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llvm-mirror/lib/Transforms/Utils/LoopSimplify.cpp
Philip Reames 647ddb3ec0 [LPM/BPI] Preserve BPI through trivial loop pass pipeline (e.g. LCSSA, LoopSimplify) 
Currently, we do not expose BPI to loop passes at all. In the old pass manager, we appear to have been ignoring the fact that LCSSA and/or LoopSimplify didn't preserve BPI, and making it available to the following loop passes anyways.  In the new one, it's invalidated before running any loop pass if either LCSSA or LoopSimplify actually make changes. If they don't make changes, then BPI is valid and available.  So, we go ahead and teach LCSSA and LoopSimplify how to preserve BPI for consistency between old and new pass managers.

This patch avoids an invalidation between the two requires in the following trivial pass pipeline:
opt -passes="requires<branch-prob>,loop(no-op-loop),requires<branch-prob>"
(when the input file is one which requires either LCSSA or LoopSimplify to canonicalize the loops)

Differential Revision: https://reviews.llvm.org/D60790

llvm-svn: 358901
2019-04-22 17:13:43 +00:00

882 lines
34 KiB
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//===- LoopSimplify.cpp - Loop Canonicalization 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 several transformations to transform natural loops into a
// simpler form, which makes subsequent analyses and transformations simpler and
// more effective.
//
// Loop pre-header insertion guarantees that there is a single, non-critical
// entry edge from outside of the loop to the loop header. This simplifies a
// number of analyses and transformations, such as LICM.
//
// Loop exit-block insertion guarantees that all exit blocks from the loop
// (blocks which are outside of the loop that have predecessors inside of the
// loop) only have predecessors from inside of the loop (and are thus dominated
// by the loop header). This simplifies transformations such as store-sinking
// that are built into LICM.
//
// This pass also guarantees that loops will have exactly one backedge.
//
// Indirectbr instructions introduce several complications. If the loop
// contains or is entered by an indirectbr instruction, it may not be possible
// to transform the loop and make these guarantees. Client code should check
// that these conditions are true before relying on them.
//
// Similar complications arise from callbr instructions, particularly in
// asm-goto where blockaddress expressions are used.
//
// Note that the simplifycfg pass will clean up blocks which are split out but
// end up being unnecessary, so usage of this pass should not pessimize
// generated code.
//
// This pass obviously modifies the CFG, but updates loop information and
// dominator information.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loop-simplify"
STATISTIC(NumNested , "Number of nested loops split out");
// If the block isn't already, move the new block to right after some 'outside
// block' block. This prevents the preheader from being placed inside the loop
// body, e.g. when the loop hasn't been rotated.
static void placeSplitBlockCarefully(BasicBlock *NewBB,
SmallVectorImpl<BasicBlock *> &SplitPreds,
Loop *L) {
// Check to see if NewBB is already well placed.
Function::iterator BBI = --NewBB->getIterator();
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
if (&*BBI == SplitPreds[i])
return;
}
// If it isn't already after an outside block, move it after one. This is
// always good as it makes the uncond branch from the outside block into a
// fall-through.
// Figure out *which* outside block to put this after. Prefer an outside
// block that neighbors a BB actually in the loop.
BasicBlock *FoundBB = nullptr;
for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
Function::iterator BBI = SplitPreds[i]->getIterator();
if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {
FoundBB = SplitPreds[i];
break;
}
}
// If our heuristic for a *good* bb to place this after doesn't find
// anything, just pick something. It's likely better than leaving it within
// the loop.
if (!FoundBB)
FoundBB = SplitPreds[0];
NewBB->moveAfter(FoundBB);
}
/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
/// preheader, this method is called to insert one. This method has two phases:
/// preheader insertion and analysis updating.
///
BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
LoopInfo *LI, bool PreserveLCSSA) {
BasicBlock *Header = L->getHeader();
// Compute the set of predecessors of the loop that are not in the loop.
SmallVector<BasicBlock*, 8> OutsideBlocks;
for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
PI != PE; ++PI) {
BasicBlock *P = *PI;
if (!L->contains(P)) { // Coming in from outside the loop?
// If the loop is branched to from an indirect terminator, we won't
// be able to fully transform the loop, because it prohibits
// edge splitting.
if (P->getTerminator()->isIndirectTerminator())
return nullptr;
// Keep track of it.
OutsideBlocks.push_back(P);
}
}
// Split out the loop pre-header.
BasicBlock *PreheaderBB;
PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
LI, nullptr, PreserveLCSSA);
if (!PreheaderBB)
return nullptr;
LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
<< PreheaderBB->getName() << "\n");
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
return PreheaderBB;
}
/// Add the specified block, and all of its predecessors, to the specified set,
/// if it's not already in there. Stop predecessor traversal when we reach
/// StopBlock.
static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
std::set<BasicBlock*> &Blocks) {
SmallVector<BasicBlock *, 8> Worklist;
Worklist.push_back(InputBB);
do {
BasicBlock *BB = Worklist.pop_back_val();
if (Blocks.insert(BB).second && BB != StopBlock)
// If BB is not already processed and it is not a stop block then
// insert its predecessor in the work list
for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
BasicBlock *WBB = *I;
Worklist.push_back(WBB);
}
} while (!Worklist.empty());
}
/// The first part of loop-nestification is to find a PHI node that tells
/// us how to partition the loops.
static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
AssumptionCache *AC) {
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
PHINode *PN = cast<PHINode>(I);
++I;
if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
// This is a degenerate PHI already, don't modify it!
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
continue;
}
// Scan this PHI node looking for a use of the PHI node by itself.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == PN &&
L->contains(PN->getIncomingBlock(i)))
// We found something tasty to remove.
return PN;
}
return nullptr;
}
/// If this loop has multiple backedges, try to pull one of them out into
/// a nested loop.
///
/// This is important for code that looks like
/// this:
///
/// Loop:
/// ...
/// br cond, Loop, Next
/// ...
/// br cond2, Loop, Out
///
/// To identify this common case, we look at the PHI nodes in the header of the
/// loop. PHI nodes with unchanging values on one backedge correspond to values
/// that change in the "outer" loop, but not in the "inner" loop.
///
/// If we are able to separate out a loop, return the new outer loop that was
/// created.
///
static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
DominatorTree *DT, LoopInfo *LI,
ScalarEvolution *SE, bool PreserveLCSSA,
AssumptionCache *AC) {
// Don't try to separate loops without a preheader.
if (!Preheader)
return nullptr;
// The header is not a landing pad; preheader insertion should ensure this.
BasicBlock *Header = L->getHeader();
assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
if (!PN) return nullptr; // No known way to partition.
// Pull out all predecessors that have varying values in the loop. This
// handles the case when a PHI node has multiple instances of itself as
// arguments.
SmallVector<BasicBlock*, 8> OuterLoopPreds;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
if (PN->getIncomingValue(i) != PN ||
!L->contains(PN->getIncomingBlock(i))) {
// We can't split indirect control flow edges.
if (PN->getIncomingBlock(i)->getTerminator()->isIndirectTerminator())
return nullptr;
OuterLoopPreds.push_back(PN->getIncomingBlock(i));
}
}
LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
// If ScalarEvolution is around and knows anything about values in
// this loop, tell it to forget them, because we're about to
// substantially change it.
if (SE)
SE->forgetLoop(L);
BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
DT, LI, nullptr, PreserveLCSSA);
// Make sure that NewBB is put someplace intelligent, which doesn't mess up
// code layout too horribly.
placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
// Create the new outer loop.
Loop *NewOuter = LI->AllocateLoop();
// Change the parent loop to use the outer loop as its child now.
if (Loop *Parent = L->getParentLoop())
Parent->replaceChildLoopWith(L, NewOuter);
else
LI->changeTopLevelLoop(L, NewOuter);
// L is now a subloop of our outer loop.
NewOuter->addChildLoop(L);
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
NewOuter->addBlockEntry(*I);
// Now reset the header in L, which had been moved by
// SplitBlockPredecessors for the outer loop.
L->moveToHeader(Header);
// Determine which blocks should stay in L and which should be moved out to
// the Outer loop now.
std::set<BasicBlock*> BlocksInL;
for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
BasicBlock *P = *PI;
if (DT->dominates(Header, P))
addBlockAndPredsToSet(P, Header, BlocksInL);
}
// Scan all of the loop children of L, moving them to OuterLoop if they are
// not part of the inner loop.
const std::vector<Loop*> &SubLoops = L->getSubLoops();
for (size_t I = 0; I != SubLoops.size(); )
if (BlocksInL.count(SubLoops[I]->getHeader()))
++I; // Loop remains in L
else
NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
SmallVector<BasicBlock *, 8> OuterLoopBlocks;
OuterLoopBlocks.push_back(NewBB);
// Now that we know which blocks are in L and which need to be moved to
// OuterLoop, move any blocks that need it.
for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
BasicBlock *BB = L->getBlocks()[i];
if (!BlocksInL.count(BB)) {
// Move this block to the parent, updating the exit blocks sets
L->removeBlockFromLoop(BB);
if ((*LI)[BB] == L) {
LI->changeLoopFor(BB, NewOuter);
OuterLoopBlocks.push_back(BB);
}
--i;
}
}
// Split edges to exit blocks from the inner loop, if they emerged in the
// process of separating the outer one.
formDedicatedExitBlocks(L, DT, LI, nullptr, PreserveLCSSA);
if (PreserveLCSSA) {
// Fix LCSSA form for L. Some values, which previously were only used inside
// L, can now be used in NewOuter loop. We need to insert phi-nodes for them
// in corresponding exit blocks.
// We don't need to form LCSSA recursively, because there cannot be uses
// inside a newly created loop of defs from inner loops as those would
// already be a use of an LCSSA phi node.
formLCSSA(*L, *DT, LI, SE);
assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) &&
"LCSSA is broken after separating nested loops!");
}
return NewOuter;
}
/// This method is called when the specified loop has more than one
/// backedge in it.
///
/// If this occurs, revector all of these backedges to target a new basic block
/// and have that block branch to the loop header. This ensures that loops
/// have exactly one backedge.
static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
DominatorTree *DT, LoopInfo *LI) {
assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
// Get information about the loop
BasicBlock *Header = L->getHeader();
Function *F = Header->getParent();
// Unique backedge insertion currently depends on having a preheader.
if (!Preheader)
return nullptr;
// The header is not an EH pad; preheader insertion should ensure this.
assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
// Figure out which basic blocks contain back-edges to the loop header.
std::vector<BasicBlock*> BackedgeBlocks;
for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
BasicBlock *P = *I;
// Indirect edges cannot be split, so we must fail if we find one.
if (P->getTerminator()->isIndirectTerminator())
return nullptr;
if (P != Preheader) BackedgeBlocks.push_back(P);
}
// Create and insert the new backedge block...
BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
Header->getName() + ".backedge", F);
BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
<< BEBlock->getName() << "\n");
// Move the new backedge block to right after the last backedge block.
Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
// Now that the block has been inserted into the function, create PHI nodes in
// the backedge block which correspond to any PHI nodes in the header block.
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
PN->getName()+".be", BETerminator);
// Loop over the PHI node, moving all entries except the one for the
// preheader over to the new PHI node.
unsigned PreheaderIdx = ~0U;
bool HasUniqueIncomingValue = true;
Value *UniqueValue = nullptr;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *IBB = PN->getIncomingBlock(i);
Value *IV = PN->getIncomingValue(i);
if (IBB == Preheader) {
PreheaderIdx = i;
} else {
NewPN->addIncoming(IV, IBB);
if (HasUniqueIncomingValue) {
if (!UniqueValue)
UniqueValue = IV;
else if (UniqueValue != IV)
HasUniqueIncomingValue = false;
}
}
}
// Delete all of the incoming values from the old PN except the preheader's
assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
if (PreheaderIdx != 0) {
PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
}
// Nuke all entries except the zero'th.
for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
PN->removeIncomingValue(e-i, false);
// Finally, add the newly constructed PHI node as the entry for the BEBlock.
PN->addIncoming(NewPN, BEBlock);
// As an optimization, if all incoming values in the new PhiNode (which is a
// subset of the incoming values of the old PHI node) have the same value,
// eliminate the PHI Node.
if (HasUniqueIncomingValue) {
NewPN->replaceAllUsesWith(UniqueValue);
BEBlock->getInstList().erase(NewPN);
}
}
// Now that all of the PHI nodes have been inserted and adjusted, modify the
// backedge blocks to jump to the BEBlock instead of the header.
// If one of the backedges has llvm.loop metadata attached, we remove
// it from the backedge and add it to BEBlock.
unsigned LoopMDKind = BEBlock->getContext().getMDKindID("llvm.loop");
MDNode *LoopMD = nullptr;
for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
Instruction *TI = BackedgeBlocks[i]->getTerminator();
if (!LoopMD)
LoopMD = TI->getMetadata(LoopMDKind);
TI->setMetadata(LoopMDKind, nullptr);
for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
if (TI->getSuccessor(Op) == Header)
TI->setSuccessor(Op, BEBlock);
}
BEBlock->getTerminator()->setMetadata(LoopMDKind, LoopMD);
//===--- Update all analyses which we must preserve now -----------------===//
// Update Loop Information - we know that this block is now in the current
// loop and all parent loops.
L->addBasicBlockToLoop(BEBlock, *LI);
// Update dominator information
DT->splitBlock(BEBlock);
return BEBlock;
}
/// Simplify one loop and queue further loops for simplification.
static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
DominatorTree *DT, LoopInfo *LI,
ScalarEvolution *SE, AssumptionCache *AC,
bool PreserveLCSSA) {
bool Changed = false;
ReprocessLoop:
// Check to see that no blocks (other than the header) in this loop have
// predecessors that are not in the loop. This is not valid for natural
// loops, but can occur if the blocks are unreachable. Since they are
// unreachable we can just shamelessly delete those CFG edges!
for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
BB != E; ++BB) {
if (*BB == L->getHeader()) continue;
SmallPtrSet<BasicBlock*, 4> BadPreds;
for (pred_iterator PI = pred_begin(*BB),
PE = pred_end(*BB); PI != PE; ++PI) {
BasicBlock *P = *PI;
if (!L->contains(P))
BadPreds.insert(P);
}
// Delete each unique out-of-loop (and thus dead) predecessor.
for (BasicBlock *P : BadPreds) {
LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
<< P->getName() << "\n");
// Zap the dead pred's terminator and replace it with unreachable.
Instruction *TI = P->getTerminator();
changeToUnreachable(TI, /*UseLLVMTrap=*/false, PreserveLCSSA);
Changed = true;
}
}
// If there are exiting blocks with branches on undef, resolve the undef in
// the direction which will exit the loop. This will help simplify loop
// trip count computations.
SmallVector<BasicBlock*, 8> ExitingBlocks;
L->getExitingBlocks(ExitingBlocks);
for (BasicBlock *ExitingBlock : ExitingBlocks)
if (BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()))
if (BI->isConditional()) {
if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
LLVM_DEBUG(dbgs()
<< "LoopSimplify: Resolving \"br i1 undef\" to exit in "
<< ExitingBlock->getName() << "\n");
BI->setCondition(ConstantInt::get(Cond->getType(),
!L->contains(BI->getSuccessor(0))));
Changed = true;
}
}
// Does the loop already have a preheader? If so, don't insert one.
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader) {
Preheader = InsertPreheaderForLoop(L, DT, LI, PreserveLCSSA);
if (Preheader)
Changed = true;
}
// Next, check to make sure that all exit nodes of the loop only have
// predecessors that are inside of the loop. This check guarantees that the
// loop preheader/header will dominate the exit blocks. If the exit block has
// predecessors from outside of the loop, split the edge now.
if (formDedicatedExitBlocks(L, DT, LI, nullptr, PreserveLCSSA))
Changed = true;
// If the header has more than two predecessors at this point (from the
// preheader and from multiple backedges), we must adjust the loop.
BasicBlock *LoopLatch = L->getLoopLatch();
if (!LoopLatch) {
// If this is really a nested loop, rip it out into a child loop. Don't do
// this for loops with a giant number of backedges, just factor them into a
// common backedge instead.
if (L->getNumBackEdges() < 8) {
if (Loop *OuterL =
separateNestedLoop(L, Preheader, DT, LI, SE, PreserveLCSSA, AC)) {
++NumNested;
// Enqueue the outer loop as it should be processed next in our
// depth-first nest walk.
Worklist.push_back(OuterL);
// This is a big restructuring change, reprocess the whole loop.
Changed = true;
// GCC doesn't tail recursion eliminate this.
// FIXME: It isn't clear we can't rely on LLVM to TRE this.
goto ReprocessLoop;
}
}
// If we either couldn't, or didn't want to, identify nesting of the loops,
// insert a new block that all backedges target, then make it jump to the
// loop header.
LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI);
if (LoopLatch)
Changed = true;
}
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
// Scan over the PHI nodes in the loop header. Since they now have only two
// incoming values (the loop is canonicalized), we may have simplified the PHI
// down to 'X = phi [X, Y]', which should be replaced with 'Y'.
PHINode *PN;
for (BasicBlock::iterator I = L->getHeader()->begin();
(PN = dyn_cast<PHINode>(I++)); )
if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
if (SE) SE->forgetValue(PN);
if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(PN, V)) {
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
}
}
// If this loop has multiple exits and the exits all go to the same
// block, attempt to merge the exits. This helps several passes, such
// as LoopRotation, which do not support loops with multiple exits.
// SimplifyCFG also does this (and this code uses the same utility
// function), however this code is loop-aware, where SimplifyCFG is
// not. That gives it the advantage of being able to hoist
// loop-invariant instructions out of the way to open up more
// opportunities, and the disadvantage of having the responsibility
// to preserve dominator information.
auto HasUniqueExitBlock = [&]() {
BasicBlock *UniqueExit = nullptr;
for (auto *ExitingBB : ExitingBlocks)
for (auto *SuccBB : successors(ExitingBB)) {
if (L->contains(SuccBB))
continue;
if (!UniqueExit)
UniqueExit = SuccBB;
else if (UniqueExit != SuccBB)
return false;
}
return true;
};
if (HasUniqueExitBlock()) {
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
BasicBlock *ExitingBlock = ExitingBlocks[i];
if (!ExitingBlock->getSinglePredecessor()) continue;
BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
if (!BI || !BI->isConditional()) continue;
CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
if (!CI || CI->getParent() != ExitingBlock) continue;
// Attempt to hoist out all instructions except for the
// comparison and the branch.
bool AllInvariant = true;
bool AnyInvariant = false;
for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) {
Instruction *Inst = &*I++;
if (Inst == CI)
continue;
if (!L->makeLoopInvariant(Inst, AnyInvariant,
Preheader ? Preheader->getTerminator()
: nullptr)) {
AllInvariant = false;
break;
}
}
if (AnyInvariant) {
Changed = true;
// The loop disposition of all SCEV expressions that depend on any
// hoisted values have also changed.
if (SE)
SE->forgetLoopDispositions(L);
}
if (!AllInvariant) continue;
// The block has now been cleared of all instructions except for
// a comparison and a conditional branch. SimplifyCFG may be able
// to fold it now.
if (!FoldBranchToCommonDest(BI))
continue;
// Success. The block is now dead, so remove it from the loop,
// update the dominator tree and delete it.
LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
<< ExitingBlock->getName() << "\n");
assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
Changed = true;
LI->removeBlock(ExitingBlock);
DomTreeNode *Node = DT->getNode(ExitingBlock);
const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
Node->getChildren();
while (!Children.empty()) {
DomTreeNode *Child = Children.front();
DT->changeImmediateDominator(Child, Node->getIDom());
}
DT->eraseNode(ExitingBlock);
BI->getSuccessor(0)->removePredecessor(
ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
BI->getSuccessor(1)->removePredecessor(
ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
ExitingBlock->eraseFromParent();
}
}
// Changing exit conditions for blocks may affect exit counts of this loop and
// any of its paretns, so we must invalidate the entire subtree if we've made
// any changes.
if (Changed && SE)
SE->forgetTopmostLoop(L);
return Changed;
}
bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
ScalarEvolution *SE, AssumptionCache *AC,
bool PreserveLCSSA) {
bool Changed = false;
#ifndef NDEBUG
// If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA
// form.
if (PreserveLCSSA) {
assert(DT && "DT not available.");
assert(LI && "LI not available.");
assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
"Requested to preserve LCSSA, but it's already broken.");
}
#endif
// Worklist maintains our depth-first queue of loops in this nest to process.
SmallVector<Loop *, 4> Worklist;
Worklist.push_back(L);
// Walk the worklist from front to back, pushing newly found sub loops onto
// the back. This will let us process loops from back to front in depth-first
// order. We can use this simple process because loops form a tree.
for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
Loop *L2 = Worklist[Idx];
Worklist.append(L2->begin(), L2->end());
}
while (!Worklist.empty())
Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
AC, PreserveLCSSA);
return Changed;
}
namespace {
struct LoopSimplify : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
LoopSimplify() : FunctionPass(ID) {
initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
// We need loop information to identify the loops...
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addPreserved<BasicAAWrapperPass>();
AU.addPreserved<AAResultsWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
AU.addPreserved<SCEVAAWrapperPass>();
AU.addPreservedID(LCSSAID);
AU.addPreserved<DependenceAnalysisWrapperPass>();
AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
AU.addPreserved<BranchProbabilityInfoWrapperPass>();
}
/// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
void verifyAnalysis() const override;
};
}
char LoopSimplify::ID = 0;
INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
"Canonicalize natural loops", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
"Canonicalize natural loops", false, false)
// Publicly exposed interface to pass...
char &llvm::LoopSimplifyID = LoopSimplify::ID;
Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
/// it in any convenient order) inserting preheaders...
///
bool LoopSimplify::runOnFunction(Function &F) {
bool Changed = false;
LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr;
AssumptionCache *AC =
&getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
// Simplify each loop nest in the function.
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
Changed |= simplifyLoop(*I, DT, LI, SE, AC, PreserveLCSSA);
#ifndef NDEBUG
if (PreserveLCSSA) {
bool InLCSSA = all_of(
*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT, *LI); });
assert(InLCSSA && "LCSSA is broken after loop-simplify.");
}
#endif
return Changed;
}
PreservedAnalyses LoopSimplifyPass::run(Function &F,
FunctionAnalysisManager &AM) {
bool Changed = false;
LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
// Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA
// after simplifying the loops.
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
Changed |= simplifyLoop(*I, DT, LI, SE, AC, /*PreserveLCSSA*/ false);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<DominatorTreeAnalysis>();
PA.preserve<LoopAnalysis>();
PA.preserve<BasicAA>();
PA.preserve<GlobalsAA>();
PA.preserve<SCEVAA>();
PA.preserve<ScalarEvolutionAnalysis>();
PA.preserve<DependenceAnalysis>();
// BPI maps conditional terminators to probabilities, LoopSimplify can insert
// blocks, but it does so only by splitting existing blocks and edges. This
// results in the interesting property that all new terminators inserted are
// unconditional branches which do not appear in BPI. All deletions are
// handled via ValueHandle callbacks w/in BPI.
PA.preserve<BranchProbabilityAnalysis>();
return PA;
}
// FIXME: Restore this code when we re-enable verification in verifyAnalysis
// below.
#if 0
static void verifyLoop(Loop *L) {
// Verify subloops.
for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
verifyLoop(*I);
// It used to be possible to just assert L->isLoopSimplifyForm(), however
// with the introduction of indirectbr, there are now cases where it's
// not possible to transform a loop as necessary. We can at least check
// that there is an indirectbr near any time there's trouble.
// Indirectbr can interfere with preheader and unique backedge insertion.
if (!L->getLoopPreheader() || !L->getLoopLatch()) {
bool HasIndBrPred = false;
for (pred_iterator PI = pred_begin(L->getHeader()),
PE = pred_end(L->getHeader()); PI != PE; ++PI)
if (isa<IndirectBrInst>((*PI)->getTerminator())) {
HasIndBrPred = true;
break;
}
assert(HasIndBrPred &&
"LoopSimplify has no excuse for missing loop header info!");
(void)HasIndBrPred;
}
// Indirectbr can interfere with exit block canonicalization.
if (!L->hasDedicatedExits()) {
bool HasIndBrExiting = false;
SmallVector<BasicBlock*, 8> ExitingBlocks;
L->getExitingBlocks(ExitingBlocks);
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
HasIndBrExiting = true;
break;
}
}
assert(HasIndBrExiting &&
"LoopSimplify has no excuse for missing exit block info!");
(void)HasIndBrExiting;
}
}
#endif
void LoopSimplify::verifyAnalysis() const {
// FIXME: This routine is being called mid-way through the loop pass manager
// as loop passes destroy this analysis. That's actually fine, but we have no
// way of expressing that here. Once all of the passes that destroy this are
// hoisted out of the loop pass manager we can add back verification here.
#if 0
for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
verifyLoop(*I);
#endif
}
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