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llvm-mirror/lib/Analysis/LoopAccessAnalysis.cpp
Florian Hahn 682cc17686 [LAA] We only need pointer checks if there are non-zero checks (NFC). 
If it turns out that we can do runtime checks, but there are no
runtime-checks to generate, set RtCheck.Need to false.

This can happen if we can prove statically that the pointers passed in
to canCheckPtrAtRT do not alias. This should not change any results, but
allows us to skip some work and assert that runtime checks are
generated, if LAA indicates that runtime checks are required.

Reviewers: anemet, Ayal

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D79969

Note: This is a recommit of 259abfc7cbc11cd98c05b1eb8e4b3fb6a9664bc0,
with some suggested renaming.
2020-05-27 12:47:36 +01:00

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//===- LoopAccessAnalysis.cpp - Loop Access Analysis Implementation --------==//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// The implementation for the loop memory dependence that was originally
// developed for the loop vectorizer.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Analysis/VectorUtils.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <iterator>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "loop-accesses"
static cl::opt<unsigned, true>
VectorizationFactor("force-vector-width", cl::Hidden,
cl::desc("Sets the SIMD width. Zero is autoselect."),
cl::location(VectorizerParams::VectorizationFactor));
unsigned VectorizerParams::VectorizationFactor;
static cl::opt<unsigned, true>
VectorizationInterleave("force-vector-interleave", cl::Hidden,
cl::desc("Sets the vectorization interleave count. "
"Zero is autoselect."),
cl::location(
VectorizerParams::VectorizationInterleave));
unsigned VectorizerParams::VectorizationInterleave;
static cl::opt<unsigned, true> RuntimeMemoryCheckThreshold(
"runtime-memory-check-threshold", cl::Hidden,
cl::desc("When performing memory disambiguation checks at runtime do not "
"generate more than this number of comparisons (default = 8)."),
cl::location(VectorizerParams::RuntimeMemoryCheckThreshold), cl::init(8));
unsigned VectorizerParams::RuntimeMemoryCheckThreshold;
/// The maximum iterations used to merge memory checks
static cl::opt<unsigned> MemoryCheckMergeThreshold(
"memory-check-merge-threshold", cl::Hidden,
cl::desc("Maximum number of comparisons done when trying to merge "
"runtime memory checks. (default = 100)"),
cl::init(100));
/// Maximum SIMD width.
const unsigned VectorizerParams::MaxVectorWidth = 64;
/// We collect dependences up to this threshold.
static cl::opt<unsigned>
MaxDependences("max-dependences", cl::Hidden,
cl::desc("Maximum number of dependences collected by "
"loop-access analysis (default = 100)"),
cl::init(100));
/// This enables versioning on the strides of symbolically striding memory
/// accesses in code like the following.
/// for (i = 0; i < N; ++i)
/// A[i * Stride1] += B[i * Stride2] ...
///
/// Will be roughly translated to
/// if (Stride1 == 1 && Stride2 == 1) {
/// for (i = 0; i < N; i+=4)
/// A[i:i+3] += ...
/// } else
/// ...
static cl::opt<bool> EnableMemAccessVersioning(
"enable-mem-access-versioning", cl::init(true), cl::Hidden,
cl::desc("Enable symbolic stride memory access versioning"));
/// Enable store-to-load forwarding conflict detection. This option can
/// be disabled for correctness testing.
static cl::opt<bool> EnableForwardingConflictDetection(
"store-to-load-forwarding-conflict-detection", cl::Hidden,
cl::desc("Enable conflict detection in loop-access analysis"),
cl::init(true));
bool VectorizerParams::isInterleaveForced() {
return ::VectorizationInterleave.getNumOccurrences() > 0;
}
Value *llvm::stripIntegerCast(Value *V) {
if (auto *CI = dyn_cast<CastInst>(V))
if (CI->getOperand(0)->getType()->isIntegerTy())
return CI->getOperand(0);
return V;
}
const SCEV *llvm::replaceSymbolicStrideSCEV(PredicatedScalarEvolution &PSE,
const ValueToValueMap &PtrToStride,
Value *Ptr, Value *OrigPtr) {
const SCEV *OrigSCEV = PSE.getSCEV(Ptr);
// If there is an entry in the map return the SCEV of the pointer with the
// symbolic stride replaced by one.
ValueToValueMap::const_iterator SI =
PtrToStride.find(OrigPtr ? OrigPtr : Ptr);
if (SI != PtrToStride.end()) {
Value *StrideVal = SI->second;
// Strip casts.
StrideVal = stripIntegerCast(StrideVal);
ScalarEvolution *SE = PSE.getSE();
const auto *U = cast<SCEVUnknown>(SE->getSCEV(StrideVal));
const auto *CT =
static_cast<const SCEVConstant *>(SE->getOne(StrideVal->getType()));
PSE.addPredicate(*SE->getEqualPredicate(U, CT));
auto *Expr = PSE.getSCEV(Ptr);
LLVM_DEBUG(dbgs() << "LAA: Replacing SCEV: " << *OrigSCEV
<< " by: " << *Expr << "\n");
return Expr;
}
// Otherwise, just return the SCEV of the original pointer.
return OrigSCEV;
}
RuntimeCheckingPtrGroup::RuntimeCheckingPtrGroup(
unsigned Index, RuntimePointerChecking &RtCheck)
: RtCheck(RtCheck), High(RtCheck.Pointers[Index].End),
Low(RtCheck.Pointers[Index].Start) {
Members.push_back(Index);
}
/// Calculate Start and End points of memory access.
/// Let's assume A is the first access and B is a memory access on N-th loop
/// iteration. Then B is calculated as:
/// B = A + Step*N .
/// Step value may be positive or negative.
/// N is a calculated back-edge taken count:
/// N = (TripCount > 0) ? RoundDown(TripCount -1 , VF) : 0
/// Start and End points are calculated in the following way:
/// Start = UMIN(A, B) ; End = UMAX(A, B) + SizeOfElt,
/// where SizeOfElt is the size of single memory access in bytes.
///
/// There is no conflict when the intervals are disjoint:
/// NoConflict = (P2.Start >= P1.End) || (P1.Start >= P2.End)
void RuntimePointerChecking::insert(Loop *Lp, Value *Ptr, bool WritePtr,
unsigned DepSetId, unsigned ASId,
const ValueToValueMap &Strides,
PredicatedScalarEvolution &PSE) {
// Get the stride replaced scev.
const SCEV *Sc = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
ScalarEvolution *SE = PSE.getSE();
const SCEV *ScStart;
const SCEV *ScEnd;
if (SE->isLoopInvariant(Sc, Lp))
ScStart = ScEnd = Sc;
else {
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Sc);
assert(AR && "Invalid addrec expression");
const SCEV *Ex = PSE.getBackedgeTakenCount();
ScStart = AR->getStart();
ScEnd = AR->evaluateAtIteration(Ex, *SE);
const SCEV *Step = AR->getStepRecurrence(*SE);
// For expressions with negative step, the upper bound is ScStart and the
// lower bound is ScEnd.
if (const auto *CStep = dyn_cast<SCEVConstant>(Step)) {
if (CStep->getValue()->isNegative())
std::swap(ScStart, ScEnd);
} else {
// Fallback case: the step is not constant, but we can still
// get the upper and lower bounds of the interval by using min/max
// expressions.
ScStart = SE->getUMinExpr(ScStart, ScEnd);
ScEnd = SE->getUMaxExpr(AR->getStart(), ScEnd);
}
// Add the size of the pointed element to ScEnd.
unsigned EltSize =
Ptr->getType()->getPointerElementType()->getScalarSizeInBits() / 8;
const SCEV *EltSizeSCEV = SE->getConstant(ScEnd->getType(), EltSize);
ScEnd = SE->getAddExpr(ScEnd, EltSizeSCEV);
}
Pointers.emplace_back(Ptr, ScStart, ScEnd, WritePtr, DepSetId, ASId, Sc);
}
SmallVector<RuntimePointerCheck, 4>
RuntimePointerChecking::generateChecks() const {
SmallVector<RuntimePointerCheck, 4> Checks;
for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
for (unsigned J = I + 1; J < CheckingGroups.size(); ++J) {
const RuntimeCheckingPtrGroup &CGI = CheckingGroups[I];
const RuntimeCheckingPtrGroup &CGJ = CheckingGroups[J];
if (needsChecking(CGI, CGJ))
Checks.push_back(std::make_pair(&CGI, &CGJ));
}
}
return Checks;
}
void RuntimePointerChecking::generateChecks(
MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
assert(Checks.empty() && "Checks is not empty");
groupChecks(DepCands, UseDependencies);
Checks = generateChecks();
}
bool RuntimePointerChecking::needsChecking(
const RuntimeCheckingPtrGroup &M, const RuntimeCheckingPtrGroup &N) const {
for (unsigned I = 0, EI = M.Members.size(); EI != I; ++I)
for (unsigned J = 0, EJ = N.Members.size(); EJ != J; ++J)
if (needsChecking(M.Members[I], N.Members[J]))
return true;
return false;
}
/// Compare \p I and \p J and return the minimum.
/// Return nullptr in case we couldn't find an answer.
static const SCEV *getMinFromExprs(const SCEV *I, const SCEV *J,
ScalarEvolution *SE) {
const SCEV *Diff = SE->getMinusSCEV(J, I);
const SCEVConstant *C = dyn_cast<const SCEVConstant>(Diff);
if (!C)
return nullptr;
if (C->getValue()->isNegative())
return J;
return I;
}
bool RuntimeCheckingPtrGroup::addPointer(unsigned Index) {
const SCEV *Start = RtCheck.Pointers[Index].Start;
const SCEV *End = RtCheck.Pointers[Index].End;
// Compare the starts and ends with the known minimum and maximum
// of this set. We need to know how we compare against the min/max
// of the set in order to be able to emit memchecks.
const SCEV *Min0 = getMinFromExprs(Start, Low, RtCheck.SE);
if (!Min0)
return false;
const SCEV *Min1 = getMinFromExprs(End, High, RtCheck.SE);
if (!Min1)
return false;
// Update the low bound expression if we've found a new min value.
if (Min0 == Start)
Low = Start;
// Update the high bound expression if we've found a new max value.
if (Min1 != End)
High = End;
Members.push_back(Index);
return true;
}
void RuntimePointerChecking::groupChecks(
MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
// We build the groups from dependency candidates equivalence classes
// because:
// - We know that pointers in the same equivalence class share
// the same underlying object and therefore there is a chance
// that we can compare pointers
// - We wouldn't be able to merge two pointers for which we need
// to emit a memcheck. The classes in DepCands are already
// conveniently built such that no two pointers in the same
// class need checking against each other.
// We use the following (greedy) algorithm to construct the groups
// For every pointer in the equivalence class:
// For each existing group:
// - if the difference between this pointer and the min/max bounds
// of the group is a constant, then make the pointer part of the
// group and update the min/max bounds of that group as required.
CheckingGroups.clear();
// If we need to check two pointers to the same underlying object
// with a non-constant difference, we shouldn't perform any pointer
// grouping with those pointers. This is because we can easily get
// into cases where the resulting check would return false, even when
// the accesses are safe.
//
// The following example shows this:
// for (i = 0; i < 1000; ++i)
// a[5000 + i * m] = a[i] + a[i + 9000]
//
// Here grouping gives a check of (5000, 5000 + 1000 * m) against
// (0, 10000) which is always false. However, if m is 1, there is no
// dependence. Not grouping the checks for a[i] and a[i + 9000] allows
// us to perform an accurate check in this case.
//
// The above case requires that we have an UnknownDependence between
// accesses to the same underlying object. This cannot happen unless
// FoundNonConstantDistanceDependence is set, and therefore UseDependencies
// is also false. In this case we will use the fallback path and create
// separate checking groups for all pointers.
// If we don't have the dependency partitions, construct a new
// checking pointer group for each pointer. This is also required
// for correctness, because in this case we can have checking between
// pointers to the same underlying object.
if (!UseDependencies) {
for (unsigned I = 0; I < Pointers.size(); ++I)
CheckingGroups.push_back(RuntimeCheckingPtrGroup(I, *this));
return;
}
unsigned TotalComparisons = 0;
DenseMap<Value *, unsigned> PositionMap;
for (unsigned Index = 0; Index < Pointers.size(); ++Index)
PositionMap[Pointers[Index].PointerValue] = Index;
// We need to keep track of what pointers we've already seen so we
// don't process them twice.
SmallSet<unsigned, 2> Seen;
// Go through all equivalence classes, get the "pointer check groups"
// and add them to the overall solution. We use the order in which accesses
// appear in 'Pointers' to enforce determinism.
for (unsigned I = 0; I < Pointers.size(); ++I) {
// We've seen this pointer before, and therefore already processed
// its equivalence class.
if (Seen.count(I))
continue;
MemoryDepChecker::MemAccessInfo Access(Pointers[I].PointerValue,
Pointers[I].IsWritePtr);
SmallVector<RuntimeCheckingPtrGroup, 2> Groups;
auto LeaderI = DepCands.findValue(DepCands.getLeaderValue(Access));
// Because DepCands is constructed by visiting accesses in the order in
// which they appear in alias sets (which is deterministic) and the
// iteration order within an equivalence class member is only dependent on
// the order in which unions and insertions are performed on the
// equivalence class, the iteration order is deterministic.
for (auto MI = DepCands.member_begin(LeaderI), ME = DepCands.member_end();
MI != ME; ++MI) {
unsigned Pointer = PositionMap[MI->getPointer()];
bool Merged = false;
// Mark this pointer as seen.
Seen.insert(Pointer);
// Go through all the existing sets and see if we can find one
// which can include this pointer.
for (RuntimeCheckingPtrGroup &Group : Groups) {
// Don't perform more than a certain amount of comparisons.
// This should limit the cost of grouping the pointers to something
// reasonable. If we do end up hitting this threshold, the algorithm
// will create separate groups for all remaining pointers.
if (TotalComparisons > MemoryCheckMergeThreshold)
break;
TotalComparisons++;
if (Group.addPointer(Pointer)) {
Merged = true;
break;
}
}
if (!Merged)
// We couldn't add this pointer to any existing set or the threshold
// for the number of comparisons has been reached. Create a new group
// to hold the current pointer.
Groups.push_back(RuntimeCheckingPtrGroup(Pointer, *this));
}
// We've computed the grouped checks for this partition.
// Save the results and continue with the next one.
llvm::copy(Groups, std::back_inserter(CheckingGroups));
}
}
bool RuntimePointerChecking::arePointersInSamePartition(
const SmallVectorImpl<int> &PtrToPartition, unsigned PtrIdx1,
unsigned PtrIdx2) {
return (PtrToPartition[PtrIdx1] != -1 &&
PtrToPartition[PtrIdx1] == PtrToPartition[PtrIdx2]);
}
bool RuntimePointerChecking::needsChecking(unsigned I, unsigned J) const {
const PointerInfo &PointerI = Pointers[I];
const PointerInfo &PointerJ = Pointers[J];
// No need to check if two readonly pointers intersect.
if (!PointerI.IsWritePtr && !PointerJ.IsWritePtr)
return false;
// Only need to check pointers between two different dependency sets.
if (PointerI.DependencySetId == PointerJ.DependencySetId)
return false;
// Only need to check pointers in the same alias set.
if (PointerI.AliasSetId != PointerJ.AliasSetId)
return false;
return true;
}
void RuntimePointerChecking::printChecks(
raw_ostream &OS, const SmallVectorImpl<RuntimePointerCheck> &Checks,
unsigned Depth) const {
unsigned N = 0;
for (const auto &Check : Checks) {
const auto &First = Check.first->Members, &Second = Check.second->Members;
OS.indent(Depth) << "Check " << N++ << ":\n";
OS.indent(Depth + 2) << "Comparing group (" << Check.first << "):\n";
for (unsigned K = 0; K < First.size(); ++K)
OS.indent(Depth + 2) << *Pointers[First[K]].PointerValue << "\n";
OS.indent(Depth + 2) << "Against group (" << Check.second << "):\n";
for (unsigned K = 0; K < Second.size(); ++K)
OS.indent(Depth + 2) << *Pointers[Second[K]].PointerValue << "\n";
}
}
void RuntimePointerChecking::print(raw_ostream &OS, unsigned Depth) const {
OS.indent(Depth) << "Run-time memory checks:\n";
printChecks(OS, Checks, Depth);
OS.indent(Depth) << "Grouped accesses:\n";
for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
const auto &CG = CheckingGroups[I];
OS.indent(Depth + 2) << "Group " << &CG << ":\n";
OS.indent(Depth + 4) << "(Low: " << *CG.Low << " High: " << *CG.High
<< ")\n";
for (unsigned J = 0; J < CG.Members.size(); ++J) {
OS.indent(Depth + 6) << "Member: " << *Pointers[CG.Members[J]].Expr
<< "\n";
}
}
}
namespace {
/// Analyses memory accesses in a loop.
///
/// Checks whether run time pointer checks are needed and builds sets for data
/// dependence checking.
class AccessAnalysis {
public:
/// Read or write access location.
typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
typedef SmallVector<MemAccessInfo, 8> MemAccessInfoList;
AccessAnalysis(const DataLayout &Dl, Loop *TheLoop, AliasAnalysis *AA,
LoopInfo *LI, MemoryDepChecker::DepCandidates &DA,
PredicatedScalarEvolution &PSE)
: DL(Dl), TheLoop(TheLoop), AST(*AA), LI(LI), DepCands(DA),
IsRTCheckAnalysisNeeded(false), PSE(PSE) {}
/// Register a load and whether it is only read from.
void addLoad(MemoryLocation &Loc, bool IsReadOnly) {
Value *Ptr = const_cast<Value*>(Loc.Ptr);
AST.add(Ptr, LocationSize::unknown(), Loc.AATags);
Accesses.insert(MemAccessInfo(Ptr, false));
if (IsReadOnly)
ReadOnlyPtr.insert(Ptr);
}
/// Register a store.
void addStore(MemoryLocation &Loc) {
Value *Ptr = const_cast<Value*>(Loc.Ptr);
AST.add(Ptr, LocationSize::unknown(), Loc.AATags);
Accesses.insert(MemAccessInfo(Ptr, true));
}
/// Check if we can emit a run-time no-alias check for \p Access.
///
/// Returns true if we can emit a run-time no alias check for \p Access.
/// If we can check this access, this also adds it to a dependence set and
/// adds a run-time to check for it to \p RtCheck. If \p Assume is true,
/// we will attempt to use additional run-time checks in order to get
/// the bounds of the pointer.
bool createCheckForAccess(RuntimePointerChecking &RtCheck,
MemAccessInfo Access,
const ValueToValueMap &Strides,
DenseMap<Value *, unsigned> &DepSetId,
Loop *TheLoop, unsigned &RunningDepId,
unsigned ASId, bool ShouldCheckStride,
bool Assume);
/// Check whether we can check the pointers at runtime for
/// non-intersection.
///
/// Returns true if we need no check or if we do and we can generate them
/// (i.e. the pointers have computable bounds).
bool canCheckPtrAtRT(RuntimePointerChecking &RtCheck, ScalarEvolution *SE,
Loop *TheLoop, const ValueToValueMap &Strides,
bool ShouldCheckWrap = false);
/// Goes over all memory accesses, checks whether a RT check is needed
/// and builds sets of dependent accesses.
void buildDependenceSets() {
processMemAccesses();
}
/// Initial processing of memory accesses determined that we need to
/// perform dependency checking.
///
/// Note that this can later be cleared if we retry memcheck analysis without
/// dependency checking (i.e. FoundNonConstantDistanceDependence).
bool isDependencyCheckNeeded() { return !CheckDeps.empty(); }
/// We decided that no dependence analysis would be used. Reset the state.
void resetDepChecks(MemoryDepChecker &DepChecker) {
CheckDeps.clear();
DepChecker.clearDependences();
}
MemAccessInfoList &getDependenciesToCheck() { return CheckDeps; }
private:
typedef SetVector<MemAccessInfo> PtrAccessSet;
/// Go over all memory access and check whether runtime pointer checks
/// are needed and build sets of dependency check candidates.
void processMemAccesses();
/// Set of all accesses.
PtrAccessSet Accesses;
const DataLayout &DL;
/// The loop being checked.
const Loop *TheLoop;
/// List of accesses that need a further dependence check.
MemAccessInfoList CheckDeps;
/// Set of pointers that are read only.
SmallPtrSet<Value*, 16> ReadOnlyPtr;
/// An alias set tracker to partition the access set by underlying object and
//intrinsic property (such as TBAA metadata).
AliasSetTracker AST;
LoopInfo *LI;
/// Sets of potentially dependent accesses - members of one set share an
/// underlying pointer. The set "CheckDeps" identfies which sets really need a
/// dependence check.
MemoryDepChecker::DepCandidates &DepCands;
/// Initial processing of memory accesses determined that we may need
/// to add memchecks. Perform the analysis to determine the necessary checks.
///
/// Note that, this is different from isDependencyCheckNeeded. When we retry
/// memcheck analysis without dependency checking
/// (i.e. FoundNonConstantDistanceDependence), isDependencyCheckNeeded is
/// cleared while this remains set if we have potentially dependent accesses.
bool IsRTCheckAnalysisNeeded;
/// The SCEV predicate containing all the SCEV-related assumptions.
PredicatedScalarEvolution &PSE;
};
} // end anonymous namespace
/// Check whether a pointer can participate in a runtime bounds check.
/// If \p Assume, try harder to prove that we can compute the bounds of \p Ptr
/// by adding run-time checks (overflow checks) if necessary.
static bool hasComputableBounds(PredicatedScalarEvolution &PSE,
const ValueToValueMap &Strides, Value *Ptr,
Loop *L, bool Assume) {
const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
// The bounds for loop-invariant pointer is trivial.
if (PSE.getSE()->isLoopInvariant(PtrScev, L))
return true;
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
if (!AR && Assume)
AR = PSE.getAsAddRec(Ptr);
if (!AR)
return false;
return AR->isAffine();
}
/// Check whether a pointer address cannot wrap.
static bool isNoWrap(PredicatedScalarEvolution &PSE,
const ValueToValueMap &Strides, Value *Ptr, Loop *L) {
const SCEV *PtrScev = PSE.getSCEV(Ptr);
if (PSE.getSE()->isLoopInvariant(PtrScev, L))
return true;
int64_t Stride = getPtrStride(PSE, Ptr, L, Strides);
if (Stride == 1 || PSE.hasNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW))
return true;
return false;
}
bool AccessAnalysis::createCheckForAccess(RuntimePointerChecking &RtCheck,
MemAccessInfo Access,
const ValueToValueMap &StridesMap,
DenseMap<Value *, unsigned> &DepSetId,
Loop *TheLoop, unsigned &RunningDepId,
unsigned ASId, bool ShouldCheckWrap,
bool Assume) {
Value *Ptr = Access.getPointer();
if (!hasComputableBounds(PSE, StridesMap, Ptr, TheLoop, Assume))
return false;
// When we run after a failing dependency check we have to make sure
// we don't have wrapping pointers.
if (ShouldCheckWrap && !isNoWrap(PSE, StridesMap, Ptr, TheLoop)) {
auto *Expr = PSE.getSCEV(Ptr);
if (!Assume || !isa<SCEVAddRecExpr>(Expr))
return false;
PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
}
// The id of the dependence set.
unsigned DepId;
if (isDependencyCheckNeeded()) {
Value *Leader = DepCands.getLeaderValue(Access).getPointer();
unsigned &LeaderId = DepSetId[Leader];
if (!LeaderId)
LeaderId = RunningDepId++;
DepId = LeaderId;
} else
// Each access has its own dependence set.
DepId = RunningDepId++;
bool IsWrite = Access.getInt();
RtCheck.insert(TheLoop, Ptr, IsWrite, DepId, ASId, StridesMap, PSE);
LLVM_DEBUG(dbgs() << "LAA: Found a runtime check ptr:" << *Ptr << '\n');
return true;
}
bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
ScalarEvolution *SE, Loop *TheLoop,
const ValueToValueMap &StridesMap,
bool ShouldCheckWrap) {
// Find pointers with computable bounds. We are going to use this information
// to place a runtime bound check.
bool CanDoRT = true;
bool MayNeedRTCheck = false;
if (!IsRTCheckAnalysisNeeded) return true;
bool IsDepCheckNeeded = isDependencyCheckNeeded();
// We assign a consecutive id to access from different alias sets.
// Accesses between different groups doesn't need to be checked.
unsigned ASId = 1;
for (auto &AS : AST) {
int NumReadPtrChecks = 0;
int NumWritePtrChecks = 0;
bool CanDoAliasSetRT = true;
// We assign consecutive id to access from different dependence sets.
// Accesses within the same set don't need a runtime check.
unsigned RunningDepId = 1;
DenseMap<Value *, unsigned> DepSetId;
SmallVector<MemAccessInfo, 4> Retries;
for (auto A : AS) {
Value *Ptr = A.getValue();
bool IsWrite = Accesses.count(MemAccessInfo(Ptr, true));
MemAccessInfo Access(Ptr, IsWrite);
if (IsWrite)
++NumWritePtrChecks;
else
++NumReadPtrChecks;
if (!createCheckForAccess(RtCheck, Access, StridesMap, DepSetId, TheLoop,
RunningDepId, ASId, ShouldCheckWrap, false)) {
LLVM_DEBUG(dbgs() << "LAA: Can't find bounds for ptr:" << *Ptr << '\n');
Retries.push_back(Access);
CanDoAliasSetRT = false;
}
}
// If we have at least two writes or one write and a read then we need to
// check them. But there is no need to checks if there is only one
// dependence set for this alias set.
//
// Note that this function computes CanDoRT and MayNeedRTCheck
// independently. For example CanDoRT=false, MayNeedRTCheck=false means that
// we have a pointer for which we couldn't find the bounds but we don't
// actually need to emit any checks so it does not matter.
bool NeedsAliasSetRTCheck = false;
if (!(IsDepCheckNeeded && CanDoAliasSetRT && RunningDepId == 2))
NeedsAliasSetRTCheck = (NumWritePtrChecks >= 2 ||
(NumReadPtrChecks >= 1 && NumWritePtrChecks >= 1));
// We need to perform run-time alias checks, but some pointers had bounds
// that couldn't be checked.
if (NeedsAliasSetRTCheck && !CanDoAliasSetRT) {
// Reset the CanDoSetRt flag and retry all accesses that have failed.
// We know that we need these checks, so we can now be more aggressive
// and add further checks if required (overflow checks).
CanDoAliasSetRT = true;
for (auto Access : Retries)
if (!createCheckForAccess(RtCheck, Access, StridesMap, DepSetId,
TheLoop, RunningDepId, ASId,
ShouldCheckWrap, /*Assume=*/true)) {
CanDoAliasSetRT = false;
break;
}
}
CanDoRT &= CanDoAliasSetRT;
MayNeedRTCheck |= NeedsAliasSetRTCheck;
++ASId;
}
// If the pointers that we would use for the bounds comparison have different
// address spaces, assume the values aren't directly comparable, so we can't
// use them for the runtime check. We also have to assume they could
// overlap. In the future there should be metadata for whether address spaces
// are disjoint.
unsigned NumPointers = RtCheck.Pointers.size();
for (unsigned i = 0; i < NumPointers; ++i) {
for (unsigned j = i + 1; j < NumPointers; ++j) {
// Only need to check pointers between two different dependency sets.
if (RtCheck.Pointers[i].DependencySetId ==
RtCheck.Pointers[j].DependencySetId)
continue;
// Only need to check pointers in the same alias set.
if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
continue;
Value *PtrI = RtCheck.Pointers[i].PointerValue;
Value *PtrJ = RtCheck.Pointers[j].PointerValue;
unsigned ASi = PtrI->getType()->getPointerAddressSpace();
unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
if (ASi != ASj) {
LLVM_DEBUG(
dbgs() << "LAA: Runtime check would require comparison between"
" different address spaces\n");
return false;
}
}
}
if (MayNeedRTCheck && CanDoRT)
RtCheck.generateChecks(DepCands, IsDepCheckNeeded);
LLVM_DEBUG(dbgs() << "LAA: We need to do " << RtCheck.getNumberOfChecks()
<< " pointer comparisons.\n");
// If we can do run-time checks, but there are no checks, no runtime checks
// are needed. This can happen when all pointers point to the same underlying
// object for example.
RtCheck.Need = CanDoRT ? RtCheck.getNumberOfChecks() != 0 : MayNeedRTCheck;
bool CanDoRTIfNeeded = !RtCheck.Need || CanDoRT;
if (!CanDoRTIfNeeded)
RtCheck.reset();
return CanDoRTIfNeeded;
}
void AccessAnalysis::processMemAccesses() {
// We process the set twice: first we process read-write pointers, last we
// process read-only pointers. This allows us to skip dependence tests for
// read-only pointers.
LLVM_DEBUG(dbgs() << "LAA: Processing memory accesses...\n");
LLVM_DEBUG(dbgs() << " AST: "; AST.dump());
LLVM_DEBUG(dbgs() << "LAA: Accesses(" << Accesses.size() << "):\n");
LLVM_DEBUG({
for (auto A : Accesses)
dbgs() << "\t" << *A.getPointer() << " (" <<
(A.getInt() ? "write" : (ReadOnlyPtr.count(A.getPointer()) ?
"read-only" : "read")) << ")\n";
});
// The AliasSetTracker has nicely partitioned our pointers by metadata
// compatibility and potential for underlying-object overlap. As a result, we
// only need to check for potential pointer dependencies within each alias
// set.
for (auto &AS : AST) {
// Note that both the alias-set tracker and the alias sets themselves used
// linked lists internally and so the iteration order here is deterministic
// (matching the original instruction order within each set).
bool SetHasWrite = false;
// Map of pointers to last access encountered.
typedef DenseMap<const Value*, MemAccessInfo> UnderlyingObjToAccessMap;
UnderlyingObjToAccessMap ObjToLastAccess;
// Set of access to check after all writes have been processed.
PtrAccessSet DeferredAccesses;
// Iterate over each alias set twice, once to process read/write pointers,
// and then to process read-only pointers.
for (int SetIteration = 0; SetIteration < 2; ++SetIteration) {
bool UseDeferred = SetIteration > 0;
PtrAccessSet &S = UseDeferred ? DeferredAccesses : Accesses;
for (auto AV : AS) {
Value *Ptr = AV.getValue();
// For a single memory access in AliasSetTracker, Accesses may contain
// both read and write, and they both need to be handled for CheckDeps.
for (auto AC : S) {
if (AC.getPointer() != Ptr)
continue;
bool IsWrite = AC.getInt();
// If we're using the deferred access set, then it contains only
// reads.
bool IsReadOnlyPtr = ReadOnlyPtr.count(Ptr) && !IsWrite;
if (UseDeferred && !IsReadOnlyPtr)
continue;
// Otherwise, the pointer must be in the PtrAccessSet, either as a
// read or a write.
assert(((IsReadOnlyPtr && UseDeferred) || IsWrite ||
S.count(MemAccessInfo(Ptr, false))) &&
"Alias-set pointer not in the access set?");
MemAccessInfo Access(Ptr, IsWrite);
DepCands.insert(Access);
// Memorize read-only pointers for later processing and skip them in
// the first round (they need to be checked after we have seen all
// write pointers). Note: we also mark pointer that are not
// consecutive as "read-only" pointers (so that we check
// "a[b[i]] +="). Hence, we need the second check for "!IsWrite".
if (!UseDeferred && IsReadOnlyPtr) {
DeferredAccesses.insert(Access);
continue;
}
// If this is a write - check other reads and writes for conflicts. If
// this is a read only check other writes for conflicts (but only if
// there is no other write to the ptr - this is an optimization to
// catch "a[i] = a[i] + " without having to do a dependence check).
if ((IsWrite || IsReadOnlyPtr) && SetHasWrite) {
CheckDeps.push_back(Access);
IsRTCheckAnalysisNeeded = true;
}
if (IsWrite)
SetHasWrite = true;
// Create sets of pointers connected by a shared alias set and
// underlying object.
typedef SmallVector<const Value *, 16> ValueVector;
ValueVector TempObjects;
GetUnderlyingObjects(Ptr, TempObjects, DL, LI);
LLVM_DEBUG(dbgs()
<< "Underlying objects for pointer " << *Ptr << "\n");
for (const Value *UnderlyingObj : TempObjects) {
// nullptr never alias, don't join sets for pointer that have "null"
// in their UnderlyingObjects list.
if (isa<ConstantPointerNull>(UnderlyingObj) &&
!NullPointerIsDefined(
TheLoop->getHeader()->getParent(),
UnderlyingObj->getType()->getPointerAddressSpace()))
continue;
UnderlyingObjToAccessMap::iterator Prev =
ObjToLastAccess.find(UnderlyingObj);
if (Prev != ObjToLastAccess.end())
DepCands.unionSets(Access, Prev->second);
ObjToLastAccess[UnderlyingObj] = Access;
LLVM_DEBUG(dbgs() << " " << *UnderlyingObj << "\n");
}
}
}
}
}
}
static bool isInBoundsGep(Value *Ptr) {
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr))
return GEP->isInBounds();
return false;
}
/// Return true if an AddRec pointer \p Ptr is unsigned non-wrapping,
/// i.e. monotonically increasing/decreasing.
static bool isNoWrapAddRec(Value *Ptr, const SCEVAddRecExpr *AR,
PredicatedScalarEvolution &PSE, const Loop *L) {
// FIXME: This should probably only return true for NUW.
if (AR->getNoWrapFlags(SCEV::NoWrapMask))
return true;
// Scalar evolution does not propagate the non-wrapping flags to values that
// are derived from a non-wrapping induction variable because non-wrapping
// could be flow-sensitive.
//
// Look through the potentially overflowing instruction to try to prove
// non-wrapping for the *specific* value of Ptr.
// The arithmetic implied by an inbounds GEP can't overflow.
auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
if (!GEP || !GEP->isInBounds())
return false;
// Make sure there is only one non-const index and analyze that.
Value *NonConstIndex = nullptr;
for (Value *Index : make_range(GEP->idx_begin(), GEP->idx_end()))
if (!isa<ConstantInt>(Index)) {
if (NonConstIndex)
return false;
NonConstIndex = Index;
}
if (!NonConstIndex)
// The recurrence is on the pointer, ignore for now.
return false;
// The index in GEP is signed. It is non-wrapping if it's derived from a NSW
// AddRec using a NSW operation.
if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
if (OBO->hasNoSignedWrap() &&
// Assume constant for other the operand so that the AddRec can be
// easily found.
isa<ConstantInt>(OBO->getOperand(1))) {
auto *OpScev = PSE.getSCEV(OBO->getOperand(0));
if (auto *OpAR = dyn_cast<SCEVAddRecExpr>(OpScev))
return OpAR->getLoop() == L && OpAR->getNoWrapFlags(SCEV::FlagNSW);
}
return false;
}
/// Check whether the access through \p Ptr has a constant stride.
int64_t llvm::getPtrStride(PredicatedScalarEvolution &PSE, Value *Ptr,
const Loop *Lp, const ValueToValueMap &StridesMap,
bool Assume, bool ShouldCheckWrap) {
Type *Ty = Ptr->getType();
assert(Ty->isPointerTy() && "Unexpected non-ptr");
// Make sure that the pointer does not point to aggregate types.
auto *PtrTy = cast<PointerType>(Ty);
if (PtrTy->getElementType()->isAggregateType()) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a pointer to a scalar type"
<< *Ptr << "\n");
return 0;
}
const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, StridesMap, Ptr);
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
if (Assume && !AR)
AR = PSE.getAsAddRec(Ptr);
if (!AR) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not an AddRecExpr pointer " << *Ptr
<< " SCEV: " << *PtrScev << "\n");
return 0;
}
// The access function must stride over the innermost loop.
if (Lp != AR->getLoop()) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
<< *Ptr << " SCEV: " << *AR << "\n");
return 0;
}
// The address calculation must not wrap. Otherwise, a dependence could be
// inverted.
// An inbounds getelementptr that is a AddRec with a unit stride
// cannot wrap per definition. The unit stride requirement is checked later.
// An getelementptr without an inbounds attribute and unit stride would have
// to access the pointer value "0" which is undefined behavior in address
// space 0, therefore we can also vectorize this case.
bool IsInBoundsGEP = isInBoundsGep(Ptr);
bool IsNoWrapAddRec = !ShouldCheckWrap ||
PSE.hasNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW) ||
isNoWrapAddRec(Ptr, AR, PSE, Lp);
if (!IsNoWrapAddRec && !IsInBoundsGEP &&
NullPointerIsDefined(Lp->getHeader()->getParent(),
PtrTy->getAddressSpace())) {
if (Assume) {
PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
IsNoWrapAddRec = true;
LLVM_DEBUG(dbgs() << "LAA: Pointer may wrap in the address space:\n"
<< "LAA: Pointer: " << *Ptr << "\n"
<< "LAA: SCEV: " << *AR << "\n"
<< "LAA: Added an overflow assumption\n");
} else {
LLVM_DEBUG(
dbgs() << "LAA: Bad stride - Pointer may wrap in the address space "
<< *Ptr << " SCEV: " << *AR << "\n");
return 0;
}
}
// Check the step is constant.
const SCEV *Step = AR->getStepRecurrence(*PSE.getSE());
// Calculate the pointer stride and check if it is constant.
const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
if (!C) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr
<< " SCEV: " << *AR << "\n");
return 0;
}
auto &DL = Lp->getHeader()->getModule()->getDataLayout();
int64_t Size = DL.getTypeAllocSize(PtrTy->getElementType());
const APInt &APStepVal = C->getAPInt();
// Huge step value - give up.
if (APStepVal.getBitWidth() > 64)
return 0;
int64_t StepVal = APStepVal.getSExtValue();
// Strided access.
int64_t Stride = StepVal / Size;
int64_t Rem = StepVal % Size;
if (Rem)
return 0;
// If the SCEV could wrap but we have an inbounds gep with a unit stride we
// know we can't "wrap around the address space". In case of address space
// zero we know that this won't happen without triggering undefined behavior.
if (!IsNoWrapAddRec && Stride != 1 && Stride != -1 &&
(IsInBoundsGEP || !NullPointerIsDefined(Lp->getHeader()->getParent(),
PtrTy->getAddressSpace()))) {
if (Assume) {
// We can avoid this case by adding a run-time check.
LLVM_DEBUG(dbgs() << "LAA: Non unit strided pointer which is not either "
<< "inbounds or in address space 0 may wrap:\n"
<< "LAA: Pointer: " << *Ptr << "\n"
<< "LAA: SCEV: " << *AR << "\n"
<< "LAA: Added an overflow assumption\n");
PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
} else
return 0;
}
return Stride;
}
bool llvm::sortPtrAccesses(ArrayRef<Value *> VL, const DataLayout &DL,
ScalarEvolution &SE,
SmallVectorImpl<unsigned> &SortedIndices) {
assert(llvm::all_of(
VL, [](const Value *V) { return V->getType()->isPointerTy(); }) &&
"Expected list of pointer operands.");
SmallVector<std::pair<int64_t, Value *>, 4> OffValPairs;
OffValPairs.reserve(VL.size());
// Walk over the pointers, and map each of them to an offset relative to
// first pointer in the array.
Value *Ptr0 = VL[0];
const SCEV *Scev0 = SE.getSCEV(Ptr0);
Value *Obj0 = GetUnderlyingObject(Ptr0, DL);
llvm::SmallSet<int64_t, 4> Offsets;
for (auto *Ptr : VL) {
// TODO: Outline this code as a special, more time consuming, version of
// computeConstantDifference() function.
if (Ptr->getType()->getPointerAddressSpace() !=
Ptr0->getType()->getPointerAddressSpace())
return false;
// If a pointer refers to a different underlying object, bail - the
// pointers are by definition incomparable.
Value *CurrObj = GetUnderlyingObject(Ptr, DL);
if (CurrObj != Obj0)
return false;
const SCEV *Scev = SE.getSCEV(Ptr);
const auto *Diff = dyn_cast<SCEVConstant>(SE.getMinusSCEV(Scev, Scev0));
// The pointers may not have a constant offset from each other, or SCEV
// may just not be smart enough to figure out they do. Regardless,
// there's nothing we can do.
if (!Diff)
return false;
// Check if the pointer with the same offset is found.
int64_t Offset = Diff->getAPInt().getSExtValue();
if (!Offsets.insert(Offset).second)
return false;
OffValPairs.emplace_back(Offset, Ptr);
}
SortedIndices.clear();
SortedIndices.resize(VL.size());
std::iota(SortedIndices.begin(), SortedIndices.end(), 0);
// Sort the memory accesses and keep the order of their uses in UseOrder.
llvm::stable_sort(SortedIndices, [&](unsigned Left, unsigned Right) {
return OffValPairs[Left].first < OffValPairs[Right].first;
});
// Check if the order is consecutive already.
if (llvm::all_of(SortedIndices, [&SortedIndices](const unsigned I) {
return I == SortedIndices[I];
}))
SortedIndices.clear();
return true;
}
/// Take the address space operand from the Load/Store instruction.
/// Returns -1 if this is not a valid Load/Store instruction.
static unsigned getAddressSpaceOperand(Value *I) {
if (LoadInst *L = dyn_cast<LoadInst>(I))
return L->getPointerAddressSpace();
if (StoreInst *S = dyn_cast<StoreInst>(I))
return S->getPointerAddressSpace();
return -1;
}
/// Returns true if the memory operations \p A and \p B are consecutive.
bool llvm::isConsecutiveAccess(Value *A, Value *B, const DataLayout &DL,
ScalarEvolution &SE, bool CheckType) {
Value *PtrA = getLoadStorePointerOperand(A);
Value *PtrB = getLoadStorePointerOperand(B);
unsigned ASA = getAddressSpaceOperand(A);
unsigned ASB = getAddressSpaceOperand(B);
// Check that the address spaces match and that the pointers are valid.
if (!PtrA || !PtrB || (ASA != ASB))
return false;
// Make sure that A and B are different pointers.
if (PtrA == PtrB)
return false;
// Make sure that A and B have the same type if required.
if (CheckType && PtrA->getType() != PtrB->getType())
return false;
unsigned IdxWidth = DL.getIndexSizeInBits(ASA);
Type *Ty = cast<PointerType>(PtrA->getType())->getElementType();
APInt OffsetA(IdxWidth, 0), OffsetB(IdxWidth, 0);
PtrA = PtrA->stripAndAccumulateInBoundsConstantOffsets(DL, OffsetA);
PtrB = PtrB->stripAndAccumulateInBoundsConstantOffsets(DL, OffsetB);
// Retrieve the address space again as pointer stripping now tracks through
// `addrspacecast`.
ASA = cast<PointerType>(PtrA->getType())->getAddressSpace();
ASB = cast<PointerType>(PtrB->getType())->getAddressSpace();
// Check that the address spaces match and that the pointers are valid.
if (ASA != ASB)
return false;
IdxWidth = DL.getIndexSizeInBits(ASA);
OffsetA = OffsetA.sextOrTrunc(IdxWidth);
OffsetB = OffsetB.sextOrTrunc(IdxWidth);
APInt Size(IdxWidth, DL.getTypeStoreSize(Ty));
// OffsetDelta = OffsetB - OffsetA;
const SCEV *OffsetSCEVA = SE.getConstant(OffsetA);
const SCEV *OffsetSCEVB = SE.getConstant(OffsetB);
const SCEV *OffsetDeltaSCEV = SE.getMinusSCEV(OffsetSCEVB, OffsetSCEVA);
const APInt &OffsetDelta = cast<SCEVConstant>(OffsetDeltaSCEV)->getAPInt();
// Check if they are based on the same pointer. That makes the offsets
// sufficient.
if (PtrA == PtrB)
return OffsetDelta == Size;
// Compute the necessary base pointer delta to have the necessary final delta
// equal to the size.
// BaseDelta = Size - OffsetDelta;
const SCEV *SizeSCEV = SE.getConstant(Size);
const SCEV *BaseDelta = SE.getMinusSCEV(SizeSCEV, OffsetDeltaSCEV);
// Otherwise compute the distance with SCEV between the base pointers.
const SCEV *PtrSCEVA = SE.getSCEV(PtrA);
const SCEV *PtrSCEVB = SE.getSCEV(PtrB);
const SCEV *X = SE.getAddExpr(PtrSCEVA, BaseDelta);
return X == PtrSCEVB;
}
MemoryDepChecker::VectorizationSafetyStatus
MemoryDepChecker::Dependence::isSafeForVectorization(DepType Type) {
switch (Type) {
case NoDep:
case Forward:
case BackwardVectorizable:
return VectorizationSafetyStatus::Safe;
case Unknown:
return VectorizationSafetyStatus::PossiblySafeWithRtChecks;
case ForwardButPreventsForwarding:
case Backward:
case BackwardVectorizableButPreventsForwarding:
return VectorizationSafetyStatus::Unsafe;
}
llvm_unreachable("unexpected DepType!");
}
bool MemoryDepChecker::Dependence::isBackward() const {
switch (Type) {
case NoDep:
case Forward:
case ForwardButPreventsForwarding:
case Unknown:
return false;
case BackwardVectorizable:
case Backward:
case BackwardVectorizableButPreventsForwarding:
return true;
}
llvm_unreachable("unexpected DepType!");
}
bool MemoryDepChecker::Dependence::isPossiblyBackward() const {
return isBackward() || Type == Unknown;
}
bool MemoryDepChecker::Dependence::isForward() const {
switch (Type) {
case Forward:
case ForwardButPreventsForwarding:
return true;
case NoDep:
case Unknown:
case BackwardVectorizable:
case Backward:
case BackwardVectorizableButPreventsForwarding:
return false;
}
llvm_unreachable("unexpected DepType!");
}
bool MemoryDepChecker::couldPreventStoreLoadForward(uint64_t Distance,
uint64_t TypeByteSize) {
// If loads occur at a distance that is not a multiple of a feasible vector
// factor store-load forwarding does not take place.
// Positive dependences might cause troubles because vectorizing them might
// prevent store-load forwarding making vectorized code run a lot slower.
// a[i] = a[i-3] ^ a[i-8];
// The stores to a[i:i+1] don't align with the stores to a[i-3:i-2] and
// hence on your typical architecture store-load forwarding does not take
// place. Vectorizing in such cases does not make sense.
// Store-load forwarding distance.
// After this many iterations store-to-load forwarding conflicts should not
// cause any slowdowns.
const uint64_t NumItersForStoreLoadThroughMemory = 8 * TypeByteSize;
// Maximum vector factor.
uint64_t MaxVFWithoutSLForwardIssues = std::min(
VectorizerParams::MaxVectorWidth * TypeByteSize, MaxSafeDepDistBytes);
// Compute the smallest VF at which the store and load would be misaligned.
for (uint64_t VF = 2 * TypeByteSize; VF <= MaxVFWithoutSLForwardIssues;
VF *= 2) {
// If the number of vector iteration between the store and the load are
// small we could incur conflicts.
if (Distance % VF && Distance / VF < NumItersForStoreLoadThroughMemory) {
MaxVFWithoutSLForwardIssues = (VF >>= 1);
break;
}
}
if (MaxVFWithoutSLForwardIssues < 2 * TypeByteSize) {
LLVM_DEBUG(
dbgs() << "LAA: Distance " << Distance
<< " that could cause a store-load forwarding conflict\n");
return true;
}
if (MaxVFWithoutSLForwardIssues < MaxSafeDepDistBytes &&
MaxVFWithoutSLForwardIssues !=
VectorizerParams::MaxVectorWidth * TypeByteSize)
MaxSafeDepDistBytes = MaxVFWithoutSLForwardIssues;
return false;
}
void MemoryDepChecker::mergeInStatus(VectorizationSafetyStatus S) {
if (Status < S)
Status = S;
}
/// Given a non-constant (unknown) dependence-distance \p Dist between two
/// memory accesses, that have the same stride whose absolute value is given
/// in \p Stride, and that have the same type size \p TypeByteSize,
/// in a loop whose takenCount is \p BackedgeTakenCount, check if it is
/// possible to prove statically that the dependence distance is larger
/// than the range that the accesses will travel through the execution of
/// the loop. If so, return true; false otherwise. This is useful for
/// example in loops such as the following (PR31098):
/// for (i = 0; i < D; ++i) {
/// = out[i];
/// out[i+D] =
/// }
static bool isSafeDependenceDistance(const DataLayout &DL, ScalarEvolution &SE,
const SCEV &BackedgeTakenCount,
const SCEV &Dist, uint64_t Stride,
uint64_t TypeByteSize) {
// If we can prove that
// (**) |Dist| > BackedgeTakenCount * Step
// where Step is the absolute stride of the memory accesses in bytes,
// then there is no dependence.
//
// Rationale:
// We basically want to check if the absolute distance (|Dist/Step|)
// is >= the loop iteration count (or > BackedgeTakenCount).
// This is equivalent to the Strong SIV Test (Practical Dependence Testing,
// Section 4.2.1); Note, that for vectorization it is sufficient to prove
// that the dependence distance is >= VF; This is checked elsewhere.
// But in some cases we can prune unknown dependence distances early, and
// even before selecting the VF, and without a runtime test, by comparing
// the distance against the loop iteration count. Since the vectorized code
// will be executed only if LoopCount >= VF, proving distance >= LoopCount
// also guarantees that distance >= VF.
//
const uint64_t ByteStride = Stride * TypeByteSize;
const SCEV *Step = SE.getConstant(BackedgeTakenCount.getType(), ByteStride);
const SCEV *Product = SE.getMulExpr(&BackedgeTakenCount, Step);
const SCEV *CastedDist = &Dist;
const SCEV *CastedProduct = Product;
uint64_t DistTypeSize = DL.getTypeAllocSize(Dist.getType());
uint64_t ProductTypeSize = DL.getTypeAllocSize(Product->getType());
// The dependence distance can be positive/negative, so we sign extend Dist;
// The multiplication of the absolute stride in bytes and the
// backedgeTakenCount is non-negative, so we zero extend Product.
if (DistTypeSize > ProductTypeSize)
CastedProduct = SE.getZeroExtendExpr(Product, Dist.getType());
else
CastedDist = SE.getNoopOrSignExtend(&Dist, Product->getType());
// Is Dist - (BackedgeTakenCount * Step) > 0 ?
// (If so, then we have proven (**) because |Dist| >= Dist)
const SCEV *Minus = SE.getMinusSCEV(CastedDist, CastedProduct);
if (SE.isKnownPositive(Minus))
return true;
// Second try: Is -Dist - (BackedgeTakenCount * Step) > 0 ?
// (If so, then we have proven (**) because |Dist| >= -1*Dist)
const SCEV *NegDist = SE.getNegativeSCEV(CastedDist);
Minus = SE.getMinusSCEV(NegDist, CastedProduct);
if (SE.isKnownPositive(Minus))
return true;
return false;
}
/// Check the dependence for two accesses with the same stride \p Stride.
/// \p Distance is the positive distance and \p TypeByteSize is type size in
/// bytes.
///
/// \returns true if they are independent.
static bool areStridedAccessesIndependent(uint64_t Distance, uint64_t Stride,
uint64_t TypeByteSize) {
assert(Stride > 1 && "The stride must be greater than 1");
assert(TypeByteSize > 0 && "The type size in byte must be non-zero");
assert(Distance > 0 && "The distance must be non-zero");
// Skip if the distance is not multiple of type byte size.
if (Distance % TypeByteSize)
return false;
uint64_t ScaledDist = Distance / TypeByteSize;
// No dependence if the scaled distance is not multiple of the stride.
// E.g.
// for (i = 0; i < 1024 ; i += 4)
// A[i+2] = A[i] + 1;
//
// Two accesses in memory (scaled distance is 2, stride is 4):
// | A[0] | | | | A[4] | | | |
// | | | A[2] | | | | A[6] | |
//
// E.g.
// for (i = 0; i < 1024 ; i += 3)
// A[i+4] = A[i] + 1;
//
// Two accesses in memory (scaled distance is 4, stride is 3):
// | A[0] | | | A[3] | | | A[6] | | |
// | | | | | A[4] | | | A[7] | |
return ScaledDist % Stride;
}
MemoryDepChecker::Dependence::DepType
MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
const MemAccessInfo &B, unsigned BIdx,
const ValueToValueMap &Strides) {
assert (AIdx < BIdx && "Must pass arguments in program order");
Value *APtr = A.getPointer();
Value *BPtr = B.getPointer();
bool AIsWrite = A.getInt();
bool BIsWrite = B.getInt();
// Two reads are independent.
if (!AIsWrite && !BIsWrite)
return Dependence::NoDep;
// We cannot check pointers in different address spaces.
if (APtr->getType()->getPointerAddressSpace() !=
BPtr->getType()->getPointerAddressSpace())
return Dependence::Unknown;
int64_t StrideAPtr = getPtrStride(PSE, APtr, InnermostLoop, Strides, true);
int64_t StrideBPtr = getPtrStride(PSE, BPtr, InnermostLoop, Strides, true);
const SCEV *Src = PSE.getSCEV(APtr);
const SCEV *Sink = PSE.getSCEV(BPtr);
// If the induction step is negative we have to invert source and sink of the
// dependence.
if (StrideAPtr < 0) {
std::swap(APtr, BPtr);
std::swap(Src, Sink);
std::swap(AIsWrite, BIsWrite);
std::swap(AIdx, BIdx);
std::swap(StrideAPtr, StrideBPtr);
}
const SCEV *Dist = PSE.getSE()->getMinusSCEV(Sink, Src);
LLVM_DEBUG(dbgs() << "LAA: Src Scev: " << *Src << "Sink Scev: " << *Sink
<< "(Induction step: " << StrideAPtr << ")\n");
LLVM_DEBUG(dbgs() << "LAA: Distance for " << *InstMap[AIdx] << " to "
<< *InstMap[BIdx] << ": " << *Dist << "\n");
// Need accesses with constant stride. We don't want to vectorize
// "A[B[i]] += ..." and similar code or pointer arithmetic that could wrap in
// the address space.
if (!StrideAPtr || !StrideBPtr || StrideAPtr != StrideBPtr){
LLVM_DEBUG(dbgs() << "Pointer access with non-constant stride\n");
return Dependence::Unknown;
}
Type *ATy = APtr->getType()->getPointerElementType();
Type *BTy = BPtr->getType()->getPointerElementType();
auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
uint64_t TypeByteSize = DL.getTypeAllocSize(ATy);
uint64_t Stride = std::abs(StrideAPtr);
const SCEVConstant *C = dyn_cast<SCEVConstant>(Dist);
if (!C) {
if (TypeByteSize == DL.getTypeAllocSize(BTy) &&
isSafeDependenceDistance(DL, *(PSE.getSE()),
*(PSE.getBackedgeTakenCount()), *Dist, Stride,
TypeByteSize))
return Dependence::NoDep;
LLVM_DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
FoundNonConstantDistanceDependence = true;
return Dependence::Unknown;
}
const APInt &Val = C->getAPInt();
int64_t Distance = Val.getSExtValue();
// Attempt to prove strided accesses independent.
if (std::abs(Distance) > 0 && Stride > 1 && ATy == BTy &&
areStridedAccessesIndependent(std::abs(Distance), Stride, TypeByteSize)) {
LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
return Dependence::NoDep;
}
// Negative distances are not plausible dependencies.
if (Val.isNegative()) {
bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
if (IsTrueDataDependence && EnableForwardingConflictDetection &&
(couldPreventStoreLoadForward(Val.abs().getZExtValue(), TypeByteSize) ||
ATy != BTy)) {
LLVM_DEBUG(dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
return Dependence::ForwardButPreventsForwarding;
}
LLVM_DEBUG(dbgs() << "LAA: Dependence is negative\n");
return Dependence::Forward;
}
// Write to the same location with the same size.
// Could be improved to assert type sizes are the same (i32 == float, etc).
if (Val == 0) {
if (ATy == BTy)
return Dependence::Forward;
LLVM_DEBUG(
dbgs() << "LAA: Zero dependence difference but different types\n");
return Dependence::Unknown;
}
assert(Val.isStrictlyPositive() && "Expect a positive value");
if (ATy != BTy) {
LLVM_DEBUG(
dbgs()
<< "LAA: ReadWrite-Write positive dependency with different types\n");
return Dependence::Unknown;
}
// Bail out early if passed-in parameters make vectorization not feasible.
unsigned ForcedFactor = (VectorizerParams::VectorizationFactor ?
VectorizerParams::VectorizationFactor : 1);
unsigned ForcedUnroll = (VectorizerParams::VectorizationInterleave ?
VectorizerParams::VectorizationInterleave : 1);
// The minimum number of iterations for a vectorized/unrolled version.
unsigned MinNumIter = std::max(ForcedFactor * ForcedUnroll, 2U);
// It's not vectorizable if the distance is smaller than the minimum distance
// needed for a vectroized/unrolled version. Vectorizing one iteration in
// front needs TypeByteSize * Stride. Vectorizing the last iteration needs
// TypeByteSize (No need to plus the last gap distance).
//
// E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
// foo(int *A) {
// int *B = (int *)((char *)A + 14);
// for (i = 0 ; i < 1024 ; i += 2)
// B[i] = A[i] + 1;
// }
//
// Two accesses in memory (stride is 2):
// | A[0] | | A[2] | | A[4] | | A[6] | |
// | B[0] | | B[2] | | B[4] |
//
// Distance needs for vectorizing iterations except the last iteration:
// 4 * 2 * (MinNumIter - 1). Distance needs for the last iteration: 4.
// So the minimum distance needed is: 4 * 2 * (MinNumIter - 1) + 4.
//
// If MinNumIter is 2, it is vectorizable as the minimum distance needed is
// 12, which is less than distance.
//
// If MinNumIter is 4 (Say if a user forces the vectorization factor to be 4),
// the minimum distance needed is 28, which is greater than distance. It is
// not safe to do vectorization.
uint64_t MinDistanceNeeded =
TypeByteSize * Stride * (MinNumIter - 1) + TypeByteSize;
if (MinDistanceNeeded > static_cast<uint64_t>(Distance)) {
LLVM_DEBUG(dbgs() << "LAA: Failure because of positive distance "
<< Distance << '\n');
return Dependence::Backward;
}
// Unsafe if the minimum distance needed is greater than max safe distance.
if (MinDistanceNeeded > MaxSafeDepDistBytes) {
LLVM_DEBUG(dbgs() << "LAA: Failure because it needs at least "
<< MinDistanceNeeded << " size in bytes");
return Dependence::Backward;
}
// Positive distance bigger than max vectorization factor.
// FIXME: Should use max factor instead of max distance in bytes, which could
// not handle different types.
// E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
// void foo (int *A, char *B) {
// for (unsigned i = 0; i < 1024; i++) {
// A[i+2] = A[i] + 1;
// B[i+2] = B[i] + 1;
// }
// }
//
// This case is currently unsafe according to the max safe distance. If we
// analyze the two accesses on array B, the max safe dependence distance
// is 2. Then we analyze the accesses on array A, the minimum distance needed
// is 8, which is less than 2 and forbidden vectorization, But actually
// both A and B could be vectorized by 2 iterations.
MaxSafeDepDistBytes =
std::min(static_cast<uint64_t>(Distance), MaxSafeDepDistBytes);
bool IsTrueDataDependence = (!AIsWrite && BIsWrite);
if (IsTrueDataDependence && EnableForwardingConflictDetection &&
couldPreventStoreLoadForward(Distance, TypeByteSize))
return Dependence::BackwardVectorizableButPreventsForwarding;
uint64_t MaxVF = MaxSafeDepDistBytes / (TypeByteSize * Stride);
LLVM_DEBUG(dbgs() << "LAA: Positive distance " << Val.getSExtValue()
<< " with max VF = " << MaxVF << '\n');
uint64_t MaxVFInBits = MaxVF * TypeByteSize * 8;
MaxSafeRegisterWidth = std::min(MaxSafeRegisterWidth, MaxVFInBits);
return Dependence::BackwardVectorizable;
}
bool MemoryDepChecker::areDepsSafe(DepCandidates &AccessSets,
MemAccessInfoList &CheckDeps,
const ValueToValueMap &Strides) {
MaxSafeDepDistBytes = -1;
SmallPtrSet<MemAccessInfo, 8> Visited;
for (MemAccessInfo CurAccess : CheckDeps) {
if (Visited.count(CurAccess))
continue;
// Get the relevant memory access set.
EquivalenceClasses<MemAccessInfo>::iterator I =
AccessSets.findValue(AccessSets.getLeaderValue(CurAccess));
// Check accesses within this set.
EquivalenceClasses<MemAccessInfo>::member_iterator AI =
AccessSets.member_begin(I);
EquivalenceClasses<MemAccessInfo>::member_iterator AE =
AccessSets.member_end();
// Check every access pair.
while (AI != AE) {
Visited.insert(*AI);
bool AIIsWrite = AI->getInt();
// Check loads only against next equivalent class, but stores also against
// other stores in the same equivalence class - to the same address.
EquivalenceClasses<MemAccessInfo>::member_iterator OI =
(AIIsWrite ? AI : std::next(AI));
while (OI != AE) {
// Check every accessing instruction pair in program order.
for (std::vector<unsigned>::iterator I1 = Accesses[*AI].begin(),
I1E = Accesses[*AI].end(); I1 != I1E; ++I1)
// Scan all accesses of another equivalence class, but only the next
// accesses of the same equivalent class.
for (std::vector<unsigned>::iterator
I2 = (OI == AI ? std::next(I1) : Accesses[*OI].begin()),
I2E = (OI == AI ? I1E : Accesses[*OI].end());
I2 != I2E; ++I2) {
auto A = std::make_pair(&*AI, *I1);
auto B = std::make_pair(&*OI, *I2);
assert(*I1 != *I2);
if (*I1 > *I2)
std::swap(A, B);
Dependence::DepType Type =
isDependent(*A.first, A.second, *B.first, B.second, Strides);
mergeInStatus(Dependence::isSafeForVectorization(Type));
// Gather dependences unless we accumulated MaxDependences
// dependences. In that case return as soon as we find the first
// unsafe dependence. This puts a limit on this quadratic
// algorithm.
if (RecordDependences) {
if (Type != Dependence::NoDep)
Dependences.push_back(Dependence(A.second, B.second, Type));
if (Dependences.size() >= MaxDependences) {
RecordDependences = false;
Dependences.clear();
LLVM_DEBUG(dbgs()
<< "Too many dependences, stopped recording\n");
}
}
if (!RecordDependences && !isSafeForVectorization())
return false;
}
++OI;
}
AI++;
}
}
LLVM_DEBUG(dbgs() << "Total Dependences: " << Dependences.size() << "\n");
return isSafeForVectorization();
}
SmallVector<Instruction *, 4>
MemoryDepChecker::getInstructionsForAccess(Value *Ptr, bool isWrite) const {
MemAccessInfo Access(Ptr, isWrite);
auto &IndexVector = Accesses.find(Access)->second;
SmallVector<Instruction *, 4> Insts;
transform(IndexVector,
std::back_inserter(Insts),
[&](unsigned Idx) { return this->InstMap[Idx]; });
return Insts;
}
const char *MemoryDepChecker::Dependence::DepName[] = {
"NoDep", "Unknown", "Forward", "ForwardButPreventsForwarding", "Backward",
"BackwardVectorizable", "BackwardVectorizableButPreventsForwarding"};
void MemoryDepChecker::Dependence::print(
raw_ostream &OS, unsigned Depth,
const SmallVectorImpl<Instruction *> &Instrs) const {
OS.indent(Depth) << DepName[Type] << ":\n";
OS.indent(Depth + 2) << *Instrs[Source] << " -> \n";
OS.indent(Depth + 2) << *Instrs[Destination] << "\n";
}
bool LoopAccessInfo::canAnalyzeLoop() {
// We need to have a loop header.
LLVM_DEBUG(dbgs() << "LAA: Found a loop in "
<< TheLoop->getHeader()->getParent()->getName() << ": "
<< TheLoop->getHeader()->getName() << '\n');
// We can only analyze innermost loops.
if (!TheLoop->empty()) {
LLVM_DEBUG(dbgs() << "LAA: loop is not the innermost loop\n");
recordAnalysis("NotInnerMostLoop") << "loop is not the innermost loop";
return false;
}
// We must have a single backedge.
if (TheLoop->getNumBackEdges() != 1) {
LLVM_DEBUG(
dbgs() << "LAA: loop control flow is not understood by analyzer\n");
recordAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by analyzer";
return false;
}
// We must have a single exiting block.
if (!TheLoop->getExitingBlock()) {
LLVM_DEBUG(
dbgs() << "LAA: loop control flow is not understood by analyzer\n");
recordAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by analyzer";
return false;
}
// We only handle bottom-tested loops, i.e. loop in which the condition is
// checked at the end of each iteration. With that we can assume that all
// instructions in the loop are executed the same number of times.
if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
LLVM_DEBUG(
dbgs() << "LAA: loop control flow is not understood by analyzer\n");
recordAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by analyzer";
return false;
}
// ScalarEvolution needs to be able to find the exit count.
const SCEV *ExitCount = PSE->getBackedgeTakenCount();
if (ExitCount == PSE->getSE()->getCouldNotCompute()) {
recordAnalysis("CantComputeNumberOfIterations")
<< "could not determine number of loop iterations";
LLVM_DEBUG(dbgs() << "LAA: SCEV could not compute the loop exit count.\n");
return false;
}
return true;
}
void LoopAccessInfo::analyzeLoop(AliasAnalysis *AA, LoopInfo *LI,
const TargetLibraryInfo *TLI,
DominatorTree *DT) {
typedef SmallPtrSet<Value*, 16> ValueSet;
// Holds the Load and Store instructions.
SmallVector<LoadInst *, 16> Loads;
SmallVector<StoreInst *, 16> Stores;
// Holds all the different accesses in the loop.
unsigned NumReads = 0;
unsigned NumReadWrites = 0;
bool HasComplexMemInst = false;
// A runtime check is only legal to insert if there are no convergent calls.
HasConvergentOp = false;
PtrRtChecking->Pointers.clear();
PtrRtChecking->Need = false;
const bool IsAnnotatedParallel = TheLoop->isAnnotatedParallel();
// For each block.
for (BasicBlock *BB : TheLoop->blocks()) {
// Scan the BB and collect legal loads and stores. Also detect any
// convergent instructions.
for (Instruction &I : *BB) {
if (auto *Call = dyn_cast<CallBase>(&I)) {
if (Call->isConvergent())
HasConvergentOp = true;
}
// With both a non-vectorizable memory instruction and a convergent
// operation, found in this loop, no reason to continue the search.
if (HasComplexMemInst && HasConvergentOp) {
CanVecMem = false;
return;
}
// Avoid hitting recordAnalysis multiple times.
if (HasComplexMemInst)
continue;
// If this is a load, save it. If this instruction can read from memory
// but is not a load, then we quit. Notice that we don't handle function
// calls that read or write.
if (I.mayReadFromMemory()) {
// Many math library functions read the rounding mode. We will only
// vectorize a loop if it contains known function calls that don't set
// the flag. Therefore, it is safe to ignore this read from memory.
auto *Call = dyn_cast<CallInst>(&I);
if (Call && getVectorIntrinsicIDForCall(Call, TLI))
continue;
// If the function has an explicit vectorized counterpart, we can safely
// assume that it can be vectorized.
if (Call && !Call->isNoBuiltin() && Call->getCalledFunction() &&
!VFDatabase::getMappings(*Call).empty())
continue;
auto *Ld = dyn_cast<LoadInst>(&I);
if (!Ld) {
recordAnalysis("CantVectorizeInstruction", Ld)
<< "instruction cannot be vectorized";
HasComplexMemInst = true;
continue;
}
if (!Ld->isSimple() && !IsAnnotatedParallel) {
recordAnalysis("NonSimpleLoad", Ld)
<< "read with atomic ordering or volatile read";
LLVM_DEBUG(dbgs() << "LAA: Found a non-simple load.\n");
HasComplexMemInst = true;
continue;
}
NumLoads++;
Loads.push_back(Ld);
DepChecker->addAccess(Ld);
if (EnableMemAccessVersioning)
collectStridedAccess(Ld);
continue;
}
// Save 'store' instructions. Abort if other instructions write to memory.
if (I.mayWriteToMemory()) {
auto *St = dyn_cast<StoreInst>(&I);
if (!St) {
recordAnalysis("CantVectorizeInstruction", St)
<< "instruction cannot be vectorized";
HasComplexMemInst = true;
continue;
}
if (!St->isSimple() && !IsAnnotatedParallel) {
recordAnalysis("NonSimpleStore", St)
<< "write with atomic ordering or volatile write";
LLVM_DEBUG(dbgs() << "LAA: Found a non-simple store.\n");
HasComplexMemInst = true;
continue;
}
NumStores++;
Stores.push_back(St);
DepChecker->addAccess(St);
if (EnableMemAccessVersioning)
collectStridedAccess(St);
}
} // Next instr.
} // Next block.
if (HasComplexMemInst) {
CanVecMem = false;
return;
}
// Now we have two lists that hold the loads and the stores.
// Next, we find the pointers that they use.
// Check if we see any stores. If there are no stores, then we don't
// care if the pointers are *restrict*.
if (!Stores.size()) {
LLVM_DEBUG(dbgs() << "LAA: Found a read-only loop!\n");
CanVecMem = true;
return;
}
MemoryDepChecker::DepCandidates DependentAccesses;
AccessAnalysis Accesses(TheLoop->getHeader()->getModule()->getDataLayout(),
TheLoop, AA, LI, DependentAccesses, *PSE);
// Holds the analyzed pointers. We don't want to call GetUnderlyingObjects
// multiple times on the same object. If the ptr is accessed twice, once
// for read and once for write, it will only appear once (on the write
// list). This is okay, since we are going to check for conflicts between
// writes and between reads and writes, but not between reads and reads.
ValueSet Seen;
// Record uniform store addresses to identify if we have multiple stores
// to the same address.
ValueSet UniformStores;
for (StoreInst *ST : Stores) {
Value *Ptr = ST->getPointerOperand();
if (isUniform(Ptr))
HasDependenceInvolvingLoopInvariantAddress |=
!UniformStores.insert(Ptr).second;
// If we did *not* see this pointer before, insert it to the read-write
// list. At this phase it is only a 'write' list.
if (Seen.insert(Ptr).second) {
++NumReadWrites;
MemoryLocation Loc = MemoryLocation::get(ST);
// The TBAA metadata could have a control dependency on the predication
// condition, so we cannot rely on it when determining whether or not we
// need runtime pointer checks.
if (blockNeedsPredication(ST->getParent(), TheLoop, DT))
Loc.AATags.TBAA = nullptr;
Accesses.addStore(Loc);
}
}
if (IsAnnotatedParallel) {
LLVM_DEBUG(
dbgs() << "LAA: A loop annotated parallel, ignore memory dependency "
<< "checks.\n");
CanVecMem = true;
return;
}
for (LoadInst *LD : Loads) {
Value *Ptr = LD->getPointerOperand();
// If we did *not* see this pointer before, insert it to the
// read list. If we *did* see it before, then it is already in
// the read-write list. This allows us to vectorize expressions
// such as A[i] += x; Because the address of A[i] is a read-write
// pointer. This only works if the index of A[i] is consecutive.
// If the address of i is unknown (for example A[B[i]]) then we may
// read a few words, modify, and write a few words, and some of the
// words may be written to the same address.
bool IsReadOnlyPtr = false;
if (Seen.insert(Ptr).second ||
!getPtrStride(*PSE, Ptr, TheLoop, SymbolicStrides)) {
++NumReads;
IsReadOnlyPtr = true;
}
// See if there is an unsafe dependency between a load to a uniform address and
// store to the same uniform address.
if (UniformStores.count(Ptr)) {
LLVM_DEBUG(dbgs() << "LAA: Found an unsafe dependency between a uniform "
"load and uniform store to the same address!\n");
HasDependenceInvolvingLoopInvariantAddress = true;
}
MemoryLocation Loc = MemoryLocation::get(LD);
// The TBAA metadata could have a control dependency on the predication
// condition, so we cannot rely on it when determining whether or not we
// need runtime pointer checks.
if (blockNeedsPredication(LD->getParent(), TheLoop, DT))
Loc.AATags.TBAA = nullptr;
Accesses.addLoad(Loc, IsReadOnlyPtr);
}
// If we write (or read-write) to a single destination and there are no
// other reads in this loop then is it safe to vectorize.
if (NumReadWrites == 1 && NumReads == 0) {
LLVM_DEBUG(dbgs() << "LAA: Found a write-only loop!\n");
CanVecMem = true;
return;
}
// Build dependence sets and check whether we need a runtime pointer bounds
// check.
Accesses.buildDependenceSets();
// Find pointers with computable bounds. We are going to use this information
// to place a runtime bound check.
bool CanDoRTIfNeeded = Accesses.canCheckPtrAtRT(*PtrRtChecking, PSE->getSE(),
TheLoop, SymbolicStrides);
if (!CanDoRTIfNeeded) {
recordAnalysis("CantIdentifyArrayBounds") << "cannot identify array bounds";
LLVM_DEBUG(dbgs() << "LAA: We can't vectorize because we can't find "
<< "the array bounds.\n");
CanVecMem = false;
return;
}
LLVM_DEBUG(
dbgs() << "LAA: May be able to perform a memory runtime check if needed.\n");
CanVecMem = true;
if (Accesses.isDependencyCheckNeeded()) {
LLVM_DEBUG(dbgs() << "LAA: Checking memory dependencies\n");
CanVecMem = DepChecker->areDepsSafe(
DependentAccesses, Accesses.getDependenciesToCheck(), SymbolicStrides);
MaxSafeDepDistBytes = DepChecker->getMaxSafeDepDistBytes();
if (!CanVecMem && DepChecker->shouldRetryWithRuntimeCheck()) {
LLVM_DEBUG(dbgs() << "LAA: Retrying with memory checks\n");
// Clear the dependency checks. We assume they are not needed.
Accesses.resetDepChecks(*DepChecker);
PtrRtChecking->reset();
PtrRtChecking->Need = true;
auto *SE = PSE->getSE();
CanDoRTIfNeeded = Accesses.canCheckPtrAtRT(*PtrRtChecking, SE, TheLoop,
SymbolicStrides, true);
// Check that we found the bounds for the pointer.
if (!CanDoRTIfNeeded) {
recordAnalysis("CantCheckMemDepsAtRunTime")
<< "cannot check memory dependencies at runtime";
LLVM_DEBUG(dbgs() << "LAA: Can't vectorize with memory checks\n");
CanVecMem = false;
return;
}
CanVecMem = true;
}
}
if (HasConvergentOp) {
recordAnalysis("CantInsertRuntimeCheckWithConvergent")
<< "cannot add control dependency to convergent operation";
LLVM_DEBUG(dbgs() << "LAA: We can't vectorize because a runtime check "
"would be needed with a convergent operation\n");
CanVecMem = false;
return;
}
if (CanVecMem)
LLVM_DEBUG(
dbgs() << "LAA: No unsafe dependent memory operations in loop. We"
<< (PtrRtChecking->Need ? "" : " don't")
<< " need runtime memory checks.\n");
else {
recordAnalysis("UnsafeMemDep")
<< "unsafe dependent memory operations in loop. Use "
"#pragma loop distribute(enable) to allow loop distribution "
"to attempt to isolate the offending operations into a separate "
"loop";
LLVM_DEBUG(dbgs() << "LAA: unsafe dependent memory operations in loop\n");
}
}
bool LoopAccessInfo::blockNeedsPredication(BasicBlock *BB, Loop *TheLoop,
DominatorTree *DT) {
assert(TheLoop->contains(BB) && "Unknown block used");
// Blocks that do not dominate the latch need predication.
BasicBlock* Latch = TheLoop->getLoopLatch();
return !DT->dominates(BB, Latch);
}
OptimizationRemarkAnalysis &LoopAccessInfo::recordAnalysis(StringRef RemarkName,
Instruction *I) {
assert(!Report && "Multiple reports generated");
Value *CodeRegion = TheLoop->getHeader();
DebugLoc DL = TheLoop->getStartLoc();
if (I) {
CodeRegion = I->getParent();
// If there is no debug location attached to the instruction, revert back to
// using the loop's.
if (I->getDebugLoc())
DL = I->getDebugLoc();
}
Report = std::make_unique<OptimizationRemarkAnalysis>(DEBUG_TYPE, RemarkName, DL,
CodeRegion);
return *Report;
}
bool LoopAccessInfo::isUniform(Value *V) const {
auto *SE = PSE->getSE();
// Since we rely on SCEV for uniformity, if the type is not SCEVable, it is
// never considered uniform.
// TODO: Is this really what we want? Even without FP SCEV, we may want some
// trivially loop-invariant FP values to be considered uniform.
if (!SE->isSCEVable(V->getType()))
return false;
return (SE->isLoopInvariant(SE->getSCEV(V), TheLoop));
}
void LoopAccessInfo::collectStridedAccess(Value *MemAccess) {
Value *Ptr = nullptr;
if (LoadInst *LI = dyn_cast<LoadInst>(MemAccess))
Ptr = LI->getPointerOperand();
else if (StoreInst *SI = dyn_cast<StoreInst>(MemAccess))
Ptr = SI->getPointerOperand();
else
return;
Value *Stride = getStrideFromPointer(Ptr, PSE->getSE(), TheLoop);
if (!Stride)
return;
LLVM_DEBUG(dbgs() << "LAA: Found a strided access that is a candidate for "
"versioning:");
LLVM_DEBUG(dbgs() << " Ptr: " << *Ptr << " Stride: " << *Stride << "\n");
// Avoid adding the "Stride == 1" predicate when we know that
// Stride >= Trip-Count. Such a predicate will effectively optimize a single
// or zero iteration loop, as Trip-Count <= Stride == 1.
//
// TODO: We are currently not making a very informed decision on when it is
// beneficial to apply stride versioning. It might make more sense that the
// users of this analysis (such as the vectorizer) will trigger it, based on
// their specific cost considerations; For example, in cases where stride
// versioning does not help resolving memory accesses/dependences, the
// vectorizer should evaluate the cost of the runtime test, and the benefit
// of various possible stride specializations, considering the alternatives
// of using gather/scatters (if available).
const SCEV *StrideExpr = PSE->getSCEV(Stride);
const SCEV *BETakenCount = PSE->getBackedgeTakenCount();
// Match the types so we can compare the stride and the BETakenCount.
// The Stride can be positive/negative, so we sign extend Stride;
// The backedgeTakenCount is non-negative, so we zero extend BETakenCount.
const DataLayout &DL = TheLoop->getHeader()->getModule()->getDataLayout();
uint64_t StrideTypeSize = DL.getTypeAllocSize(StrideExpr->getType());
uint64_t BETypeSize = DL.getTypeAllocSize(BETakenCount->getType());
const SCEV *CastedStride = StrideExpr;
const SCEV *CastedBECount = BETakenCount;
ScalarEvolution *SE = PSE->getSE();
if (BETypeSize >= StrideTypeSize)
CastedStride = SE->getNoopOrSignExtend(StrideExpr, BETakenCount->getType());
else
CastedBECount = SE->getZeroExtendExpr(BETakenCount, StrideExpr->getType());
const SCEV *StrideMinusBETaken = SE->getMinusSCEV(CastedStride, CastedBECount);
// Since TripCount == BackEdgeTakenCount + 1, checking:
// "Stride >= TripCount" is equivalent to checking:
// Stride - BETakenCount > 0
if (SE->isKnownPositive(StrideMinusBETaken)) {
LLVM_DEBUG(
dbgs() << "LAA: Stride>=TripCount; No point in versioning as the "
"Stride==1 predicate will imply that the loop executes "
"at most once.\n");
return;
}
LLVM_DEBUG(dbgs() << "LAA: Found a strided access that we can version.");
SymbolicStrides[Ptr] = Stride;
StrideSet.insert(Stride);
}
LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
const TargetLibraryInfo *TLI, AliasAnalysis *AA,
DominatorTree *DT, LoopInfo *LI)
: PSE(std::make_unique<PredicatedScalarEvolution>(*SE, *L)),
PtrRtChecking(std::make_unique<RuntimePointerChecking>(SE)),
DepChecker(std::make_unique<MemoryDepChecker>(*PSE, L)), TheLoop(L),
NumLoads(0), NumStores(0), MaxSafeDepDistBytes(-1), CanVecMem(false),
HasConvergentOp(false),
HasDependenceInvolvingLoopInvariantAddress(false) {
if (canAnalyzeLoop())
analyzeLoop(AA, LI, TLI, DT);
}
void LoopAccessInfo::print(raw_ostream &OS, unsigned Depth) const {
if (CanVecMem) {
OS.indent(Depth) << "Memory dependences are safe";
if (MaxSafeDepDistBytes != -1ULL)
OS << " with a maximum dependence distance of " << MaxSafeDepDistBytes
<< " bytes";
if (PtrRtChecking->Need)
OS << " with run-time checks";
OS << "\n";
}
if (HasConvergentOp)
OS.indent(Depth) << "Has convergent operation in loop\n";
if (Report)
OS.indent(Depth) << "Report: " << Report->getMsg() << "\n";
if (auto *Dependences = DepChecker->getDependences()) {
OS.indent(Depth) << "Dependences:\n";
for (auto &Dep : *Dependences) {
Dep.print(OS, Depth + 2, DepChecker->getMemoryInstructions());
OS << "\n";
}
} else
OS.indent(Depth) << "Too many dependences, not recorded\n";
// List the pair of accesses need run-time checks to prove independence.
PtrRtChecking->print(OS, Depth);
OS << "\n";
OS.indent(Depth) << "Non vectorizable stores to invariant address were "
<< (HasDependenceInvolvingLoopInvariantAddress ? "" : "not ")
<< "found in loop.\n";
OS.indent(Depth) << "SCEV assumptions:\n";
PSE->getUnionPredicate().print(OS, Depth);
OS << "\n";
OS.indent(Depth) << "Expressions re-written:\n";
PSE->print(OS, Depth);
}
LoopAccessLegacyAnalysis::LoopAccessLegacyAnalysis() : FunctionPass(ID) {
initializeLoopAccessLegacyAnalysisPass(*PassRegistry::getPassRegistry());
}
const LoopAccessInfo &LoopAccessLegacyAnalysis::getInfo(Loop *L) {
auto &LAI = LoopAccessInfoMap[L];
if (!LAI)
LAI = std::make_unique<LoopAccessInfo>(L, SE, TLI, AA, DT, LI);
return *LAI.get();
}
void LoopAccessLegacyAnalysis::print(raw_ostream &OS, const Module *M) const {
LoopAccessLegacyAnalysis &LAA = *const_cast<LoopAccessLegacyAnalysis *>(this);
for (Loop *TopLevelLoop : *LI)
for (Loop *L : depth_first(TopLevelLoop)) {
OS.indent(2) << L->getHeader()->getName() << ":\n";
auto &LAI = LAA.getInfo(L);
LAI.print(OS, 4);
}
}
bool LoopAccessLegacyAnalysis::runOnFunction(Function &F) {
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
TLI = TLIP ? &TLIP->getTLI(F) : nullptr;
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
return false;
}
void LoopAccessLegacyAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.setPreservesAll();
}
char LoopAccessLegacyAnalysis::ID = 0;
static const char laa_name[] = "Loop Access Analysis";
#define LAA_NAME "loop-accesses"
INITIALIZE_PASS_BEGIN(LoopAccessLegacyAnalysis, LAA_NAME, laa_name, false, true)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(LoopAccessLegacyAnalysis, LAA_NAME, laa_name, false, true)
AnalysisKey LoopAccessAnalysis::Key;
LoopAccessInfo LoopAccessAnalysis::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR) {
return LoopAccessInfo(&L, &AR.SE, &AR.TLI, &AR.AA, &AR.DT, &AR.LI);
}
namespace llvm {
Pass *createLAAPass() {
return new LoopAccessLegacyAnalysis();
}
} // end namespace llvm
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