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//===- LoopVectorizationLegality.cpp --------------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file provides loop vectorization legality analysis. Original code
// resided in LoopVectorize.cpp for a long time.
//
// At this point, it is implemented as a utility class, not as an analysis
// pass. It should be easy to create an analysis pass around it if there
// is a need (but D45420 needs to happen first).
//
#include "llvm/Transforms/Vectorize/LoopVectorizationLegality.h"
#include "llvm/Analysis/VectorUtils.h"
#include "llvm/IR/IntrinsicInst.h"
using namespace llvm;
#define LV_NAME "loop-vectorize"
#define DEBUG_TYPE LV_NAME
static cl::opt<bool>
EnableIfConversion("enable-if-conversion", cl::init(true), cl::Hidden,
cl::desc("Enable if-conversion during vectorization."));
static cl::opt<unsigned> PragmaVectorizeMemoryCheckThreshold(
"pragma-vectorize-memory-check-threshold", cl::init(128), cl::Hidden,
cl::desc("The maximum allowed number of runtime memory checks with a "
"vectorize(enable) pragma."));
static cl::opt<unsigned> VectorizeSCEVCheckThreshold(
"vectorize-scev-check-threshold", cl::init(16), cl::Hidden,
cl::desc("The maximum number of SCEV checks allowed."));
static cl::opt<unsigned> PragmaVectorizeSCEVCheckThreshold(
"pragma-vectorize-scev-check-threshold", cl::init(128), cl::Hidden,
cl::desc("The maximum number of SCEV checks allowed with a "
"vectorize(enable) pragma"));
/// Maximum vectorization interleave count.
static const unsigned MaxInterleaveFactor = 16;
namespace llvm {
OptimizationRemarkAnalysis createLVMissedAnalysis(const char *PassName,
StringRef RemarkName,
Loop *TheLoop,
Instruction *I) {
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();
}
OptimizationRemarkAnalysis R(PassName, RemarkName, DL, CodeRegion);
R << "loop not vectorized: ";
return R;
}
bool LoopVectorizeHints::Hint::validate(unsigned Val) {
switch (Kind) {
case HK_WIDTH:
return isPowerOf2_32(Val) && Val <= VectorizerParams::MaxVectorWidth;
case HK_UNROLL:
return isPowerOf2_32(Val) && Val <= MaxInterleaveFactor;
case HK_FORCE:
return (Val <= 1);
case HK_ISVECTORIZED:
return (Val == 0 || Val == 1);
}
return false;
}
LoopVectorizeHints::LoopVectorizeHints(const Loop *L, bool DisableInterleaving,
OptimizationRemarkEmitter &ORE)
: Width("vectorize.width", VectorizerParams::VectorizationFactor, HK_WIDTH),
Interleave("interleave.count", DisableInterleaving, HK_UNROLL),
Force("vectorize.enable", FK_Undefined, HK_FORCE),
IsVectorized("isvectorized", 0, HK_ISVECTORIZED), TheLoop(L), ORE(ORE) {
// Populate values with existing loop metadata.
getHintsFromMetadata();
// force-vector-interleave overrides DisableInterleaving.
if (VectorizerParams::isInterleaveForced())
Interleave.Value = VectorizerParams::VectorizationInterleave;
if (IsVectorized.Value != 1)
// If the vectorization width and interleaving count are both 1 then
// consider the loop to have been already vectorized because there's
// nothing more that we can do.
IsVectorized.Value = Width.Value == 1 && Interleave.Value == 1;
LLVM_DEBUG(if (DisableInterleaving && Interleave.Value == 1) dbgs()
<< "LV: Interleaving disabled by the pass manager\n");
}
bool LoopVectorizeHints::allowVectorization(Function *F, Loop *L,
bool AlwaysVectorize) const {
if (getForce() == LoopVectorizeHints::FK_Disabled) {
LLVM_DEBUG(dbgs() << "LV: Not vectorizing: #pragma vectorize disable.\n");
emitRemarkWithHints();
return false;
}
if (!AlwaysVectorize && getForce() != LoopVectorizeHints::FK_Enabled) {
LLVM_DEBUG(dbgs() << "LV: Not vectorizing: No #pragma vectorize enable.\n");
emitRemarkWithHints();
return false;
}
if (getIsVectorized() == 1) {
LLVM_DEBUG(dbgs() << "LV: Not vectorizing: Disabled/already vectorized.\n");
// FIXME: Add interleave.disable metadata. This will allow
// vectorize.disable to be used without disabling the pass and errors
// to differentiate between disabled vectorization and a width of 1.
ORE.emit([&]() {
return OptimizationRemarkAnalysis(vectorizeAnalysisPassName(),
"AllDisabled", L->getStartLoc(),
L->getHeader())
<< "loop not vectorized: vectorization and interleaving are "
"explicitly disabled, or the loop has already been "
"vectorized";
});
return false;
}
return true;
}
void LoopVectorizeHints::emitRemarkWithHints() const {
using namespace ore;
ORE.emit([&]() {
if (Force.Value == LoopVectorizeHints::FK_Disabled)
return OptimizationRemarkMissed(LV_NAME, "MissedExplicitlyDisabled",
TheLoop->getStartLoc(),
TheLoop->getHeader())
<< "loop not vectorized: vectorization is explicitly disabled";
else {
OptimizationRemarkMissed R(LV_NAME, "MissedDetails",
TheLoop->getStartLoc(), TheLoop->getHeader());
R << "loop not vectorized";
if (Force.Value == LoopVectorizeHints::FK_Enabled) {
R << " (Force=" << NV("Force", true);
if (Width.Value != 0)
R << ", Vector Width=" << NV("VectorWidth", Width.Value);
if (Interleave.Value != 0)
R << ", Interleave Count=" << NV("InterleaveCount", Interleave.Value);
R << ")";
}
return R;
}
});
}
const char *LoopVectorizeHints::vectorizeAnalysisPassName() const {
if (getWidth() == 1)
return LV_NAME;
if (getForce() == LoopVectorizeHints::FK_Disabled)
return LV_NAME;
if (getForce() == LoopVectorizeHints::FK_Undefined && getWidth() == 0)
return LV_NAME;
return OptimizationRemarkAnalysis::AlwaysPrint;
}
void LoopVectorizeHints::getHintsFromMetadata() {
MDNode *LoopID = TheLoop->getLoopID();
if (!LoopID)
return;
// First operand should refer to the loop id itself.
assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
const MDString *S = nullptr;
SmallVector<Metadata *, 4> Args;
// The expected hint is either a MDString or a MDNode with the first
// operand a MDString.
if (const MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i))) {
if (!MD || MD->getNumOperands() == 0)
continue;
S = dyn_cast<MDString>(MD->getOperand(0));
for (unsigned i = 1, ie = MD->getNumOperands(); i < ie; ++i)
Args.push_back(MD->getOperand(i));
} else {
S = dyn_cast<MDString>(LoopID->getOperand(i));
assert(Args.size() == 0 && "too many arguments for MDString");
}
if (!S)
continue;
// Check if the hint starts with the loop metadata prefix.
StringRef Name = S->getString();
if (Args.size() == 1)
setHint(Name, Args[0]);
}
}
void LoopVectorizeHints::setHint(StringRef Name, Metadata *Arg) {
if (!Name.startswith(Prefix()))
return;
Name = Name.substr(Prefix().size(), StringRef::npos);
const ConstantInt *C = mdconst::dyn_extract<ConstantInt>(Arg);
if (!C)
return;
unsigned Val = C->getZExtValue();
Hint *Hints[] = {&Width, &Interleave, &Force, &IsVectorized};
for (auto H : Hints) {
if (Name == H->Name) {
if (H->validate(Val))
H->Value = Val;
else
LLVM_DEBUG(dbgs() << "LV: ignoring invalid hint '" << Name << "'\n");
break;
}
}
}
MDNode *LoopVectorizeHints::createHintMetadata(StringRef Name,
unsigned V) const {
LLVMContext &Context = TheLoop->getHeader()->getContext();
Metadata *MDs[] = {
MDString::get(Context, Name),
ConstantAsMetadata::get(ConstantInt::get(Type::getInt32Ty(Context), V))};
return MDNode::get(Context, MDs);
}
bool LoopVectorizeHints::matchesHintMetadataName(MDNode *Node,
ArrayRef<Hint> HintTypes) {
MDString *Name = dyn_cast<MDString>(Node->getOperand(0));
if (!Name)
return false;
for (auto H : HintTypes)
if (Name->getString().endswith(H.Name))
return true;
return false;
}
void LoopVectorizeHints::writeHintsToMetadata(ArrayRef<Hint> HintTypes) {
if (HintTypes.empty())
return;
// Reserve the first element to LoopID (see below).
SmallVector<Metadata *, 4> MDs(1);
// If the loop already has metadata, then ignore the existing operands.
MDNode *LoopID = TheLoop->getLoopID();
if (LoopID) {
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
MDNode *Node = cast<MDNode>(LoopID->getOperand(i));
// If node in update list, ignore old value.
if (!matchesHintMetadataName(Node, HintTypes))
MDs.push_back(Node);
}
}
// Now, add the missing hints.
for (auto H : HintTypes)
MDs.push_back(createHintMetadata(Twine(Prefix(), H.Name).str(), H.Value));
// Replace current metadata node with new one.
LLVMContext &Context = TheLoop->getHeader()->getContext();
MDNode *NewLoopID = MDNode::get(Context, MDs);
// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
TheLoop->setLoopID(NewLoopID);
}
bool LoopVectorizationRequirements::doesNotMeet(
Function *F, Loop *L, const LoopVectorizeHints &Hints) {
const char *PassName = Hints.vectorizeAnalysisPassName();
bool Failed = false;
if (UnsafeAlgebraInst && !Hints.allowReordering()) {
ORE.emit([&]() {
return OptimizationRemarkAnalysisFPCommute(
PassName, "CantReorderFPOps", UnsafeAlgebraInst->getDebugLoc(),
UnsafeAlgebraInst->getParent())
<< "loop not vectorized: cannot prove it is safe to reorder "
"floating-point operations";
});
Failed = true;
}
// Test if runtime memcheck thresholds are exceeded.
bool PragmaThresholdReached =
NumRuntimePointerChecks > PragmaVectorizeMemoryCheckThreshold;
bool ThresholdReached =
NumRuntimePointerChecks > VectorizerParams::RuntimeMemoryCheckThreshold;
if ((ThresholdReached && !Hints.allowReordering()) ||
PragmaThresholdReached) {
ORE.emit([&]() {
return OptimizationRemarkAnalysisAliasing(PassName, "CantReorderMemOps",
L->getStartLoc(),
L->getHeader())
<< "loop not vectorized: cannot prove it is safe to reorder "
"memory operations";
});
LLVM_DEBUG(dbgs() << "LV: Too many memory checks needed.\n");
Failed = true;
}
return Failed;
}
// Return true if the inner loop \p Lp is uniform with regard to the outer loop
// \p OuterLp (i.e., if the outer loop is vectorized, all the vector lanes
// executing the inner loop will execute the same iterations). This check is
// very constrained for now but it will be relaxed in the future. \p Lp is
// considered uniform if it meets all the following conditions:
// 1) it has a canonical IV (starting from 0 and with stride 1),
// 2) its latch terminator is a conditional branch and,
// 3) its latch condition is a compare instruction whose operands are the
// canonical IV and an OuterLp invariant.
// This check doesn't take into account the uniformity of other conditions not
// related to the loop latch because they don't affect the loop uniformity.
//
// NOTE: We decided to keep all these checks and its associated documentation
// together so that we can easily have a picture of the current supported loop
// nests. However, some of the current checks don't depend on \p OuterLp and
// would be redundantly executed for each \p Lp if we invoked this function for
// different candidate outer loops. This is not the case for now because we
// don't currently have the infrastructure to evaluate multiple candidate outer
// loops and \p OuterLp will be a fixed parameter while we only support explicit
// outer loop vectorization. It's also very likely that these checks go away
// before introducing the aforementioned infrastructure. However, if this is not
// the case, we should move the \p OuterLp independent checks to a separate
// function that is only executed once for each \p Lp.
static bool isUniformLoop(Loop *Lp, Loop *OuterLp) {
assert(Lp->getLoopLatch() && "Expected loop with a single latch.");
// If Lp is the outer loop, it's uniform by definition.
if (Lp == OuterLp)
return true;
assert(OuterLp->contains(Lp) && "OuterLp must contain Lp.");
// 1.
PHINode *IV = Lp->getCanonicalInductionVariable();
if (!IV) {
LLVM_DEBUG(dbgs() << "LV: Canonical IV not found.\n");
return false;
}
// 2.
BasicBlock *Latch = Lp->getLoopLatch();
auto *LatchBr = dyn_cast<BranchInst>(Latch->getTerminator());
if (!LatchBr || LatchBr->isUnconditional()) {
LLVM_DEBUG(dbgs() << "LV: Unsupported loop latch branch.\n");
return false;
}
// 3.
auto *LatchCmp = dyn_cast<CmpInst>(LatchBr->getCondition());
if (!LatchCmp) {
LLVM_DEBUG(
dbgs() << "LV: Loop latch condition is not a compare instruction.\n");
return false;
}
Value *CondOp0 = LatchCmp->getOperand(0);
Value *CondOp1 = LatchCmp->getOperand(1);
Value *IVUpdate = IV->getIncomingValueForBlock(Latch);
if (!(CondOp0 == IVUpdate && OuterLp->isLoopInvariant(CondOp1)) &&
!(CondOp1 == IVUpdate && OuterLp->isLoopInvariant(CondOp0))) {
LLVM_DEBUG(dbgs() << "LV: Loop latch condition is not uniform.\n");
return false;
}
return true;
}
// Return true if \p Lp and all its nested loops are uniform with regard to \p
// OuterLp.
static bool isUniformLoopNest(Loop *Lp, Loop *OuterLp) {
if (!isUniformLoop(Lp, OuterLp))
return false;
// Check if nested loops are uniform.
for (Loop *SubLp : *Lp)
if (!isUniformLoopNest(SubLp, OuterLp))
return false;
return true;
}
/// Check whether it is safe to if-convert this phi node.
///
/// Phi nodes with constant expressions that can trap are not safe to if
/// convert.
static bool canIfConvertPHINodes(BasicBlock *BB) {
for (PHINode &Phi : BB->phis()) {
for (Value *V : Phi.incoming_values())
if (auto *C = dyn_cast<Constant>(V))
if (C->canTrap())
return false;
}
return true;
}
static Type *convertPointerToIntegerType(const DataLayout &DL, Type *Ty) {
if (Ty->isPointerTy())
return DL.getIntPtrType(Ty);
// It is possible that char's or short's overflow when we ask for the loop's
// trip count, work around this by changing the type size.
if (Ty->getScalarSizeInBits() < 32)
return Type::getInt32Ty(Ty->getContext());
return Ty;
}
static Type *getWiderType(const DataLayout &DL, Type *Ty0, Type *Ty1) {
Ty0 = convertPointerToIntegerType(DL, Ty0);
Ty1 = convertPointerToIntegerType(DL, Ty1);
if (Ty0->getScalarSizeInBits() > Ty1->getScalarSizeInBits())
return Ty0;
return Ty1;
}
/// Check that the instruction has outside loop users and is not an
/// identified reduction variable.
static bool hasOutsideLoopUser(const Loop *TheLoop, Instruction *Inst,
SmallPtrSetImpl<Value *> &AllowedExit) {
// Reductions, Inductions and non-header phis are allowed to have exit users. All
// other instructions must not have external users.
if (!AllowedExit.count(Inst))
// Check that all of the users of the loop are inside the BB.
for (User *U : Inst->users()) {
Instruction *UI = cast<Instruction>(U);
// This user may be a reduction exit value.
if (!TheLoop->contains(UI)) {
LLVM_DEBUG(dbgs() << "LV: Found an outside user for : " << *UI << '\n');
return true;
}
}
return false;
}
int LoopVectorizationLegality::isConsecutivePtr(Value *Ptr) {
const ValueToValueMap &Strides =
getSymbolicStrides() ? *getSymbolicStrides() : ValueToValueMap();
int Stride = getPtrStride(PSE, Ptr, TheLoop, Strides, true, false);
if (Stride == 1 || Stride == -1)
return Stride;
return 0;
}
bool LoopVectorizationLegality::isUniform(Value *V) {
return LAI->isUniform(V);
}
bool LoopVectorizationLegality::canVectorizeOuterLoop() {
assert(!TheLoop->empty() && "We are not vectorizing an outer loop.");
// Store the result and return it at the end instead of exiting early, in case
// allowExtraAnalysis is used to report multiple reasons for not vectorizing.
bool Result = true;
bool DoExtraAnalysis = ORE->allowExtraAnalysis(DEBUG_TYPE);
for (BasicBlock *BB : TheLoop->blocks()) {
// Check whether the BB terminator is a BranchInst. Any other terminator is
// not supported yet.
auto *Br = dyn_cast<BranchInst>(BB->getTerminator());
if (!Br) {
LLVM_DEBUG(dbgs() << "LV: Unsupported basic block terminator.\n");
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// Check whether the BranchInst is a supported one. Only unconditional
// branches, conditional branches with an outer loop invariant condition or
// backedges are supported.
if (Br && Br->isConditional() &&
!TheLoop->isLoopInvariant(Br->getCondition()) &&
!LI->isLoopHeader(Br->getSuccessor(0)) &&
!LI->isLoopHeader(Br->getSuccessor(1))) {
LLVM_DEBUG(dbgs() << "LV: Unsupported conditional branch.\n");
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
}
// Check whether inner loops are uniform. At this point, we only support
// simple outer loops scenarios with uniform nested loops.
if (!isUniformLoopNest(TheLoop /*loop nest*/,
TheLoop /*context outer loop*/)) {
LLVM_DEBUG(
dbgs()
<< "LV: Not vectorizing: Outer loop contains divergent loops.\n");
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// Check whether we are able to set up outer loop induction.
if (!setupOuterLoopInductions()) {
LLVM_DEBUG(
dbgs() << "LV: Not vectorizing: Unsupported outer loop Phi(s).\n");
ORE->emit(createMissedAnalysis("UnsupportedPhi")
<< "Unsupported outer loop Phi(s)");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
return Result;
}
void LoopVectorizationLegality::addInductionPhi(
PHINode *Phi, const InductionDescriptor &ID,
SmallPtrSetImpl<Value *> &AllowedExit) {
Inductions[Phi] = ID;
// In case this induction also comes with casts that we know we can ignore
// in the vectorized loop body, record them here. All casts could be recorded
// here for ignoring, but suffices to record only the first (as it is the
// only one that may bw used outside the cast sequence).
const SmallVectorImpl<Instruction *> &Casts = ID.getCastInsts();
if (!Casts.empty())
InductionCastsToIgnore.insert(*Casts.begin());
Type *PhiTy = Phi->getType();
const DataLayout &DL = Phi->getModule()->getDataLayout();
// Get the widest type.
if (!PhiTy->isFloatingPointTy()) {
if (!WidestIndTy)
WidestIndTy = convertPointerToIntegerType(DL, PhiTy);
else
WidestIndTy = getWiderType(DL, PhiTy, WidestIndTy);
}
// Int inductions are special because we only allow one IV.
if (ID.getKind() == InductionDescriptor::IK_IntInduction &&
ID.getConstIntStepValue() && ID.getConstIntStepValue()->isOne() &&
isa<Constant>(ID.getStartValue()) &&
cast<Constant>(ID.getStartValue())->isNullValue()) {
// Use the phi node with the widest type as induction. Use the last
// one if there are multiple (no good reason for doing this other
// than it is expedient). We've checked that it begins at zero and
// steps by one, so this is a canonical induction variable.
if (!PrimaryInduction || PhiTy == WidestIndTy)
PrimaryInduction = Phi;
}
// Both the PHI node itself, and the "post-increment" value feeding
// back into the PHI node may have external users.
// We can allow those uses, except if the SCEVs we have for them rely
// on predicates that only hold within the loop, since allowing the exit
// currently means re-using this SCEV outside the loop (see PR33706 for more
// details).
if (PSE.getUnionPredicate().isAlwaysTrue()) {
AllowedExit.insert(Phi);
AllowedExit.insert(Phi->getIncomingValueForBlock(TheLoop->getLoopLatch()));
}
LLVM_DEBUG(dbgs() << "LV: Found an induction variable.\n");
}
bool LoopVectorizationLegality::setupOuterLoopInductions() {
BasicBlock *Header = TheLoop->getHeader();
// Returns true if a given Phi is a supported induction.
auto isSupportedPhi = [&](PHINode &Phi) -> bool {
InductionDescriptor ID;
if (InductionDescriptor::isInductionPHI(&Phi, TheLoop, PSE, ID) &&
ID.getKind() == InductionDescriptor::IK_IntInduction) {
addInductionPhi(&Phi, ID, AllowedExit);
return true;
} else {
// Bail out for any Phi in the outer loop header that is not a supported
// induction.
LLVM_DEBUG(
dbgs()
<< "LV: Found unsupported PHI for outer loop vectorization.\n");
return false;
}
};
if (llvm::all_of(Header->phis(), isSupportedPhi))
return true;
else
return false;
}
bool LoopVectorizationLegality::canVectorizeInstrs() {
BasicBlock *Header = TheLoop->getHeader();
// Look for the attribute signaling the absence of NaNs.
Function &F = *Header->getParent();
HasFunNoNaNAttr =
F.getFnAttribute("no-nans-fp-math").getValueAsString() == "true";
// For each block in the loop.
for (BasicBlock *BB : TheLoop->blocks()) {
// Scan the instructions in the block and look for hazards.
for (Instruction &I : *BB) {
if (auto *Phi = dyn_cast<PHINode>(&I)) {
Type *PhiTy = Phi->getType();
// Check that this PHI type is allowed.
if (!PhiTy->isIntegerTy() && !PhiTy->isFloatingPointTy() &&
!PhiTy->isPointerTy()) {
ORE->emit(createMissedAnalysis("CFGNotUnderstood", Phi)
<< "loop control flow is not understood by vectorizer");
LLVM_DEBUG(dbgs() << "LV: Found an non-int non-pointer PHI.\n");
return false;
}
// If this PHINode is not in the header block, then we know that we
// can convert it to select during if-conversion. No need to check if
// the PHIs in this block are induction or reduction variables.
if (BB != Header) {
// Non-header phi nodes that have outside uses can be vectorized. Add
// them to the list of allowed exits.
// Unsafe cyclic dependencies with header phis are identified during
// legalization for reduction, induction and first order
// recurrences.
continue;
}
// We only allow if-converted PHIs with exactly two incoming values.
if (Phi->getNumIncomingValues() != 2) {
ORE->emit(createMissedAnalysis("CFGNotUnderstood", Phi)
<< "control flow not understood by vectorizer");
LLVM_DEBUG(dbgs() << "LV: Found an invalid PHI.\n");
return false;
}
RecurrenceDescriptor RedDes;
if (RecurrenceDescriptor::isReductionPHI(Phi, TheLoop, RedDes, DB, AC,
DT)) {
if (RedDes.hasUnsafeAlgebra())
Requirements->addUnsafeAlgebraInst(RedDes.getUnsafeAlgebraInst());
AllowedExit.insert(RedDes.getLoopExitInstr());
Reductions[Phi] = RedDes;
continue;
}
// TODO: Instead of recording the AllowedExit, it would be good to record the
// complementary set: NotAllowedExit. These include (but may not be
// limited to):
// 1. Reduction phis as they represent the one-before-last value, which
// is not available when vectorized
// 2. Induction phis and increment when SCEV predicates cannot be used
// outside the loop - see addInductionPhi
// 3. Non-Phis with outside uses when SCEV predicates cannot be used
// outside the loop - see call to hasOutsideLoopUser in the non-phi
// handling below
// 4. FirstOrderRecurrence phis that can possibly be handled by
// extraction.
// By recording these, we can then reason about ways to vectorize each
// of these NotAllowedExit.
InductionDescriptor ID;
if (InductionDescriptor::isInductionPHI(Phi, TheLoop, PSE, ID)) {
addInductionPhi(Phi, ID, AllowedExit);
if (ID.hasUnsafeAlgebra() && !HasFunNoNaNAttr)
Requirements->addUnsafeAlgebraInst(ID.getUnsafeAlgebraInst());
continue;
}
if (RecurrenceDescriptor::isFirstOrderRecurrence(Phi, TheLoop,
SinkAfter, DT)) {
FirstOrderRecurrences.insert(Phi);
continue;
}
// As a last resort, coerce the PHI to a AddRec expression
// and re-try classifying it a an induction PHI.
if (InductionDescriptor::isInductionPHI(Phi, TheLoop, PSE, ID, true)) {
addInductionPhi(Phi, ID, AllowedExit);
continue;
}
ORE->emit(createMissedAnalysis("NonReductionValueUsedOutsideLoop", Phi)
<< "value that could not be identified as "
"reduction is used outside the loop");
LLVM_DEBUG(dbgs() << "LV: Found an unidentified PHI." << *Phi << "\n");
return false;
} // end of PHI handling
// We handle calls that:
// * Are debug info intrinsics.
// * Have a mapping to an IR intrinsic.
// * Have a vector version available.
auto *CI = dyn_cast<CallInst>(&I);
if (CI && !getVectorIntrinsicIDForCall(CI, TLI) &&
!isa<DbgInfoIntrinsic>(CI) &&
!(CI->getCalledFunction() && TLI &&
TLI->isFunctionVectorizable(CI->getCalledFunction()->getName()))) {
ORE->emit(createMissedAnalysis("CantVectorizeCall", CI)
<< "call instruction cannot be vectorized");
LLVM_DEBUG(
dbgs() << "LV: Found a non-intrinsic, non-libfunc callsite.\n");
return false;
}
// Intrinsics such as powi,cttz and ctlz are legal to vectorize if the
// second argument is the same (i.e. loop invariant)
if (CI && hasVectorInstrinsicScalarOpd(
getVectorIntrinsicIDForCall(CI, TLI), 1)) {
auto *SE = PSE.getSE();
if (!SE->isLoopInvariant(PSE.getSCEV(CI->getOperand(1)), TheLoop)) {
ORE->emit(createMissedAnalysis("CantVectorizeIntrinsic", CI)
<< "intrinsic instruction cannot be vectorized");
LLVM_DEBUG(dbgs()
<< "LV: Found unvectorizable intrinsic " << *CI << "\n");
return false;
}
}
// Check that the instruction return type is vectorizable.
// Also, we can't vectorize extractelement instructions.
if ((!VectorType::isValidElementType(I.getType()) &&
!I.getType()->isVoidTy()) ||
isa<ExtractElementInst>(I)) {
ORE->emit(createMissedAnalysis("CantVectorizeInstructionReturnType", &I)
<< "instruction return type cannot be vectorized");
LLVM_DEBUG(dbgs() << "LV: Found unvectorizable type.\n");
return false;
}
// Check that the stored type is vectorizable.
if (auto *ST = dyn_cast<StoreInst>(&I)) {
Type *T = ST->getValueOperand()->getType();
if (!VectorType::isValidElementType(T)) {
ORE->emit(createMissedAnalysis("CantVectorizeStore", ST)
<< "store instruction cannot be vectorized");
return false;
}
// FP instructions can allow unsafe algebra, thus vectorizable by
// non-IEEE-754 compliant SIMD units.
// This applies to floating-point math operations and calls, not memory
// operations, shuffles, or casts, as they don't change precision or
// semantics.
} else if (I.getType()->isFloatingPointTy() && (CI || I.isBinaryOp()) &&
!I.isFast()) {
LLVM_DEBUG(dbgs() << "LV: Found FP op with unsafe algebra.\n");
Hints->setPotentiallyUnsafe();
}
// Reduction instructions are allowed to have exit users.
// All other instructions must not have external users.
if (hasOutsideLoopUser(TheLoop, &I, AllowedExit)) {
// We can safely vectorize loops where instructions within the loop are
// used outside the loop only if the SCEV predicates within the loop is
// same as outside the loop. Allowing the exit means reusing the SCEV
// outside the loop.
if (PSE.getUnionPredicate().isAlwaysTrue()) {
AllowedExit.insert(&I);
continue;
}
ORE->emit(createMissedAnalysis("ValueUsedOutsideLoop", &I)
<< "value cannot be used outside the loop");
return false;
}
} // next instr.
}
if (!PrimaryInduction) {
LLVM_DEBUG(dbgs() << "LV: Did not find one integer induction var.\n");
if (Inductions.empty()) {
ORE->emit(createMissedAnalysis("NoInductionVariable")
<< "loop induction variable could not be identified");
return false;
} else if (!WidestIndTy) {
ORE->emit(createMissedAnalysis("NoIntegerInductionVariable")
<< "integer loop induction variable could not be identified");
return false;
}
}
// Now we know the widest induction type, check if our found induction
// is the same size. If it's not, unset it here and InnerLoopVectorizer
// will create another.
if (PrimaryInduction && WidestIndTy != PrimaryInduction->getType())
PrimaryInduction = nullptr;
return true;
}
bool LoopVectorizationLegality::canVectorizeMemory() {
LAI = &(*GetLAA)(*TheLoop);
const OptimizationRemarkAnalysis *LAR = LAI->getReport();
if (LAR) {
ORE->emit([&]() {
return OptimizationRemarkAnalysis(Hints->vectorizeAnalysisPassName(),
"loop not vectorized: ", *LAR);
});
}
if (!LAI->canVectorizeMemory())
return false;
if (LAI->hasMultipleStoresToLoopInvariantAddress()) {
ORE->emit(createMissedAnalysis("CantVectorizeStoreToLoopInvariantAddress")
<< "multiple writes to a loop invariant address could not "
"be vectorized");
LLVM_DEBUG(
dbgs() << "LV: We don't allow multiple stores to a uniform address\n");
return false;
}
Requirements->addRuntimePointerChecks(LAI->getNumRuntimePointerChecks());
PSE.addPredicate(LAI->getPSE().getUnionPredicate());
return true;
}
bool LoopVectorizationLegality::isInductionPhi(const Value *V) {
Value *In0 = const_cast<Value *>(V);
PHINode *PN = dyn_cast_or_null<PHINode>(In0);
if (!PN)
return false;
return Inductions.count(PN);
}
bool LoopVectorizationLegality::isCastedInductionVariable(const Value *V) {
auto *Inst = dyn_cast<Instruction>(V);
return (Inst && InductionCastsToIgnore.count(Inst));
}
bool LoopVectorizationLegality::isInductionVariable(const Value *V) {
return isInductionPhi(V) || isCastedInductionVariable(V);
}
bool LoopVectorizationLegality::isFirstOrderRecurrence(const PHINode *Phi) {
return FirstOrderRecurrences.count(Phi);
}
bool LoopVectorizationLegality::blockNeedsPredication(BasicBlock *BB) {
return LoopAccessInfo::blockNeedsPredication(BB, TheLoop, DT);
}
bool LoopVectorizationLegality::blockCanBePredicated(
BasicBlock *BB, SmallPtrSetImpl<Value *> &SafePtrs) {
const bool IsAnnotatedParallel = TheLoop->isAnnotatedParallel();
for (Instruction &I : *BB) {
// Check that we don't have a constant expression that can trap as operand.
for (Value *Operand : I.operands()) {
if (auto *C = dyn_cast<Constant>(Operand))
if (C->canTrap())
return false;
}
// We might be able to hoist the load.
if (I.mayReadFromMemory()) {
auto *LI = dyn_cast<LoadInst>(&I);
if (!LI)
return false;
if (!SafePtrs.count(LI->getPointerOperand())) {
// !llvm.mem.parallel_loop_access implies if-conversion safety.
// Otherwise, record that the load needs (real or emulated) masking
// and let the cost model decide.
if (!IsAnnotatedParallel)
MaskedOp.insert(LI);
continue;
}
}
if (I.mayWriteToMemory()) {
auto *SI = dyn_cast<StoreInst>(&I);
if (!SI)
return false;
// Predicated store requires some form of masking:
// 1) masked store HW instruction,
// 2) emulation via load-blend-store (only if safe and legal to do so,
// be aware on the race conditions), or
// 3) element-by-element predicate check and scalar store.
MaskedOp.insert(SI);
continue;
}
if (I.mayThrow())
return false;
}
return true;
}
bool LoopVectorizationLegality::canVectorizeWithIfConvert() {
if (!EnableIfConversion) {
ORE->emit(createMissedAnalysis("IfConversionDisabled")
<< "if-conversion is disabled");
return false;
}
assert(TheLoop->getNumBlocks() > 1 && "Single block loops are vectorizable");
// A list of pointers that we can safely read and write to.
SmallPtrSet<Value *, 8> SafePointes;
// Collect safe addresses.
for (BasicBlock *BB : TheLoop->blocks()) {
if (blockNeedsPredication(BB))
continue;
for (Instruction &I : *BB)
if (auto *Ptr = getLoadStorePointerOperand(&I))
SafePointes.insert(Ptr);
}
// Collect the blocks that need predication.
BasicBlock *Header = TheLoop->getHeader();
for (BasicBlock *BB : TheLoop->blocks()) {
// We don't support switch statements inside loops.
if (!isa<BranchInst>(BB->getTerminator())) {
ORE->emit(createMissedAnalysis("LoopContainsSwitch", BB->getTerminator())
<< "loop contains a switch statement");
return false;
}
// We must be able to predicate all blocks that need to be predicated.
if (blockNeedsPredication(BB)) {
if (!blockCanBePredicated(BB, SafePointes)) {
ORE->emit(createMissedAnalysis("NoCFGForSelect", BB->getTerminator())
<< "control flow cannot be substituted for a select");
return false;
}
} else if (BB != Header && !canIfConvertPHINodes(BB)) {
ORE->emit(createMissedAnalysis("NoCFGForSelect", BB->getTerminator())
<< "control flow cannot be substituted for a select");
return false;
}
}
// We can if-convert this loop.
return true;
}
// Helper function to canVectorizeLoopNestCFG.
bool LoopVectorizationLegality::canVectorizeLoopCFG(Loop *Lp,
bool UseVPlanNativePath) {
assert((UseVPlanNativePath || Lp->empty()) &&
"VPlan-native path is not enabled.");
// TODO: ORE should be improved to show more accurate information when an
// outer loop can't be vectorized because a nested loop is not understood or
// legal. Something like: "outer_loop_location: loop not vectorized:
// (inner_loop_location) loop control flow is not understood by vectorizer".
// Store the result and return it at the end instead of exiting early, in case
// allowExtraAnalysis is used to report multiple reasons for not vectorizing.
bool Result = true;
bool DoExtraAnalysis = ORE->allowExtraAnalysis(DEBUG_TYPE);
// We must have a loop in canonical form. Loops with indirectbr in them cannot
// be canonicalized.
if (!Lp->getLoopPreheader()) {
LLVM_DEBUG(dbgs() << "LV: Loop doesn't have a legal pre-header.\n");
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// We must have a single backedge.
if (Lp->getNumBackEdges() != 1) {
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// We must have a single exiting block.
if (!Lp->getExitingBlock()) {
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
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 (Lp->getExitingBlock() != Lp->getLoopLatch()) {
ORE->emit(createMissedAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by vectorizer");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
return Result;
}
bool LoopVectorizationLegality::canVectorizeLoopNestCFG(
Loop *Lp, bool UseVPlanNativePath) {
// Store the result and return it at the end instead of exiting early, in case
// allowExtraAnalysis is used to report multiple reasons for not vectorizing.
bool Result = true;
bool DoExtraAnalysis = ORE->allowExtraAnalysis(DEBUG_TYPE);
if (!canVectorizeLoopCFG(Lp, UseVPlanNativePath)) {
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// Recursively check whether the loop control flow of nested loops is
// understood.
for (Loop *SubLp : *Lp)
if (!canVectorizeLoopNestCFG(SubLp, UseVPlanNativePath)) {
if (DoExtraAnalysis)
Result = false;
else
return false;
}
return Result;
}
bool LoopVectorizationLegality::canVectorize(bool UseVPlanNativePath) {
// Store the result and return it at the end instead of exiting early, in case
// allowExtraAnalysis is used to report multiple reasons for not vectorizing.
bool Result = true;
bool DoExtraAnalysis = ORE->allowExtraAnalysis(DEBUG_TYPE);
// Check whether the loop-related control flow in the loop nest is expected by
// vectorizer.
if (!canVectorizeLoopNestCFG(TheLoop, UseVPlanNativePath)) {
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// We need to have a loop header.
LLVM_DEBUG(dbgs() << "LV: Found a loop: " << TheLoop->getHeader()->getName()
<< '\n');
// Specific checks for outer loops. We skip the remaining legal checks at this
// point because they don't support outer loops.
if (!TheLoop->empty()) {
assert(UseVPlanNativePath && "VPlan-native path is not enabled.");
if (!canVectorizeOuterLoop()) {
LLVM_DEBUG(dbgs() << "LV: Not vectorizing: Unsupported outer loop.\n");
// TODO: Implement DoExtraAnalysis when subsequent legal checks support
// outer loops.
return false;
}
LLVM_DEBUG(dbgs() << "LV: We can vectorize this outer loop!\n");
return Result;
}
assert(TheLoop->empty() && "Inner loop expected.");
// Check if we can if-convert non-single-bb loops.
unsigned NumBlocks = TheLoop->getNumBlocks();
if (NumBlocks != 1 && !canVectorizeWithIfConvert()) {
LLVM_DEBUG(dbgs() << "LV: Can't if-convert the loop.\n");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// Check if we can vectorize the instructions and CFG in this loop.
if (!canVectorizeInstrs()) {
LLVM_DEBUG(dbgs() << "LV: Can't vectorize the instructions or CFG\n");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// Go over each instruction and look at memory deps.
if (!canVectorizeMemory()) {
LLVM_DEBUG(dbgs() << "LV: Can't vectorize due to memory conflicts\n");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
LLVM_DEBUG(dbgs() << "LV: We can vectorize this loop"
<< (LAI->getRuntimePointerChecking()->Need
? " (with a runtime bound check)"
: "")
<< "!\n");
unsigned SCEVThreshold = VectorizeSCEVCheckThreshold;
if (Hints->getForce() == LoopVectorizeHints::FK_Enabled)
SCEVThreshold = PragmaVectorizeSCEVCheckThreshold;
if (PSE.getUnionPredicate().getComplexity() > SCEVThreshold) {
ORE->emit(createMissedAnalysis("TooManySCEVRunTimeChecks")
<< "Too many SCEV assumptions need to be made and checked "
<< "at runtime");
LLVM_DEBUG(dbgs() << "LV: Too many SCEV checks needed.\n");
if (DoExtraAnalysis)
Result = false;
else
return false;
}
// Okay! We've done all the tests. If any have failed, return false. Otherwise
// we can vectorize, and at this point we don't have any other mem analysis
// which may limit our maximum vectorization factor, so just return true with
// no restrictions.
return Result;
}
bool LoopVectorizationLegality::canFoldTailByMasking() {
LLVM_DEBUG(dbgs() << "LV: checking if tail can be folded by masking.\n");
if (!PrimaryInduction) {
ORE->emit(createMissedAnalysis("NoPrimaryInduction")
<< "Missing a primary induction variable in the loop, which is "
<< "needed in order to fold tail by masking as required.");
LLVM_DEBUG(dbgs() << "LV: No primary induction, cannot fold tail by "
<< "masking.\n");
return false;
}
// TODO: handle reductions when tail is folded by masking.
if (!Reductions.empty()) {
ORE->emit(createMissedAnalysis("ReductionFoldingTailByMasking")
<< "Cannot fold tail by masking in the presence of reductions.");
LLVM_DEBUG(dbgs() << "LV: Loop has reductions, cannot fold tail by "
<< "masking.\n");
return false;
}
// TODO: handle outside users when tail is folded by masking.
for (auto *AE : AllowedExit) {
// Check that all users of allowed exit values are inside the loop.
for (User *U : AE->users()) {
Instruction *UI = cast<Instruction>(U);
if (TheLoop->contains(UI))
continue;
ORE->emit(createMissedAnalysis("LiveOutFoldingTailByMasking")
<< "Cannot fold tail by masking in the presence of live outs.");
LLVM_DEBUG(dbgs() << "LV: Cannot fold tail by masking, loop has an "
<< "outside user for : " << *UI << '\n');
return false;
}
}
// The list of pointers that we can safely read and write to remains empty.
SmallPtrSet<Value *, 8> SafePointers;
// Check and mark all blocks for predication, including those that ordinarily
// do not need predication such as the header block.
for (BasicBlock *BB : TheLoop->blocks()) {
if (!blockCanBePredicated(BB, SafePointers)) {
ORE->emit(createMissedAnalysis("NoCFGForSelect", BB->getTerminator())
<< "control flow cannot be substituted for a select");
LLVM_DEBUG(dbgs() << "LV: Cannot fold tail by masking as required.\n");
return false;
}
}
LLVM_DEBUG(dbgs() << "LV: can fold tail by masking.\n");
return true;
}
} // namespace llvm