CurLoop->hasLoopInvariantOperands(PN))
return false;
// We can hoist phis if the block they are in is the target of hoistable
// branches which cover all of the predecessors of the block.
SmallPtrSet<BasicBlock *, 8> PredecessorBlocks;
BasicBlock *BB = PN->getParent();
for (BasicBlock *PredBB : predecessors(BB))
PredecessorBlocks.insert(PredBB);
// If we have less predecessor blocks than predecessors then the phi will
// have more than one incoming value for the same block which we can't
// handle.
// TODO: This could be handled be erasing some of the duplicate incoming
// values.
if (PredecessorBlocks.size() != pred_size(BB))
return false;
for (auto &Pair : HoistableBranches) {
if (Pair.second == BB) {
// Which blocks are predecessors via this branch depends on if the
// branch is triangle-like or diamond-like.
if (Pair.first->getSuccessor(0) == BB) {
PredecessorBlocks.erase(Pair.first->getParent());
PredecessorBlocks.erase(Pair.first->getSuccessor(1));
} else if (Pair.first->getSuccessor(1) == BB) {
PredecessorBlocks.erase(Pair.first->getParent());
PredecessorBlocks.erase(Pair.first->getSuccessor(0));
} else {
PredecessorBlocks.erase(Pair.first->getSuccessor(0));
PredecessorBlocks.erase(Pair.first->getSuccessor(1));
}
}
}
// PredecessorBlocks will now be empty if for every predecessor of BB we
// found a hoistable branch source.
return PredecessorBlocks.empty();
}
BasicBlock *getOrCreateHoistedBlock(BasicBlock *BB) {
if (!ControlFlowHoisting)
return CurLoop->getLoopPreheader();
// If BB has already been hoisted, return that
if (HoistDestinationMap.count(BB))
return HoistDestinationMap[BB];
// Check if this block is conditional based on a pending branch
auto HasBBAsSuccessor =
[&](DenseMap<BranchInst *, BasicBlock *>::value_type &Pair) {
return BB != Pair.second && (Pair.first->getSuccessor(0) == BB ||
Pair.first->getSuccessor(1) == BB);
};
auto It = llvm::find_if(HoistableBranches, HasBBAsSuccessor);
// If not involved in a pending branch, hoist to preheader
BasicBlock *InitialPreheader = CurLoop->getLoopPreheader();
if (It == HoistableBranches.end()) {
LLVM_DEBUG(dbgs() << "LICM using "
<< InitialPreheader->getNameOrAsOperand()
<< " as hoist destination for "
<< BB->getNameOrAsOperand() << "\n");
HoistDestinationMap[BB] = InitialPreheader;
return InitialPreheader;
}
BranchInst *BI = It->first;
assert(std::find_if(++It, HoistableBranches.end(), HasBBAsSuccessor) ==
HoistableBranches.end() &&
"BB is expected to be the target of at most one branch");
LLVMContext &C = BB->getContext();
BasicBlock *TrueDest = BI->getSuccessor(0);
BasicBlock *FalseDest = BI->getSuccessor(1);
BasicBlock *CommonSucc = HoistableBranches[BI];
BasicBlock *HoistTarget = getOrCreateHoistedBlock(BI->getParent());
// Create hoisted versions of blocks that currently don't have them
auto CreateHoistedBlock = [&](BasicBlock *Orig) {
if (HoistDestinationMap.count(Orig))
return HoistDestinationMap[Orig];
BasicBlock *New =
BasicBlock::Create(C, Orig->getName() + ".licm", Orig->getParent());
HoistDestinationMap[Orig] = New;
DT->addNewBlock(New, HoistTarget);
if (CurLoop->getParentLoop())
CurLoop->getParentLoop()->addBasicBlockToLoop(New, *LI);
++NumCreatedBlocks;
LLVM_DEBUG(dbgs() << "LICM created " << New->getName()
<< " as hoist destination for " << Orig->getName()
<< "\n");
return New;
};
BasicBlock *HoistTrueDest = CreateHoistedBlock(TrueDest);
BasicBlock *HoistFalseDest = CreateHoistedBlock(FalseDest);
BasicBlock *HoistCommonSucc = CreateHoistedBlock(CommonSucc);
// Link up these blocks with branches.
if (!HoistCommonSucc->getTerminator()) {
// The new common successor we've generated will branch to whatever that
// hoist target branched to.
BasicBlock *TargetSucc = HoistTarget->getSingleSuccessor();
assert(TargetSucc && "Expected hoist target to have a single successor");
HoistCommonSucc->moveBefore(TargetSucc);
BranchInst::Create(TargetSucc, HoistCommonSucc);
}
if (!HoistTrueDest->getTerminator()) {
HoistTrueDest->moveBefore(HoistCommonSucc);
BranchInst::Create(HoistCommonSucc, HoistTrueDest);
}
if (!HoistFalseDest->getTerminator()) {
HoistFalseDest->moveBefore(HoistCommonSucc);
BranchInst::Create(HoistCommonSucc, HoistFalseDest);
}
// If BI is being cloned to what was originally the preheader then
// HoistCommonSucc will now be the new preheader.
if (HoistTarget == InitialPreheader) {
// Phis in the loop header now need to use the new preheader.
InitialPreheader->replaceSuccessorsPhiUsesWith(HoistCommonSucc);
if (MSSAU)
MSSAU->wireOldPredecessorsToNewImmediatePredecessor(
HoistTarget->getSingleSuccessor(), HoistCommonSucc, {HoistTarget});
// The new preheader dominates the loop header.
DomTreeNode *PreheaderNode = DT->getNode(HoistCommonSucc);
DomTreeNode *HeaderNode = DT->getNode(CurLoop->getHeader());
DT->changeImmediateDominator(HeaderNode, PreheaderNode);
// The preheader hoist destination is now the new preheader, with the
// exception of the hoist destination of this branch.
for (auto &Pair : HoistDestinationMap)
if (Pair.second == InitialPreheader && Pair.first != BI->getParent())
Pair.second = HoistCommonSucc;
}
// Now finally clone BI.
ReplaceInstWithInst(
HoistTarget->getTerminator(),
BranchInst::Create(HoistTrueDest, HoistFalseDest, BI->getCondition()));
++NumClonedBranches;
assert(CurLoop->getLoopPreheader() &&
"Hoisting blocks should not have destroyed preheader");
return HoistDestinationMap[BB];
}
};
} // namespace
// Hoisting/sinking instruction out of a loop isn't always beneficial. It's only
// only worthwhile if the destination block is actually colder than current
// block.
static bool worthSinkOrHoistInst(Instruction &I, BasicBlock *DstBlock,
OptimizationRemarkEmitter *ORE,
BlockFrequencyInfo *BFI) {
// Check block frequency only when runtime profile is available
// to avoid pathological cases. With static profile, lean towards
// hosting because it helps canonicalize the loop for vectorizer.
if (!DstBlock->getParent()->hasProfileData())
return true;
if (!HoistSinkColdnessThreshold || !BFI)
return true;
BasicBlock *SrcBlock = I.getParent();
if (BFI->getBlockFreq(DstBlock).getFrequency() / HoistSinkColdnessThreshold >
BFI->getBlockFreq(SrcBlock).getFrequency()) {
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "SinkHoistInst", &I)
<< "failed to sink or hoist instruction because containing block "
"has lower frequency than destination block";
});
return false;
}
return true;
}
/// Walk the specified region of the CFG (defined by all blocks dominated by
/// the specified block, and that are in the current loop) in depth first
/// order w.r.t the DominatorTree. This allows us to visit definitions before
/// uses, allowing us to hoist a loop body in one pass without iteration.
///
bool llvm::hoistRegion(DomTreeNode *N, AAResults *AA, LoopInfo *LI,
DominatorTree *DT, BlockFrequencyInfo *BFI,
TargetLibraryInfo *TLI, Loop *CurLoop,
AliasSetTracker *CurAST, MemorySSAUpdater *MSSAU,
ScalarEvolution *SE, ICFLoopSafetyInfo *SafetyInfo,
SinkAndHoistLICMFlags &Flags,
OptimizationRemarkEmitter *ORE) {
// Verify inputs.
assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
CurLoop != nullptr && SafetyInfo != nullptr &&
"Unexpected input to hoistRegion.");
assert(((CurAST != nullptr) ^ (MSSAU != nullptr)) &&
"Either AliasSetTracker or MemorySSA should be initialized.");
ControlFlowHoister CFH(LI, DT, CurLoop, MSSAU);
// Keep track of instructions that have been hoisted, as they may need to be
// re-hoisted if they end up not dominating all of their uses.
SmallVector<Instruction *, 16> HoistedInstructions;
// For PHI hoisting to work we need to hoist blocks before their successors.
// We can do this by iterating through the blocks in the loop in reverse
// post-order.
LoopBlocksRPO Worklist(CurLoop);
Worklist.perform(LI);
bool Changed = false;
for (BasicBlock *BB : Worklist) {
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (inSubLoop(BB, CurLoop, LI))
continue;
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
Instruction &I = *II++;
// Try constant folding this instruction. If all the operands are
// constants, it is technically hoistable, but it would be better to
// just fold it.
if (Constant *C = ConstantFoldInstruction(
&I, I.getModule()->getDataLayout(), TLI)) {
LLVM_DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C
<< '\n');
if (CurAST)
CurAST->copyValue(&I, C);
// FIXME MSSA: Such replacements may make accesses unoptimized (D51960).
I.replaceAllUsesWith(C);
if (isInstructionTriviallyDead(&I, TLI))
eraseInstruction(I, *SafetyInfo, CurAST, MSSAU);
Changed = true;
continue;
}
// Try hoisting the instruction out to the preheader. We can only do
// this if all of the operands of the instruction are loop invariant and
// if it is safe to hoist the instruction. We also check block frequency
// to make sure instruction only gets hoisted into colder blocks.
// TODO: It may be safe to hoist if we are hoisting to a conditional block
// and we have accurately duplicated the control flow from the loop header
// to that block.
if (CurLoop->hasLoopInvariantOperands(&I) &&
canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, MSSAU, true, &Flags,
ORE) &&
worthSinkOrHoistInst(I, CurLoop->getLoopPreheader(), ORE, BFI) &&
isSafeToExecuteUnconditionally(
I, DT, TLI, CurLoop, SafetyInfo, ORE,
CurLoop->getLoopPreheader()->getTerminator())) {
hoist(I, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo,
MSSAU, SE, ORE);
HoistedInstructions.push_back(&I);
Changed = true;
continue;
}
// Attempt to remove floating point division out of the loop by
// converting it to a reciprocal multiplication.
if (I.getOpcode() == Instruction::FDiv && I.hasAllowReciprocal() &&
CurLoop->isLoopInvariant(I.getOperand(1))) {
auto Divisor = I.getOperand(1);
auto One = llvm::ConstantFP::get(Divisor->getType(), 1.0);
auto ReciprocalDivisor = BinaryOperator::CreateFDiv(One, Divisor);
ReciprocalDivisor->setFastMathFlags(I.getFastMathFlags());
SafetyInfo->insertInstructionTo(ReciprocalDivisor, I.getParent());
ReciprocalDivisor->insertBefore(&I);
auto Product =
BinaryOperator::CreateFMul(I.getOperand(0), ReciprocalDivisor);
Product->setFastMathFlags(I.getFastMathFlags());
SafetyInfo->insertInstructionTo(Product, I.getParent());
Product->insertAfter(&I);
I.replaceAllUsesWith(Product);
eraseInstruction(I, *SafetyInfo, CurAST, MSSAU);
hoist(*ReciprocalDivisor, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB),
SafetyInfo, MSSAU, SE, ORE);
HoistedInstructions.push_back(ReciprocalDivisor);
Changed = true;
continue;
}
auto IsInvariantStart = [&](Instruction &I) {
using namespace PatternMatch;
return I.use_empty() &&
match(&I, m_Intrinsic<Intrinsic::invariant_start>());
};
auto MustExecuteWithoutWritesBefore = [&](Instruction &I) {
return SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop) &&
SafetyInfo->doesNotWriteMemoryBefore(I, CurLoop);
};
if ((IsInvariantStart(I) || isGuard(&I)) &&
CurLoop->hasLoopInvariantOperands(&I) &&
MustExecuteWithoutWritesBefore(I)) {
hoist(I, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo,
MSSAU, SE, ORE);
HoistedInstructions.push_back(&I);
Changed = true;
continue;
}
if (PHINode *PN = dyn_cast<PHINode>(&I)) {
if (CFH.canHoistPHI(PN)) {
// Redirect incoming blocks first to ensure that we create hoisted
// versions of those blocks before we hoist the phi.
for (unsigned int i = 0; i < PN->getNumIncomingValues(); ++i)
PN->setIncomingBlock(
i, CFH.getOrCreateHoistedBlock(PN->getIncomingBlock(i)));
hoist(*PN, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo,
MSSAU, SE, ORE);
assert(DT->dominates(PN, BB) && "Conditional PHIs not expected");
Changed = true;
continue;
}
}
// Remember possibly hoistable branches so we can actually hoist them
// later if needed.
if (BranchInst *BI = dyn_cast<BranchInst>(&I))
CFH.registerPossiblyHoistableBranch(BI);
}
}
// If we hoisted instructions to a conditional block they may not dominate
// their uses that weren't hoisted (such as phis where some operands are not
// loop invariant). If so make them unconditional by moving them to their
// immediate dominator. We iterate through the instructions in reverse order
// which ensures that when we rehoist an instruction we rehoist its operands,
// and also keep track of where in the block we are rehoisting to to make sure
// that we rehoist instructions before the instructions that use them.
Instruction *HoistPoint = nullptr;
if (ControlFlowHoisting) {
for (Instruction *I : reverse(HoistedInstructions)) {
if (!llvm::all_of(I->uses(),
[&](Use &U) { return DT->dominates(I, U); })) {
BasicBlock *Dominator =
DT->getNode(I->getParent())->getIDom()->getBlock();
if (!HoistPoint || !DT->dominates(HoistPoint->getParent(), Dominator)) {
if (HoistPoint)
assert(DT->dominates(Dominator, HoistPoint->getParent()) &&
"New hoist point expected to dominate old hoist point");
HoistPoint = Dominator->getTerminator();
}
LLVM_DEBUG(dbgs() << "LICM rehoisting to "
<< HoistPoint->getParent()->getNameOrAsOperand()
<< ": " << *I << "\n");
moveInstructionBefore(*I, *HoistPoint, *SafetyInfo, MSSAU, SE);
HoistPoint = I;
Changed = true;
}
}
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Now that we've finished hoisting make sure that LI and DT are still
// valid.
#ifdef EXPENSIVE_CHECKS
if (Changed) {
assert(DT->verify(DominatorTree::VerificationLevel::Fast) &&
"Dominator tree verification failed");
LI->verify(*DT);
}
#endif
return Changed;
}
// Return true if LI is invariant within scope of the loop. LI is invariant if
// CurLoop is dominated by an invariant.start representing the same memory
// location and size as the memory location LI loads from, and also the
// invariant.start has no uses.
static bool isLoadInvariantInLoop(LoadInst *LI, DominatorTree *DT,
Loop *CurLoop) {
Value *Addr = LI->getOperand(0);
const DataLayout &DL = LI->getModule()->getDataLayout();
const TypeSize LocSizeInBits = DL.getTypeSizeInBits(LI->getType());
// It is not currently possible for clang to generate an invariant.start
// intrinsic with scalable vector types because we don't support thread local
// sizeless types and we don't permit sizeless types in structs or classes.
// Furthermore, even if support is added for this in future the intrinsic
// itself is defined to have a size of -1 for variable sized objects. This
// makes it impossible to verify if the intrinsic envelops our region of
// interest. For example, both <vscale x 32 x i8> and <vscale x 16 x i8>
// types would have a -1 parameter, but the former is clearly double the size
// of the latter.
if (LocSizeInBits.isScalable())
return false;
// if the type is i8 addrspace(x)*, we know this is the type of
// llvm.invariant.start operand
auto *PtrInt8Ty = PointerType::get(Type::getInt8Ty(LI->getContext()),
LI->getPointerAddressSpace());
unsigned BitcastsVisited = 0;
// Look through bitcasts until we reach the i8* type (this is invariant.start
// operand type).
while (Addr->getType() != PtrInt8Ty) {
auto *BC = dyn_cast<BitCastInst>(Addr);
// Avoid traversing high number of bitcast uses.
if (++BitcastsVisited > MaxNumUsesTraversed || !BC)
return false;
Addr = BC->getOperand(0);
}
unsigned UsesVisited = 0;
// Traverse all uses of the load operand value, to see if invariant.start is
// one of the uses, and whether it dominates the load instruction.
for (auto *U : Addr->users()) {
// Avoid traversing for Load operand with high number of users.
if (++UsesVisited > MaxNumUsesTraversed)
return false;
IntrinsicInst *II = dyn_cast<IntrinsicInst>(U);
// If there are escaping uses of invariant.start instruction, the load maybe
// non-invariant.
if (!II || II->getIntrinsicID() != Intrinsic::invariant_start ||
!II->use_empty())
continue;
ConstantInt *InvariantSize = cast<ConstantInt>(II->getArgOperand(0));
// The intrinsic supports having a -1 argument for variable sized objects
// so we should check for that here.
if (InvariantSize->isNegative())
continue;
uint64_t InvariantSizeInBits = InvariantSize->getSExtValue() * 8;
// Confirm the invariant.start location size contains the load operand size
// in bits. Also, the invariant.start should dominate the load, and we
// should not hoist the load out of a loop that contains this dominating
// invariant.start.
if (LocSizeInBits.getFixedSize() <= InvariantSizeInBits &&
DT->properlyDominates(II->getParent(), CurLoop->getHeader()))
return true;
}
return false;
}
namespace {
/// Return true if-and-only-if we know how to (mechanically) both hoist and
/// sink a given instruction out of a loop. Does not address legality
/// concerns such as aliasing or speculation safety.
bool isHoistableAndSinkableInst(Instruction &I) {
// Only these instructions are hoistable/sinkable.
return (isa<LoadInst>(I) || isa<StoreInst>(I) || isa<CallInst>(I) ||
isa<FenceInst>(I) || isa<CastInst>(I) || isa<UnaryOperator>(I) ||
isa<BinaryOperator>(I) || isa<SelectInst>(I) ||
isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
isa<ShuffleVectorInst>(I) || isa<ExtractValueInst>(I) ||
isa<InsertValueInst>(I) || isa<FreezeInst>(I));
}
/// Return true if all of the alias sets within this AST are known not to
/// contain a Mod, or if MSSA knows there are no MemoryDefs in the loop.
bool isReadOnly(AliasSetTracker *CurAST, const MemorySSAUpdater *MSSAU,
const Loop *L) {
if (CurAST) {
for (AliasSet &AS : *CurAST) {
if (!AS.isForwardingAliasSet() && AS.isMod()) {
return false;
}
}
return true;
} else { /*MSSAU*/
for (auto *BB : L->getBlocks())
if (MSSAU->getMemorySSA()->getBlockDefs(BB))
return false;
return true;
}
}
/// Return true if I is the only Instruction with a MemoryAccess in L.
bool isOnlyMemoryAccess(const Instruction *I, const Loop *L,
const MemorySSAUpdater *MSSAU) {
for (auto *BB : L->getBlocks())
if (auto *Accs = MSSAU->getMemorySSA()->getBlockAccesses(BB)) {
int NotAPhi = 0;
for (const auto &Acc : *Accs) {
if (isa<MemoryPhi>(&Acc))
continue;
const auto *MUD = cast<MemoryUseOrDef>(&Acc);
if (MUD->getMemoryInst() != I || NotAPhi++ == 1)
return false;
}
}
return true;
}
}
bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
Loop *CurLoop, AliasSetTracker *CurAST,
MemorySSAUpdater *MSSAU,
bool TargetExecutesOncePerLoop,
SinkAndHoistLICMFlags *Flags,
OptimizationRemarkEmitter *ORE) {
assert(((CurAST != nullptr) ^ (MSSAU != nullptr)) &&
"Either AliasSetTracker or MemorySSA should be initialized.");
// If we don't understand the instruction, bail early.
if (!isHoistableAndSinkableInst(I))
return false;
MemorySSA *MSSA = MSSAU ? MSSAU->getMemorySSA() : nullptr;
if (MSSA)
assert(Flags != nullptr && "Flags cannot be null.");
// Loads have extra constraints we have to verify before we can hoist them.
if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
if (!LI->isUnordered())
return false; // Don't sink/hoist volatile or ordered atomic loads!
// Loads from constant memory are always safe to move, even if they end up
// in the same alias set as something that ends up being modified.
if (AA->pointsToConstantMemory(LI->getOperand(0)))
return true;
if (LI->hasMetadata(LLVMContext::MD_invariant_load))
return true;
if (LI->isAtomic() && !TargetExecutesOncePerLoop)
return false; // Don't risk duplicating unordered loads
// This checks for an invariant.start dominating the load.
if (isLoadInvariantInLoop(LI, DT, CurLoop))
return true;
bool Invalidated;
if (CurAST)
Invalidated = pointerInvalidatedByLoop(MemoryLocation::get(LI), CurAST,
CurLoop, AA);
else
Invalidated = pointerInvalidatedByLoopWithMSSA(
MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(LI)), CurLoop, I, *Flags);
// Check loop-invariant address because this may also be a sinkable load
// whose address is not necessarily loop-invariant.
if (ORE && Invalidated && CurLoop->isLoopInvariant(LI->getPointerOperand()))
ORE->emit([&]() {
return OptimizationRemarkMissed(
DEBUG_TYPE, "LoadWithLoopInvariantAddressInvalidated", LI)
<< "failed to move load with loop-invariant address "
"because the loop may invalidate its value";
});
return !Invalidated;
} else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
// Don't sink or hoist dbg info; it's legal, but not useful.
if (isa<DbgInfoIntrinsic>(I))
return false;
// Don't sink calls which can throw.
if (CI->mayThrow())
return false;
// Convergent attribute has been used on operations that involve
// inter-thread communication which results are implicitly affected by the
// enclosing control flows. It is not safe to hoist or sink such operations
// across control flow.
if (CI->isConvergent())
return false;
using namespace PatternMatch;
if (match(CI, m_Intrinsic<Intrinsic::assume>()))
// Assumes don't actually alias anything or throw
return true;
if (match(CI, m_Intrinsic<Intrinsic::experimental_widenable_condition>()))
// Widenable conditions don't actually alias anything or throw
return true;
// Handle simple cases by querying alias analysis.
FunctionModRefBehavior Behavior = AA->getModRefBehavior(CI);
if (Behavior == FMRB_DoesNotAccessMemory)
return true;
if (AAResults::onlyReadsMemory(Behavior)) {
// A readonly argmemonly function only reads from memory pointed to by
// it's arguments with arbitrary offsets. If we can prove there are no
// writes to this memory in the loop, we can hoist or sink.
if (AAResults::onlyAccessesArgPointees(Behavior)) {
// TODO: expand to writeable arguments
for (Value *Op : CI->arg_operands())
if (Op->getType()->isPointerTy()) {
bool Invalidated;
if (CurAST)
Invalidated = pointerInvalidatedByLoop(
MemoryLocation::getBeforeOrAfter(Op), CurAST, CurLoop, AA);
else
Invalidated = pointerInvalidatedByLoopWithMSSA(
MSSA, cast<MemoryUse>(MSSA->getMemoryAccess(CI)), CurLoop, I,
*Flags);
if (Invalidated)
return false;
}
return true;
}
// If this call only reads from memory and there are no writes to memory
// in the loop, we can hoist or sink the call as appropriate.
if (isReadOnly(CurAST, MSSAU, CurLoop))
return true;
}
// FIXME: This should use mod/ref information to see if we can hoist or
// sink the call.
return false;
} else if (auto *FI = dyn_cast<FenceInst>(&I)) {
// Fences alias (most) everything to provide ordering. For the moment,
// just give up if there are any other memory operations in the loop.
if (CurAST) {
auto Begin = CurAST->begin();
assert(Begin != CurAST->end() && "must contain FI");
if (std::next(Begin) != CurAST->end())
// constant memory for instance, TODO: handle better
return false;
auto *UniqueI = Begin->getUniqueInstruction();
if (!UniqueI)
// other memory op, give up
return false;
(void)FI; // suppress unused variable warning
assert(UniqueI == FI && "AS must contain FI");
return true;
} else // MSSAU
return isOnlyMemoryAccess(FI, CurLoop, MSSAU);
} else if (auto *SI = dyn_cast<StoreInst>(&I)) {
if (!SI->isUnordered())
return false; // Don't sink/hoist volatile or ordered atomic store!
// We can only hoist a store that we can prove writes a value which is not
// read or overwritten within the loop. For those cases, we fallback to
// load store promotion instead. TODO: We can extend this to cases where
// there is exactly one write to the location and that write dominates an
// arbitrary number of reads in the loop.
if (CurAST) {
auto &AS = CurAST->getAliasSetFor(MemoryLocation::get(SI));
if (AS.isRef() || !AS.isMustAlias())
// Quick exit test, handled by the full path below as well.
return false;
auto *UniqueI = AS.getUniqueInstruction();
if (!UniqueI)
// other memory op, give up
return false;
assert(UniqueI == SI && "AS must contain SI");
return true;
} else { // MSSAU
if (isOnlyMemoryAccess(SI, CurLoop, MSSAU))
return true;
// If there are more accesses than the Promotion cap or no "quota" to
// check clobber, then give up as we're not walking a list that long.
if (Flags->tooManyMemoryAccesses() || Flags->tooManyClobberingCalls())
return false;
// If there are interfering Uses (i.e. their defining access is in the
// loop), or ordered loads (stored as Defs!), don't move this store.
// Could do better here, but this is conservatively correct.
// TODO: Cache set of Uses on the first walk in runOnLoop, update when
// moving accesses. Can also extend to dominating uses.
auto *SIMD = MSSA->getMemoryAccess(SI);
for (auto *BB : CurLoop->getBlocks())
if (auto *Accesses = MSSA->getBlockAccesses(BB)) {
for (const auto &MA : *Accesses)
if (const auto *MU = dyn_cast<MemoryUse>(&MA)) {
auto *MD = MU->getDefiningAccess();
if (!MSSA->isLiveOnEntryDef(MD) &&
CurLoop->contains(MD->getBlock()))
return false;
// Disable hoisting past potentially interfering loads. Optimized
// Uses may point to an access outside the loop, as getClobbering
// checks the previous iteration when walking the backedge.
// FIXME: More precise: no Uses that alias SI.
if (!Flags->getIsSink() && !MSSA->dominates(SIMD, MU))
return false;
} else if (const auto *MD = dyn_cast<MemoryDef>(&MA)) {
if (auto *LI = dyn_cast<LoadInst>(MD->getMemoryInst())) {
(void)LI; // Silence warning.
assert(!LI->isUnordered() && "Expected unordered load");
return false;
}
// Any call, while it may not be clobbering SI, it may be a use.
if (auto *CI = dyn_cast<CallInst>(MD->getMemoryInst())) {
// Check if the call may read from the memory location written
// to by SI. Check CI's attributes and arguments; the number of
// such checks performed is limited above by NoOfMemAccTooLarge.
ModRefInfo MRI = AA->getModRefInfo(CI, MemoryLocation::get(SI));
if (isModOrRefSet(MRI))
return false;
}
}
}
auto *Source = MSSA->getSkipSelfWalker()->getClobberingMemoryAccess(SI);
Flags->incrementClobberingCalls();
// If there are no clobbering Defs in the loop, store is safe to hoist.
return MSSA->isLiveOnEntryDef(Source) ||
!CurLoop->contains(Source->getBlock());
}
}
assert(!I.mayReadOrWriteMemory() && "unhandled aliasing");
// We've established mechanical ability and aliasing, it's up to the caller
// to check fault safety
return true;
}
/// Returns true if a PHINode is a trivially replaceable with an
/// Instruction.
/// This is true when all incoming values are that instruction.
/// This pattern occurs most often with LCSSA PHI nodes.
///
static bool isTriviallyReplaceablePHI(const PHINode &PN, const Instruction &I) {
for (const Value *IncValue : PN.incoming_values())
if (IncValue != &I)
return false;
return true;
}
/// Return true if the instruction is free in the loop.
static bool isFreeInLoop(const Instruction &I, const Loop *CurLoop,
const TargetTransformInfo *TTI) {
if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(&I)) {
if (TTI->getUserCost(GEP, TargetTransformInfo::TCK_SizeAndLatency) !=
TargetTransformInfo::TCC_Free)
return false;
// For a GEP, we cannot simply use getUserCost because currently it
// optimistically assume that a GEP will fold into addressing mode
// regardless of its users.
const BasicBlock *BB = GEP->getParent();
for (const User *U : GEP->users()) {
const Instruction *UI = cast<Instruction>(U);
if (CurLoop->contains(UI) &&
(BB != UI->getParent() ||
(!isa<StoreInst>(UI) && !isa<LoadInst>(UI))))
return false;
}
return true;
} else
return TTI->getUserCost(&I, TargetTransformInfo::TCK_SizeAndLatency) ==
TargetTransformInfo::TCC_Free;
}
/// Return true if the only users of this instruction are outside of
/// the loop. If this is true, we can sink the instruction to the exit
/// blocks of the loop.
///
/// We also return true if the instruction could be folded away in lowering.
/// (e.g., a GEP can be folded into a load as an addressing mode in the loop).
static bool isNotUsedOrFreeInLoop(const Instruction &I, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
TargetTransformInfo *TTI, bool &FreeInLoop) {
const auto &BlockColors = SafetyInfo->getBlockColors();
bool IsFree = isFreeInLoop(I, CurLoop, TTI);
for (const User *U : I.users()) {
const Instruction *UI = cast<Instruction>(U);
if (const PHINode *PN = dyn_cast<PHINode>(UI)) {
const BasicBlock *BB = PN->getParent();
// We cannot sink uses in catchswitches.
if (isa<CatchSwitchInst>(BB->getTerminator()))
return false;
// We need to sink a callsite to a unique funclet. Avoid sinking if the
// phi use is too muddled.
if (isa<CallInst>(I))
if (!BlockColors.empty() &&
BlockColors.find(const_cast<BasicBlock *>(BB))->second.size() != 1)
return false;
}
if (CurLoop->contains(UI)) {
if (IsFree) {
FreeInLoop = true;
continue;
}
return false;
}
}
return true;
}
static Instruction *cloneInstructionInExitBlock(
Instruction &I, BasicBlock &ExitBlock, PHINode &PN, const LoopInfo *LI,
const LoopSafetyInfo *SafetyInfo, MemorySSAUpdater *MSSAU) {
Instruction *New;
if (auto *CI = dyn_cast<CallInst>(&I)) {
const auto &BlockColors = SafetyInfo->getBlockColors();
// Sinking call-sites need to be handled differently from other
// instructions. The cloned call-site needs a funclet bundle operand
// appropriate for its location in the CFG.
SmallVector<OperandBundleDef, 1> OpBundles;
for (unsigned BundleIdx = 0, BundleEnd = CI->getNumOperandBundles();
BundleIdx != BundleEnd; ++BundleIdx) {
OperandBundleUse Bundle = CI->getOperandBundleAt(BundleIdx);
if (Bundle.getTagID() == LLVMContext::OB_funclet)
continue;
OpBundles.emplace_back(Bundle);
}
if (!BlockColors.empty()) {
const ColorVector &CV = BlockColors.find(&ExitBlock)->second;
assert(CV.size() == 1 && "non-unique color for exit block!");
BasicBlock *BBColor = CV.front();
Instruction *EHPad = BBColor->getFirstNonPHI();
if (EHPad->isEHPad())
OpBundles.emplace_back("funclet", EHPad);
}
New = CallInst::Create(CI, OpBundles);
} else {
New = I.clone();
}
ExitBlock.getInstList().insert(ExitBlock.getFirstInsertionPt(), New);
if (!I.getName().empty())
New->setName(I.getName() + ".le");
if (MSSAU && MSSAU->getMemorySSA()->getMemoryAccess(&I)) {
// Create a new MemoryAccess and let MemorySSA set its defining access.
MemoryAccess *NewMemAcc = MSSAU->createMemoryAccessInBB(
New, nullptr, New->getParent(), MemorySSA::Beginning);
if (NewMemAcc) {
if (auto *MemDef = dyn_cast<MemoryDef>(NewMemAcc))
MSSAU->insertDef(MemDef, /*RenameUses=*/true);
else {
auto *MemUse = cast<MemoryUse>(NewMemAcc);
MSSAU->insertUse(MemUse, /*RenameUses=*/true);
}
}
}
// Build LCSSA PHI nodes for any in-loop operands (if legal). Note that
// this is particularly cheap because we can rip off the PHI node that we're
// replacing for the number and blocks of the predecessors.
// OPT: If this shows up in a profile, we can instead finish sinking all
// invariant instructions, and then walk their operands to re-establish
// LCSSA. That will eliminate creating PHI nodes just to nuke them when
// sinking bottom-up.
for (Use &Op : New->operands())
if (LI->wouldBeOutOfLoopUseRequiringLCSSA(Op.get(), PN.getParent())) {
auto *OInst = cast<Instruction>(Op.get());
PHINode *OpPN =
PHINode::Create(OInst->getType(), PN.getNumIncomingValues(),
OInst->getName() + ".lcssa", &ExitBlock.front());
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
OpPN->addIncoming(OInst, PN.getIncomingBlock(i));
Op = OpPN;
}
return New;
}
static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
AliasSetTracker *AST, MemorySSAUpdater *MSSAU) {
if (AST)
AST->deleteValue(&I);
if (MSSAU)
MSSAU->removeMemoryAccess(&I);
SafetyInfo.removeInstruction(&I);
I.eraseFromParent();
}
static void moveInstructionBefore(Instruction &I, Instruction &Dest,
ICFLoopSafetyInfo &SafetyInfo,
MemorySSAUpdater *MSSAU,
ScalarEvolution *SE) {
SafetyInfo.removeInstruction(&I);
SafetyInfo.insertInstructionTo(&I, Dest.getParent());
I.moveBefore(&Dest);
if (MSSAU)
if (MemoryUseOrDef *OldMemAcc = cast_or_null<MemoryUseOrDef>(
MSSAU->getMemorySSA()->getMemoryAccess(&I)))
MSSAU->moveToPlace(OldMemAcc, Dest.getParent(),
MemorySSA::BeforeTerminator);
if (SE)
SE->forgetValue(&I);
}
static Instruction *sinkThroughTriviallyReplaceablePHI(
PHINode *TPN, Instruction *I, LoopInfo *LI,
SmallDenseMap<BasicBlock *, Instruction *, 32> &SunkCopies,
const LoopSafetyInfo *SafetyInfo, const Loop *CurLoop,
MemorySSAUpdater *MSSAU) {
assert(isTriviallyReplaceablePHI(*TPN, *I) &&
"Expect only trivially replaceable PHI");
BasicBlock *ExitBlock = TPN->getParent();
Instruction *New;
auto It = SunkCopies.find(ExitBlock);
if (It != SunkCopies.end())
New = It->second;
else
New = SunkCopies[ExitBlock] = cloneInstructionInExitBlock(
*I, *ExitBlock, *TPN, LI, SafetyInfo, MSSAU);
return New;
}
static bool canSplitPredecessors(PHINode *PN, LoopSafetyInfo *SafetyInfo) {
BasicBlock *BB = PN->getParent();
if (!BB->canSplitPredecessors())
return false;
// It's not impossible to split EHPad blocks, but if BlockColors already exist
// it require updating BlockColors for all offspring blocks accordingly. By
// skipping such corner case, we can make updating BlockColors after splitting
// predecessor fairly simple.
if (!SafetyInfo->getBlockColors().empty() && BB->getFirstNonPHI()->isEHPad())
return false;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *BBPred = *PI;
if (isa<IndirectBrInst>(BBPred->getTerminator()) ||
isa<CallBrInst>(BBPred->getTerminator()))
return false;
}
return true;
}
static void splitPredecessorsOfLoopExit(PHINode *PN, DominatorTree *DT,
LoopInfo *LI, const Loop *CurLoop,
LoopSafetyInfo *SafetyInfo,
MemorySSAUpdater *MSSAU) {
#ifndef NDEBUG
SmallVector<BasicBlock *, 32> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(),
ExitBlocks.end());
#endif
BasicBlock *ExitBB = PN->getParent();
assert(ExitBlockSet.count(ExitBB) && "Expect the PHI is in an exit block.");
// Split predecessors of the loop exit to make instructions in the loop are
// exposed to exit blocks through trivially replaceable PHIs while keeping the
// loop in the canonical form where each predecessor of each exit block should
// be contained within the loop. For example, this will convert the loop below
// from
//
// LB1:
// %v1 =
// br %LE, %LB2
// LB2:
// %v2 =
// br %LE, %LB1
// LE:
// %p = phi [%v1, %LB1], [%v2, %LB2] <-- non-trivially replaceable
//
// to
//
// LB1:
// %v1 =
// br %LE.split, %LB2
// LB2:
// %v2 =
// br %LE.split2, %LB1
// LE.split:
// %p1 = phi [%v1, %LB1] <-- trivially replaceable
// br %LE
// LE.split2:
// %p2 = phi [%v2, %LB2] <-- trivially replaceable
// br %LE
// LE:
// %p = phi [%p1, %LE.split], [%p2, %LE.split2]
//
const auto &BlockColors = SafetyInfo->getBlockColors();
SmallSetVector<BasicBlock *, 8> PredBBs(pred_begin(ExitBB), pred_end(ExitBB));
while (!PredBBs.empty()) {
BasicBlock *PredBB = *PredBBs.begin();
assert(CurLoop->contains(PredBB) &&
"Expect all predecessors are in the loop");
if (PN->getBasicBlockIndex(PredBB) >= 0) {
BasicBlock *NewPred = SplitBlockPredecessors(
ExitBB, PredBB, ".split.loop.exit", DT, LI, MSSAU, true);
// Since we do not allow splitting EH-block with BlockColors in
// canSplitPredecessors(), we can simply assign predecessor's color to
// the new block.
if (!BlockColors.empty())
// Grab a reference to the ColorVector to be inserted before getting the
// reference to the vector we are copying because inserting the new
// element in BlockColors might cause the map to be reallocated.
SafetyInfo->copyColors(NewPred, PredBB);
}
PredBBs.remove(PredBB);
}
}
/// When an instruction is found to only be used outside of the loop, this
/// function moves it to the exit blocks and patches up SSA form as needed.
/// This method is guaranteed to remove the original instruction from its
/// position, and may either delete it or move it to outside of the loop.
///
static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
BlockFrequencyInfo *BFI, const Loop *CurLoop,
ICFLoopSafetyInfo *SafetyInfo, MemorySSAUpdater *MSSAU,
OptimizationRemarkEmitter *ORE) {
bool Changed = false;
LLVM_DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
// Iterate over users to be ready for actual sinking. Replace users via
// unreachable blocks with undef and make all user PHIs trivially replaceable.
SmallPtrSet<Instruction *, 8> VisitedUsers;
for (Value::user_iterator UI = I.user_begin(), UE = I.user_end(); UI != UE;) {
auto *User = cast<Instruction>(*UI);
Use &U = UI.getUse();
++UI;
if (VisitedUsers.count(User) || CurLoop->contains(User))
continue;
if (!DT->isReachableFromEntry(User->getParent())) {
U = UndefValue::get(I.getType());
Changed = true;
continue;
}
// The user must be a PHI node.
PHINode *PN = cast<PHINode>(User);
// Surprisingly, instructions can be used outside of loops without any
// exits. This can only happen in PHI nodes if the incoming block is
// unreachable.
BasicBlock *BB = PN->getIncomingBlock(U);
if (!DT->isReachableFromEntry(BB)) {
U = UndefValue::get(I.getType());
Changed = true;
continue;
}
VisitedUsers.insert(PN);
if (isTriviallyReplaceablePHI(*PN, I))
continue;
if (!canSplitPredecessors(PN, SafetyInfo))
return Changed;
// Split predecessors of the PHI so that we can make users trivially
// replaceable.
splitPredecessorsOfLoopExit(PN, DT, LI, CurLoop, SafetyInfo, MSSAU);
// Should rebuild the iterators, as they may be invalidated by
// splitPredecessorsOfLoopExit().
UI = I.user_begin();
UE = I.user_end();
}
if (VisitedUsers.empty())
return Changed;
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "InstSunk", &I)
<< "sinking " << ore::NV("Inst", &I);
});
if (isa<LoadInst>(I))
++NumMovedLoads;
else if (isa<CallInst>(I))
++NumMovedCalls;
++NumSunk;
#ifndef NDEBUG
SmallVector<BasicBlock *, 32> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(),
ExitBlocks.end());
#endif
// Clones of this instruction. Don't create more than one per exit block!
SmallDenseMap<BasicBlock *, Instruction *, 32> SunkCopies;
// If this instruction is only used outside of the loop, then all users are
// PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
// the instruction.
// First check if I is worth sinking for all uses. Sink only when it is worth
// across all uses.
SmallSetVector<User*, 8> Users(I.user_begin(), I.user_end());
SmallVector<PHINode *, 8> ExitPNs;
for (auto *UI : Users) {
auto *User = cast<Instruction>(UI);
if (CurLoop->contains(User))
continue;
PHINode *PN = cast<PHINode>(User);
assert(ExitBlockSet.count(PN->getParent()) &&
"The LCSSA PHI is not in an exit block!");
if (!worthSinkOrHoistInst(I, PN->getParent(), ORE, BFI)) {
return Changed;
}
ExitPNs.push_back(PN);
}
for (auto *PN : ExitPNs) {
// The PHI must be trivially replaceable.
Instruction *New = sinkThroughTriviallyReplaceablePHI(
PN, &I, LI, SunkCopies, SafetyInfo, CurLoop, MSSAU);
PN->replaceAllUsesWith(New);
eraseInstruction(*PN, *SafetyInfo, nullptr, nullptr);
Changed = true;
}
return Changed;
}
/// When an instruction is found to only use loop invariant operands that
/// is safe to hoist, this instruction is called to do the dirty work.
///
static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
BasicBlock *Dest, ICFLoopSafetyInfo *SafetyInfo,
MemorySSAUpdater *MSSAU, ScalarEvolution *SE,
OptimizationRemarkEmitter *ORE) {
LLVM_DEBUG(dbgs() << "LICM hoisting to " << Dest->getNameOrAsOperand() << ": "
<< I << "\n");
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "Hoisted", &I) << "hoisting "
<< ore::NV("Inst", &I);
});
// Metadata can be dependent on conditions we are hoisting above.
// Conservatively strip all metadata on the instruction unless we were
// guaranteed to execute I if we entered the loop, in which case the metadata
// is valid in the loop preheader.
if (I.hasMetadataOtherThanDebugLoc() &&
// The check on hasMetadataOtherThanDebugLoc is to prevent us from burning
// time in isGuaranteedToExecute if we don't actually have anything to
// drop. It is a compile time optimization, not required for correctness.
!SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop))
I.dropUnknownNonDebugMetadata();
if (isa<PHINode>(I))
// Move the new node to the end of the phi list in the destination block.
moveInstructionBefore(I, *Dest->getFirstNonPHI(), *SafetyInfo, MSSAU, SE);
else
// Move the new node to the destination block, before its terminator.
moveInstructionBefore(I, *Dest->getTerminator(), *SafetyInfo, MSSAU, SE);
I.updateLocationAfterHoist();
if (isa<LoadInst>(I))
++NumMovedLoads;
else if (isa<CallInst>(I))
++NumMovedCalls;
++NumHoisted;
}
/// Only sink or hoist an instruction if it is not a trapping instruction,
/// or if the instruction is known not to trap when moved to the preheader.
/// or if it is a trapping instruction and is guaranteed to execute.
static bool isSafeToExecuteUnconditionally(Instruction &Inst,
const DominatorTree *DT,
const TargetLibraryInfo *TLI,
const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE,
const Instruction *CtxI) {
if (isSafeToSpeculativelyExecute(&Inst, CtxI, DT, TLI))
return true;
bool GuaranteedToExecute =
SafetyInfo->isGuaranteedToExecute(Inst, DT, CurLoop);
if (!GuaranteedToExecute) {
auto *LI = dyn_cast<LoadInst>(&Inst);
if (LI && CurLoop->isLoopInvariant(LI->getPointerOperand()))
ORE->emit([&]() {
return OptimizationRemarkMissed(
DEBUG_TYPE, "LoadWithLoopInvariantAddressCondExecuted", LI)
<< "failed to hoist load with loop-invariant address "
"because load is conditionally executed";
});
}
return GuaranteedToExecute;
}
namespace {
class LoopPromoter : public LoadAndStorePromoter {
Value *SomePtr; // Designated pointer to store to.
const SmallSetVector<Value *, 8> &PointerMustAliases;
SmallVectorImpl<BasicBlock *> &LoopExitBlocks;
SmallVectorImpl<Instruction *> &LoopInsertPts;
SmallVectorImpl<MemoryAccess *> &MSSAInsertPts;
PredIteratorCache &PredCache;
AliasSetTracker *AST;
MemorySSAUpdater *MSSAU;
LoopInfo &LI;
DebugLoc DL;
int Alignment;
bool UnorderedAtomic;
AAMDNodes AATags;
ICFLoopSafetyInfo &SafetyInfo;
// We're about to add a use of V in a loop exit block. Insert an LCSSA phi
// (if legal) if doing so would add an out-of-loop use to an instruction
// defined in-loop.
Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
if (!LI.wouldBeOutOfLoopUseRequiringLCSSA(V, BB))
return V;
Instruction *I = cast<Instruction>(V);
// We need to create an LCSSA PHI node for the incoming value and
// store that.
PHINode *PN = PHINode::Create(I->getType(), PredCache.size(BB),
I->getName() + ".lcssa", &BB->front());
for (BasicBlock *Pred : PredCache.get(BB))
PN->addIncoming(I, Pred);
return PN;
}
public:
LoopPromoter(Value *SP, ArrayRef<const Instruction *> Insts, SSAUpdater &S,
const SmallSetVector<Value *, 8> &PMA,
SmallVectorImpl<BasicBlock *> &LEB,
SmallVectorImpl<Instruction *> &LIP,
SmallVectorImpl<MemoryAccess *> &MSSAIP, PredIteratorCache &PIC,
AliasSetTracker *ast, MemorySSAUpdater *MSSAU, LoopInfo &li,
DebugLoc dl, int alignment, bool UnorderedAtomic,
const AAMDNodes &AATags, ICFLoopSafetyInfo &SafetyInfo)
: LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA),
LoopExitBlocks(LEB), LoopInsertPts(LIP), MSSAInsertPts(MSSAIP),
PredCache(PIC), AST(ast), MSSAU(MSSAU), LI(li), DL(std::move(dl)),
Alignment(alignment), UnorderedAtomic(UnorderedAtomic), AATags(AATags),
SafetyInfo(SafetyInfo) {}
bool isInstInList(Instruction *I,
const SmallVectorImpl<Instruction *> &) const override {
Value *Ptr;
if (LoadInst *LI = dyn_cast<LoadInst>(I))
Ptr = LI->getOperand(0);
else
Ptr = cast<StoreInst>(I)->getPointerOperand();
return PointerMustAliases.count(Ptr);
}
void doExtraRewritesBeforeFinalDeletion() override {
// Insert stores after in the loop exit blocks. Each exit block gets a
// store of the live-out values that feed them. Since we've already told
// the SSA updater about the defs in the loop and the preheader
// definition, it is all set and we can start using it.
for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = LoopExitBlocks[i];
Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
LiveInValue = maybeInsertLCSSAPHI(LiveInValue, ExitBlock);
Value *Ptr = maybeInsertLCSSAPHI(SomePtr, ExitBlock);
Instruction *InsertPos = LoopInsertPts[i];
StoreInst *NewSI = new StoreInst(LiveInValue, Ptr, InsertPos);
if (UnorderedAtomic)
NewSI->setOrdering(AtomicOrdering::Unordered);
NewSI->setAlignment(Align(Alignment));
NewSI->setDebugLoc(DL);
if (AATags)
NewSI->setAAMetadata(AATags);
if (MSSAU) {
MemoryAccess *MSSAInsertPoint = MSSAInsertPts[i];
MemoryAccess *NewMemAcc;
if (!MSSAInsertPoint) {
NewMemAcc = MSSAU->createMemoryAccessInBB(
NewSI, nullptr, NewSI->getParent(), MemorySSA::Beginning);
} else {
NewMemAcc =
MSSAU->createMemoryAccessAfter(NewSI, nullptr, MSSAInsertPoint);
}
MSSAInsertPts[i] = NewMemAcc;
MSSAU->insertDef(cast<MemoryDef>(NewMemAcc), true);
// FIXME: true for safety, false may still be correct.
}
}
}
void replaceLoadWithValue(LoadInst *LI, Value *V) const override {
// Update alias analysis.
if (AST)
AST->copyValue(LI, V);
}
void instructionDeleted(Instruction *I) const override {
SafetyInfo.removeInstruction(I);
if (AST)
AST->deleteValue(I);
if (MSSAU)
MSSAU->removeMemoryAccess(I);
}
};
bool isNotCapturedBeforeOrInLoop(const Value *V, const Loop *L,
DominatorTree *DT) {
// We can perform the captured-before check against any instruction in the
// loop header, as the loop header is reachable from any instruction inside
// the loop.
// TODO: ReturnCaptures=true shouldn't be necessary here.
return !PointerMayBeCapturedBefore(V, /* ReturnCaptures */ true,
/* StoreCaptures */ true,
L->getHeader()->getTerminator(), DT);
}
/// Return true iff we can prove that a caller of this function can not inspect
/// the contents of the provided object in a well defined program.
bool isKnownNonEscaping(Value *Object, const Loop *L,
const TargetLibraryInfo *TLI, DominatorTree *DT) {
if (isa<AllocaInst>(Object))
// Since the alloca goes out of scope, we know the caller can't retain a
// reference to it and be well defined. Thus, we don't need to check for
// capture.
return true;
// For all other objects we need to know that the caller can't possibly
// have gotten a reference to the object. There are two components of
// that:
// 1) Object can't be escaped by this function. This is what
// PointerMayBeCaptured checks.
// 2) Object can't have been captured at definition site. For this, we
// need to know the return value is noalias. At the moment, we use a
// weaker condition and handle only AllocLikeFunctions (which are
// known to be noalias). TODO
return isAllocLikeFn(Object, TLI) &&
isNotCapturedBeforeOrInLoop(Object, L, DT);
}
} // namespace
/// Try to promote memory values to scalars by sinking stores out of the
/// loop and moving loads to before the loop. We do this by looping over
/// the stores in the loop, looking for stores to Must pointers which are
/// loop invariant.
///
bool llvm::promoteLoopAccessesToScalars(
const SmallSetVector<Value *, 8> &PointerMustAliases,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
SmallVectorImpl<Instruction *> &InsertPts,
SmallVectorImpl<MemoryAccess *> &MSSAInsertPts, PredIteratorCache &PIC,
LoopInfo *LI, DominatorTree *DT, const TargetLibraryInfo *TLI,
Loop *CurLoop, AliasSetTracker *CurAST, MemorySSAUpdater *MSSAU,
ICFLoopSafetyInfo *SafetyInfo, OptimizationRemarkEmitter *ORE) {
// Verify inputs.
assert(LI != nullptr && DT != nullptr && CurLoop != nullptr &&
SafetyInfo != nullptr &&
"Unexpected Input to promoteLoopAccessesToScalars");
Value *SomePtr = *PointerMustAliases.begin();
BasicBlock *Preheader = CurLoop->getLoopPreheader();
// It is not safe to promote a load/store from the loop if the load/store is
// conditional. For example, turning:
//
// for () { if (c) *P += 1; }
//
// into:
//
// tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
//
// is not safe, because *P may only be valid to access if 'c' is true.
//
// The safety property divides into two parts:
// p1) The memory may not be dereferenceable on entry to the loop. In this
// case, we can't insert the required load in the preheader.
// p2) The memory model does not allow us to insert a store along any dynamic
// path which did not originally have one.
//
// If at least one store is guaranteed to execute, both properties are
// satisfied, and promotion is legal.
//
// This, however, is not a necessary condition. Even if no store/load is
// guaranteed to execute, we can still establish these properties.
// We can establish (p1) by proving that hoisting the load into the preheader
// is safe (i.e. proving dereferenceability on all paths through the loop). We
// can use any access within the alias set to prove dereferenceability,
// since they're all must alias.
//
// There are two ways establish (p2):
// a) Prove the location is thread-local. In this case the memory model
// requirement does not apply, and stores are safe to insert.
// b) Prove a store dominates every exit block. In this case, if an exit
// blocks is reached, the original dynamic path would have taken us through
// the store, so inserting a store into the exit block is safe. Note that this
// is different from the store being guaranteed to execute. For instance,
// if an exception is thrown on the first iteration of the loop, the original
// store is never executed, but the exit blocks are not executed either.
bool DereferenceableInPH = false;
bool SafeToInsertStore = false;
SmallVector<Instruction *, 64> LoopUses;
// We start with an alignment of one and try to find instructions that allow
// us to prove better alignment.
Align Alignment;
// Keep track of which types of access we see
bool SawUnorderedAtomic = false;
bool SawNotAtomic = false;
AAMDNodes AATags;
const DataLayout &MDL = Preheader->getModule()->getDataLayout();
bool IsKnownThreadLocalObject = false;
if (SafetyInfo->anyBlockMayThrow()) {
// If a loop can throw, we have to insert a store along each unwind edge.
// That said, we can't actually make the unwind edge explicit. Therefore,
// we have to prove that the store is dead along the unwind edge. We do
// this by proving that the caller can't have a reference to the object
// after return and thus can't possibly load from the object.
Value *Object = getUnderlyingObject(SomePtr);
if (!isKnownNonEscaping(Object, CurLoop, TLI, DT))
return false;
// Subtlety: Alloca's aren't visible to callers, but *are* potentially
// visible to other threads if captured and used during their lifetimes.
IsKnownThreadLocalObject = !isa<AllocaInst>(Object);
}
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes. While we are at it, collect alignment and AA info.
for (Value *ASIV : PointerMustAliases) {
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes.
if (SomePtr->getType() != ASIV->getType())
return false;
for (User *U : ASIV->users()) {
// Ignore instructions that are outside the loop.
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI || !CurLoop->contains(UI))
continue;
// If there is an non-load/store instruction in the loop, we can't promote
// it.
if (LoadInst *Load = dyn_cast<LoadInst>(UI)) {
if (!Load->isUnordered())
return false;
SawUnorderedAtomic |= Load->isAtomic();
SawNotAtomic |= !Load->isAtomic();
Align InstAlignment = Load->getAlign();
// Note that proving a load safe to speculate requires proving
// sufficient alignment at the target location. Proving it guaranteed
// to execute does as well. Thus we can increase our guaranteed
// alignment as well.
if (!DereferenceableInPH || (InstAlignment > Alignment))
if (isSafeToExecuteUnconditionally(*Load, DT, TLI, CurLoop,
SafetyInfo, ORE,
Preheader->getTerminator())) {
DereferenceableInPH = true;
Alignment = std::max(Alignment, InstAlignment);
}
} else if (const StoreInst *Store = dyn_cast<StoreInst>(UI)) {
// Stores *of* the pointer are not interesting, only stores *to* the
// pointer.
if (UI->getOperand(1) != ASIV)
continue;
if (!Store->isUnordered())
return false;
SawUnorderedAtomic |= Store->isAtomic();
SawNotAtomic |= !Store->isAtomic();
// If the store is guaranteed to execute, both properties are satisfied.
// We may want to check if a store is guaranteed to execute even if we
// already know that promotion is safe, since it may have higher
// alignment than any other guaranteed stores, in which case we can
// raise the alignment on the promoted store.
Align InstAlignment = Store->getAlign();
if (!DereferenceableInPH || !SafeToInsertStore ||
(InstAlignment > Alignment)) {
if (SafetyInfo->isGuaranteedToExecute(*UI, DT, CurLoop)) {
DereferenceableInPH = true;
SafeToInsertStore = true;
Alignment = std::max(Alignment, InstAlignment);
}
}
// If a store dominates all exit blocks, it is safe to sink.
// As explained above, if an exit block was executed, a dominating
// store must have been executed at least once, so we are not
// introducing stores on paths that did not have them.
// Note that this only looks at explicit exit blocks. If we ever
// start sinking stores into unwind edges (see above), this will break.
if (!SafeToInsertStore)
SafeToInsertStore = llvm::all_of(ExitBlocks, [&](BasicBlock *Exit) {
return DT->dominates(Store->getParent(), Exit);
});
// If the store is not guaranteed to execute, we may still get
// deref info through it.
if (!DereferenceableInPH) {
DereferenceableInPH = isDereferenceableAndAlignedPointer(
Store->getPointerOperand(), Store->getValueOperand()->getType(),
Store->getAlign(), MDL, Preheader->getTerminator(), DT, TLI);
}
} else
return false; // Not a load or store.
// Merge the AA tags.
if (LoopUses.empty()) {
// On the first load/store, just take its AA tags.
UI->getAAMetadata(AATags);
} else if (AATags) {
UI->getAAMetadata(AATags, /* Merge = */ true);
}
LoopUses.push_back(UI);
}
}
// If we found both an unordered atomic instruction and a non-atomic memory
// access, bail. We can't blindly promote non-atomic to atomic since we
// might not be able to lower the result. We can't downgrade since that
// would violate memory model. Also, align 0 is an error for atomics.
if (SawUnorderedAtomic && SawNotAtomic)
return false;
// If we're inserting an atomic load in the preheader, we must be able to
// lower it. We're only guaranteed to be able to lower naturally aligned
// atomics.
auto *SomePtrElemType = SomePtr->getType()->getPointerElementType();
if (SawUnorderedAtomic &&
Alignment < MDL.getTypeStoreSize(SomePtrElemType))
return false;
// If we couldn't prove we can hoist the load, bail.
if (!DereferenceableInPH)
return false;
// We know we can hoist the load, but don't have a guaranteed store.
// Check whether the location is thread-local. If it is, then we can insert
// stores along paths which originally didn't have them without violating the
// memory model.
if (!SafeToInsertStore) {
if (IsKnownThreadLocalObject)
SafeToInsertStore = true;
else {
Value *Object = getUnderlyingObject(SomePtr);
SafeToInsertStore =
(isAllocLikeFn(Object, TLI) || isa<AllocaInst>(Object)) &&
isNotCapturedBeforeOrInLoop(Object, CurLoop, DT);
}
}
// If we've still failed to prove we can sink the store, give up.
if (!SafeToInsertStore)
return false;
// Otherwise, this is safe to promote, lets do it!
LLVM_DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " << *SomePtr
<< '\n');
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "PromoteLoopAccessesToScalar",
LoopUses[0])
<< "Moving accesses to memory location out of the loop";
});
++NumPromoted;
// Look at all the loop uses, and try to merge their locations.
std::vector<const DILocation *> LoopUsesLocs;
for (auto U : LoopUses)
LoopUsesLocs.push_back(U->getDebugLoc().get());
auto DL = DebugLoc(DILocation::getMergedLocations(LoopUsesLocs));
// We use the SSAUpdater interface to insert phi nodes as required.
SmallVector<PHINode *, 16> NewPHIs;
SSAUpdater SSA(&NewPHIs);
LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
InsertPts, MSSAInsertPts, PIC, CurAST, MSSAU, *LI, DL,
Alignment.value(), SawUnorderedAtomic, AATags,
*SafetyInfo);
// Set up the preheader to have a definition of the value. It is the live-out
// value from the preheader that uses in the loop will use.
LoadInst *PreheaderLoad = new LoadInst(
SomePtr->getType()->getPointerElementType(), SomePtr,
SomePtr->getName() + ".promoted", Preheader->getTerminator());
if (SawUnorderedAtomic)
PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
PreheaderLoad->setAlignment(Alignment);
PreheaderLoad->setDebugLoc(DebugLoc());
if (AATags)
PreheaderLoad->setAAMetadata(AATags);
SSA.AddAvailableValue(Preheader, PreheaderLoad);
if (MSSAU) {
MemoryAccess *PreheaderLoadMemoryAccess = MSSAU->createMemoryAccessInBB(
PreheaderLoad, nullptr, PreheaderLoad->getParent(), MemorySSA::End);
MemoryUse *NewMemUse = cast<MemoryUse>(PreheaderLoadMemoryAccess);
MSSAU->insertUse(NewMemUse, /*RenameUses=*/true);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Rewrite all the loads in the loop and remember all the definitions from
// stores in the loop.
Promoter.run(LoopUses);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// If the SSAUpdater didn't use the load in the preheader, just zap it now.
if (PreheaderLoad->use_empty())
eraseInstruction(*PreheaderLoad, *SafetyInfo, CurAST, MSSAU);
return true;
}
static void foreachMemoryAccess(MemorySSA *MSSA, Loop *L,
function_ref<void(Instruction *)> Fn) {
for (const BasicBlock *BB : L->blocks())
if (const auto *Accesses = MSSA->getBlockAccesses(BB))
for (const auto &Access : *Accesses)
if (const auto *MUD = dyn_cast<MemoryUseOrDef>(&Access))
Fn(MUD->getMemoryInst());
}
static SmallVector<SmallSetVector<Value *, 8>, 0>
collectPromotionCandidates(MemorySSA *MSSA, AliasAnalysis *AA, Loop *L) {
AliasSetTracker AST(*AA);
auto IsPotentiallyPromotable = [L](const Instruction *I) {
if (const auto *SI = dyn_cast<StoreInst>(I))
return L->isLoopInvariant(SI->getPointerOperand());
if (const auto *LI = dyn_cast<LoadInst>(I))
return L->isLoopInvariant(LI->getPointerOperand());
return false;
};
// Populate AST with potentially promotable accesses and remove them from
// MaybePromotable, so they will not be checked again on the next iteration.
SmallPtrSet<Value *, 16> AttemptingPromotion;
foreachMemoryAccess(MSSA, L, [&](Instruction *I) {
if (IsPotentiallyPromotable(I)) {
AttemptingPromotion.insert(I);
AST.add(I);
}
});
// We're only interested in must-alias sets that contain a mod.
SmallVector<const AliasSet *, 8> Sets;
for (AliasSet &AS : AST)
if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias())
Sets.push_back(&AS);
if (Sets.empty())
return {}; // Nothing to promote...
// Discard any sets for which there is an aliasing non-promotable access.
foreachMemoryAccess(MSSA, L, [&](Instruction *I) {
if (AttemptingPromotion.contains(I))
return;
llvm::erase_if(Sets, [&](const AliasSet *AS) {
return AS->aliasesUnknownInst(I, *AA);
});
});
SmallVector<SmallSetVector<Value *, 8>, 0> Result;
for (const AliasSet *Set : Sets) {
SmallSetVector<Value *, 8> PointerMustAliases;
for (const auto &ASI : *Set)
PointerMustAliases.insert(ASI.getValue());
Result.push_back(std::move(PointerMustAliases));
}
return Result;
}
/// Returns an owning pointer to an alias set which incorporates aliasing info
/// from L and all subloops of L.
std::unique_ptr<AliasSetTracker>
LoopInvariantCodeMotion::collectAliasInfoForLoop(Loop *L, LoopInfo *LI,
AAResults *AA) {
auto CurAST = std::make_unique<AliasSetTracker>(*AA);
// Add everything from all the sub loops.
for (Loop *InnerL : L->getSubLoops())
for (BasicBlock *BB : InnerL->blocks())
CurAST->add(*BB);
// And merge in this loop (without anything from inner loops).
for (BasicBlock *BB : L->blocks())
if (LI->getLoopFor(BB) == L)
CurAST->add(*BB);
return CurAST;
}
static bool pointerInvalidatedByLoop(MemoryLocation MemLoc,
AliasSetTracker *CurAST, Loop *CurLoop,
AAResults *AA) {
// First check to see if any of the basic blocks in CurLoop invalidate *V.
bool isInvalidatedAccordingToAST = CurAST->getAliasSetFor(MemLoc).isMod();
if (!isInvalidatedAccordingToAST || !LICMN2Theshold)
return isInvalidatedAccordingToAST;
// Check with a diagnostic analysis if we can refine the information above.
// This is to identify the limitations of using the AST.
// The alias set mechanism used by LICM has a major weakness in that it
// combines all things which may alias into a single set *before* asking
// modref questions. As a result, a single readonly call within a loop will
// collapse all loads and stores into a single alias set and report
// invalidation if the loop contains any store. For example, readonly calls
// with deopt states have this form and create a general alias set with all
// loads and stores. In order to get any LICM in loops containing possible
// deopt states we need a more precise invalidation of checking the mod ref
// info of each instruction within the loop and LI. This has a complexity of
// O(N^2), so currently, it is used only as a diagnostic tool since the
// default value of LICMN2Threshold is zero.
// Don't look at nested loops.
if (CurLoop->begin() != CurLoop->end())
return true;
int N = 0;
for (BasicBlock *BB : CurLoop->getBlocks())
for (Instruction &I : *BB) {
if (N >= LICMN2Theshold) {
LLVM_DEBUG(dbgs() << "Alasing N2 threshold exhausted for "
<< *(MemLoc.Ptr) << "\n");
return true;
}
N++;
auto Res = AA->getModRefInfo(&I, MemLoc);
if (isModSet(Res)) {
LLVM_DEBUG(dbgs() << "Aliasing failed on " << I << " for "
<< *(MemLoc.Ptr) << "\n");
return true;
}
}
LLVM_DEBUG(dbgs() << "Aliasing okay for " << *(MemLoc.Ptr) << "\n");
return false;
}
bool pointerInvalidatedByLoopWithMSSA(MemorySSA *MSSA, MemoryUse *MU,
Loop *CurLoop, Instruction &I,
SinkAndHoistLICMFlags &Flags) {
// For hoisting, use the walker to determine safety
if (!Flags.getIsSink()) {
MemoryAccess *Source;
// See declaration of SetLicmMssaOptCap for usage details.
if (Flags.tooManyClobberingCalls())
Source = MU->getDefiningAccess();
else {
Source = MSSA->getSkipSelfWalker()->getClobberingMemoryAccess(MU);
Flags.incrementClobberingCalls();
}
return !MSSA->isLiveOnEntryDef(Source) &&
CurLoop->contains(Source->getBlock());
}
// For sinking, we'd need to check all Defs below this use. The getClobbering
// call will look on the backedge of the loop, but will check aliasing with
// the instructions on the previous iteration.
// For example:
// for (i ... )
// load a[i] ( Use (LoE)
// store a[i] ( 1 = Def (2), with 2 = Phi for the loop.
// i++;
// The load sees no clobbering inside the loop, as the backedge alias check
// does phi translation, and will check aliasing against store a[i-1].
// However sinking the load outside the loop, below the store is incorrect.
// For now, only sink if there are no Defs in the loop, and the existing ones
// precede the use and are in the same block.
// FIXME: Increase precision: Safe to sink if Use post dominates the Def;
// needs PostDominatorTreeAnalysis.
// FIXME: More precise: no Defs that alias this Use.
if (Flags.tooManyMemoryAccesses())
return true;
for (auto *BB : CurLoop->getBlocks())
if (pointerInvalidatedByBlockWithMSSA(*BB, *MSSA, *MU))
return true;
// When sinking, the source block may not be part of the loop so check it.
if (!CurLoop->contains(&I))
return pointerInvalidatedByBlockWithMSSA(*I.getParent(), *MSSA, *MU);
return false;
}
bool pointerInvalidatedByBlockWithMSSA(BasicBlock &BB, MemorySSA &MSSA,
MemoryUse &MU) {
if (const auto *Accesses = MSSA.getBlockDefs(&BB))
for (const auto &MA : *Accesses)
if (const auto *MD = dyn_cast<MemoryDef>(&MA))
if (MU.getBlock() != MD->getBlock() || !MSSA.locallyDominates(MD, &MU))
return true;
return false;
}
/// Little predicate that returns true if the specified basic block is in
/// a subloop of the current one, not the current one itself.
///
static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI) {
assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
return LI->getLoopFor(BB) != CurLoop;
}
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