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//===----- CodeGen/ExpandVectorPredication.cpp - Expand VP intrinsics -----===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass implements IR expansion for vector predication intrinsics, allowing
// targets to enable vector predication until just before codegen.
#include "llvm/CodeGen/ExpandVectorPredication.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/MathExtras.h"
using namespace llvm;
using VPLegalization = TargetTransformInfo::VPLegalization;
using VPTransform = TargetTransformInfo::VPLegalization::VPTransform;
// Keep this in sync with TargetTransformInfo::VPLegalization.
#define VPINTERNAL_CASE(X) "|" #X
// Override options.
static cl::opt<std::string> EVLTransformOverride(
"expandvp-override-evl-transform", cl::init(""), cl::Hidden,
cl::desc("Options: <empty>" VPINTERNAL_VPLEGAL_CASES
". If non-empty, ignore "
"TargetTransformInfo and "
"always use this transformation for the %evl parameter (Used in "
static cl::opt<std::string> MaskTransformOverride(
"expandvp-override-mask-transform", cl::init(""), cl::Hidden,
cl::desc("Options: <empty>" VPINTERNAL_VPLEGAL_CASES
". If non-empty, Ignore "
"TargetTransformInfo and "
"always use this transformation for the %mask parameter (Used in "
#define VPINTERNAL_CASE(X) .Case(#X, VPLegalization::X)
static VPTransform parseOverrideOption(const std::string &TextOpt) {
return StringSwitch<VPTransform>(TextOpt) VPINTERNAL_VPLEGAL_CASES;
// Whether any override options are set.
static bool anyExpandVPOverridesSet() {
return !EVLTransformOverride.empty() || !MaskTransformOverride.empty();
#define DEBUG_TYPE "expandvp"
STATISTIC(NumFoldedVL, "Number of folded vector length params");
STATISTIC(NumLoweredVPOps, "Number of folded vector predication operations");
///// Helpers {
/// \returns Whether the vector mask \p MaskVal has all lane bits set.
static bool isAllTrueMask(Value *MaskVal) {
auto *ConstVec = dyn_cast<ConstantVector>(MaskVal);
return ConstVec && ConstVec->isAllOnesValue();
/// \returns A non-excepting divisor constant for this type.
static Constant *getSafeDivisor(Type *DivTy) {
assert(DivTy->isIntOrIntVectorTy() && "Unsupported divisor type");
return ConstantInt::get(DivTy, 1u, false);
/// Transfer operation properties from \p OldVPI to \p NewVal.
static void transferDecorations(Value &NewVal, VPIntrinsic &VPI) {
auto *NewInst = dyn_cast<Instruction>(&NewVal);
if (!NewInst || !isa<FPMathOperator>(NewVal))
auto *OldFMOp = dyn_cast<FPMathOperator>(&VPI);
if (!OldFMOp)
/// Transfer all properties from \p OldOp to \p NewOp and replace all uses.
/// OldVP gets erased.
static void replaceOperation(Value &NewOp, VPIntrinsic &OldOp) {
transferDecorations(NewOp, OldOp);
//// } Helpers
namespace {
// Expansion pass state at function scope.
struct CachingVPExpander {
Function &F;
const TargetTransformInfo &TTI;
/// \returns A (fixed length) vector with ascending integer indices
/// (<0, 1, ..., NumElems-1>).
/// \p Builder
/// Used for instruction creation.
/// \p LaneTy
/// Integer element type of the result vector.
/// \p NumElems
/// Number of vector elements.
Value *createStepVector(IRBuilder<> &Builder, Type *LaneTy,
unsigned NumElems);
/// \returns A bitmask that is true where the lane position is less-than \p
/// EVLParam
/// \p Builder
/// Used for instruction creation.
/// \p VLParam
/// The explicit vector length parameter to test against the lane
/// positions.
/// \p ElemCount
/// Static (potentially scalable) number of vector elements.
Value *convertEVLToMask(IRBuilder<> &Builder, Value *EVLParam,
ElementCount ElemCount);
Value *foldEVLIntoMask(VPIntrinsic &VPI);
/// "Remove" the %evl parameter of \p PI by setting it to the static vector
/// length of the operation.
void discardEVLParameter(VPIntrinsic &PI);
/// \brief Lower this VP binary operator to a unpredicated binary operator.
Value *expandPredicationInBinaryOperator(IRBuilder<> &Builder,
VPIntrinsic &PI);
/// \brief Lower this VP reduction to a call to an unpredicated reduction
/// intrinsic.
Value *expandPredicationInReduction(IRBuilder<> &Builder,
VPReductionIntrinsic &PI);
/// \brief Query TTI and expand the vector predication in \p P accordingly.
Value *expandPredication(VPIntrinsic &PI);
/// \brief Determine how and whether the VPIntrinsic \p VPI shall be
/// expanded. This overrides TTI with the cl::opts listed at the top of this
/// file.
VPLegalization getVPLegalizationStrategy(const VPIntrinsic &VPI) const;
bool UsingTTIOverrides;
CachingVPExpander(Function &F, const TargetTransformInfo &TTI)
: F(F), TTI(TTI), UsingTTIOverrides(anyExpandVPOverridesSet()) {}
bool expandVectorPredication();
//// CachingVPExpander {
Value *CachingVPExpander::createStepVector(IRBuilder<> &Builder, Type *LaneTy,
unsigned NumElems) {
// TODO add caching
SmallVector<Constant *, 16> ConstElems;
for (unsigned Idx = 0; Idx < NumElems; ++Idx)
ConstElems.push_back(ConstantInt::get(LaneTy, Idx, false));
return ConstantVector::get(ConstElems);
Value *CachingVPExpander::convertEVLToMask(IRBuilder<> &Builder,
Value *EVLParam,
ElementCount ElemCount) {
// TODO add caching
// Scalable vector %evl conversion.
if (ElemCount.isScalable()) {
auto *M = Builder.GetInsertBlock()->getModule();
Type *BoolVecTy = VectorType::get(Builder.getInt1Ty(), ElemCount);
Function *ActiveMaskFunc = Intrinsic::getDeclaration(
M, Intrinsic::get_active_lane_mask, {BoolVecTy, EVLParam->getType()});
// `get_active_lane_mask` performs an implicit less-than comparison.
Value *ConstZero = Builder.getInt32(0);
return Builder.CreateCall(ActiveMaskFunc, {ConstZero, EVLParam});
// Fixed vector %evl conversion.
Type *LaneTy = EVLParam->getType();
unsigned NumElems = ElemCount.getFixedValue();
Value *VLSplat = Builder.CreateVectorSplat(NumElems, EVLParam);
Value *IdxVec = createStepVector(Builder, LaneTy, NumElems);
return Builder.CreateICmp(CmpInst::ICMP_ULT, IdxVec, VLSplat);
Value *
CachingVPExpander::expandPredicationInBinaryOperator(IRBuilder<> &Builder,
VPIntrinsic &VPI) {
assert((isSafeToSpeculativelyExecute(&VPI) ||
VPI.canIgnoreVectorLengthParam()) &&
"Implicitly dropping %evl in non-speculatable operator!");
auto OC = static_cast<Instruction::BinaryOps>(*VPI.getFunctionalOpcode());
Value *Op0 = VPI.getOperand(0);
Value *Op1 = VPI.getOperand(1);
Value *Mask = VPI.getMaskParam();
// Blend in safe operands.
if (Mask && !isAllTrueMask(Mask)) {
switch (OC) {
// Can safely ignore the predicate.
// Division operators need a safe divisor on masked-off lanes (1).
case Instruction::UDiv:
case Instruction::SDiv:
case Instruction::URem:
case Instruction::SRem:
// 2nd operand must not be zero.
Value *SafeDivisor = getSafeDivisor(VPI.getType());
Op1 = Builder.CreateSelect(Mask, Op1, SafeDivisor);
Value *NewBinOp = Builder.CreateBinOp(OC, Op0, Op1, VPI.getName());
replaceOperation(*NewBinOp, VPI);
return NewBinOp;
static Value *getNeutralReductionElement(const VPReductionIntrinsic &VPI,
Type *EltTy) {
bool Negative = false;
unsigned EltBits = EltTy->getScalarSizeInBits();
switch (VPI.getIntrinsicID()) {
llvm_unreachable("Expecting a VP reduction intrinsic");
case Intrinsic::vp_reduce_add:
case Intrinsic::vp_reduce_or:
case Intrinsic::vp_reduce_xor:
case Intrinsic::vp_reduce_umax:
return Constant::getNullValue(EltTy);
case Intrinsic::vp_reduce_mul:
return ConstantInt::get(EltTy, 1, /*IsSigned*/ false);
case Intrinsic::vp_reduce_and:
case Intrinsic::vp_reduce_umin:
return ConstantInt::getAllOnesValue(EltTy);
case Intrinsic::vp_reduce_smin:
return ConstantInt::get(EltTy->getContext(),
case Intrinsic::vp_reduce_smax:
return ConstantInt::get(EltTy->getContext(),
case Intrinsic::vp_reduce_fmax:
Negative = true;
case Intrinsic::vp_reduce_fmin: {
FastMathFlags Flags = VPI.getFastMathFlags();
const fltSemantics &Semantics = EltTy->getFltSemantics();
return !Flags.noNaNs() ? ConstantFP::getQNaN(EltTy, Negative)
: !Flags.noInfs()
? ConstantFP::getInfinity(EltTy, Negative)
: ConstantFP::get(EltTy,
APFloat::getLargest(Semantics, Negative));
case Intrinsic::vp_reduce_fadd:
return ConstantFP::getNegativeZero(EltTy);
case Intrinsic::vp_reduce_fmul:
return ConstantFP::get(EltTy, 1.0);
Value *
CachingVPExpander::expandPredicationInReduction(IRBuilder<> &Builder,
VPReductionIntrinsic &VPI) {
assert((isSafeToSpeculativelyExecute(&VPI) ||
VPI.canIgnoreVectorLengthParam()) &&
"Implicitly dropping %evl in non-speculatable operator!");
Value *Mask = VPI.getMaskParam();
Value *RedOp = VPI.getOperand(VPI.getVectorParamPos());
// Insert neutral element in masked-out positions
if (Mask && !isAllTrueMask(Mask)) {
auto *NeutralElt = getNeutralReductionElement(VPI, VPI.getType());
auto *NeutralVector = Builder.CreateVectorSplat(
cast<VectorType>(RedOp->getType())->getElementCount(), NeutralElt);
RedOp = Builder.CreateSelect(Mask, RedOp, NeutralVector);
Value *Reduction;
Value *Start = VPI.getOperand(VPI.getStartParamPos());
switch (VPI.getIntrinsicID()) {
llvm_unreachable("Impossible reduction kind");
case Intrinsic::vp_reduce_add:
Reduction = Builder.CreateAddReduce(RedOp);
Reduction = Builder.CreateAdd(Reduction, Start);
case Intrinsic::vp_reduce_mul:
Reduction = Builder.CreateMulReduce(RedOp);
Reduction = Builder.CreateMul(Reduction, Start);
case Intrinsic::vp_reduce_and:
Reduction = Builder.CreateAndReduce(RedOp);
Reduction = Builder.CreateAnd(Reduction, Start);
case Intrinsic::vp_reduce_or:
Reduction = Builder.CreateOrReduce(RedOp);
Reduction = Builder.CreateOr(Reduction, Start);
case Intrinsic::vp_reduce_xor:
Reduction = Builder.CreateXorReduce(RedOp);
Reduction = Builder.CreateXor(Reduction, Start);
case Intrinsic::vp_reduce_smax:
Reduction = Builder.CreateIntMaxReduce(RedOp, /*IsSigned*/ true);
Reduction =
Builder.CreateBinaryIntrinsic(Intrinsic::smax, Reduction, Start);
case Intrinsic::vp_reduce_smin:
Reduction = Builder.CreateIntMinReduce(RedOp, /*IsSigned*/ true);
Reduction =
Builder.CreateBinaryIntrinsic(Intrinsic::smin, Reduction, Start);
case Intrinsic::vp_reduce_umax:
Reduction = Builder.CreateIntMaxReduce(RedOp, /*IsSigned*/ false);
Reduction =
Builder.CreateBinaryIntrinsic(Intrinsic::umax, Reduction, Start);
case Intrinsic::vp_reduce_umin:
Reduction = Builder.CreateIntMinReduce(RedOp, /*IsSigned*/ false);
Reduction =
Builder.CreateBinaryIntrinsic(Intrinsic::umin, Reduction, Start);
case Intrinsic::vp_reduce_fmax:
Reduction = Builder.CreateFPMaxReduce(RedOp);
transferDecorations(*Reduction, VPI);
Reduction =
Builder.CreateBinaryIntrinsic(Intrinsic::maxnum, Reduction, Start);
case Intrinsic::vp_reduce_fmin:
Reduction = Builder.CreateFPMinReduce(RedOp);
transferDecorations(*Reduction, VPI);
Reduction =
Builder.CreateBinaryIntrinsic(Intrinsic::minnum, Reduction, Start);
case Intrinsic::vp_reduce_fadd:
Reduction = Builder.CreateFAddReduce(Start, RedOp);
case Intrinsic::vp_reduce_fmul:
Reduction = Builder.CreateFMulReduce(Start, RedOp);
replaceOperation(*Reduction, VPI);
return Reduction;
void CachingVPExpander::discardEVLParameter(VPIntrinsic &VPI) {
LLVM_DEBUG(dbgs() << "Discard EVL parameter in " << VPI << "\n");
if (VPI.canIgnoreVectorLengthParam())
Value *EVLParam = VPI.getVectorLengthParam();
if (!EVLParam)
ElementCount StaticElemCount = VPI.getStaticVectorLength();
Value *MaxEVL = nullptr;
Type *Int32Ty = Type::getInt32Ty(VPI.getContext());
if (StaticElemCount.isScalable()) {
// TODO add caching
auto *M = VPI.getModule();
Function *VScaleFunc =
Intrinsic::getDeclaration(M, Intrinsic::vscale, Int32Ty);
IRBuilder<> Builder(VPI.getParent(), VPI.getIterator());
Value *FactorConst = Builder.getInt32(StaticElemCount.getKnownMinValue());
Value *VScale = Builder.CreateCall(VScaleFunc, {}, "vscale");
MaxEVL = Builder.CreateMul(VScale, FactorConst, "scalable_size",
/*NUW*/ true, /*NSW*/ false);
} else {
MaxEVL = ConstantInt::get(Int32Ty, StaticElemCount.getFixedValue(), false);
Value *CachingVPExpander::foldEVLIntoMask(VPIntrinsic &VPI) {
LLVM_DEBUG(dbgs() << "Folding vlen for " << VPI << '\n');
IRBuilder<> Builder(&VPI);
// Ineffective %evl parameter and so nothing to do here.
if (VPI.canIgnoreVectorLengthParam())
return &VPI;
// Only VP intrinsics can have an %evl parameter.
Value *OldMaskParam = VPI.getMaskParam();
Value *OldEVLParam = VPI.getVectorLengthParam();
assert(OldMaskParam && "no mask param to fold the vl param into");
assert(OldEVLParam && "no EVL param to fold away");
LLVM_DEBUG(dbgs() << "OLD evl: " << *OldEVLParam << '\n');
LLVM_DEBUG(dbgs() << "OLD mask: " << *OldMaskParam << '\n');
// Convert the %evl predication into vector mask predication.
ElementCount ElemCount = VPI.getStaticVectorLength();
Value *VLMask = convertEVLToMask(Builder, OldEVLParam, ElemCount);
Value *NewMaskParam = Builder.CreateAnd(VLMask, OldMaskParam);
// Drop the %evl parameter.
assert(VPI.canIgnoreVectorLengthParam() &&
"transformation did not render the evl param ineffective!");
// Reassess the modified instruction.
return &VPI;
Value *CachingVPExpander::expandPredication(VPIntrinsic &VPI) {
LLVM_DEBUG(dbgs() << "Lowering to unpredicated op: " << VPI << '\n');
IRBuilder<> Builder(&VPI);
// Try lowering to a LLVM instruction first.
auto OC = VPI.getFunctionalOpcode();
if (OC && Instruction::isBinaryOp(*OC))
return expandPredicationInBinaryOperator(Builder, VPI);
if (auto *VPRI = dyn_cast<VPReductionIntrinsic>(&VPI))
return expandPredicationInReduction(Builder, *VPRI);
return &VPI;
//// } CachingVPExpander
struct TransformJob {
VPIntrinsic *PI;
TargetTransformInfo::VPLegalization Strategy;
TransformJob(VPIntrinsic *PI, TargetTransformInfo::VPLegalization InitStrat)
: PI(PI), Strategy(InitStrat) {}
bool isDone() const { return Strategy.shouldDoNothing(); }
void sanitizeStrategy(Instruction &I, VPLegalization &LegalizeStrat) {
// Speculatable instructions do not strictly need predication.
if (isSafeToSpeculativelyExecute(&I)) {
// Converting a speculatable VP intrinsic means dropping %mask and %evl.
// No need to expand %evl into the %mask only to ignore that code.
if (LegalizeStrat.OpStrategy == VPLegalization::Convert)
LegalizeStrat.EVLParamStrategy = VPLegalization::Discard;
// We have to preserve the predicating effect of %evl for this
// non-speculatable VP intrinsic.
// 1) Never discard %evl.
// 2) If this VP intrinsic will be expanded to non-VP code, make sure that
// %evl gets folded into %mask.
if ((LegalizeStrat.EVLParamStrategy == VPLegalization::Discard) ||
(LegalizeStrat.OpStrategy == VPLegalization::Convert)) {
LegalizeStrat.EVLParamStrategy = VPLegalization::Convert;
CachingVPExpander::getVPLegalizationStrategy(const VPIntrinsic &VPI) const {
auto VPStrat = TTI.getVPLegalizationStrategy(VPI);
if (LLVM_LIKELY(!UsingTTIOverrides)) {
// No overrides - we are in production.
return VPStrat;
// Overrides set - we are in testing, the following does not need to be
// efficient.
VPStrat.EVLParamStrategy = parseOverrideOption(EVLTransformOverride);
VPStrat.OpStrategy = parseOverrideOption(MaskTransformOverride);
return VPStrat;
/// \brief Expand llvm.vp.* intrinsics as requested by \p TTI.
bool CachingVPExpander::expandVectorPredication() {
SmallVector<TransformJob, 16> Worklist;
// Collect all VPIntrinsics that need expansion and determine their expansion
// strategy.
for (auto &I : instructions(F)) {
auto *VPI = dyn_cast<VPIntrinsic>(&I);
if (!VPI)
auto VPStrat = getVPLegalizationStrategy(*VPI);
sanitizeStrategy(I, VPStrat);
if (!VPStrat.shouldDoNothing())
Worklist.emplace_back(VPI, VPStrat);
if (Worklist.empty())
return false;
// Transform all VPIntrinsics on the worklist.
LLVM_DEBUG(dbgs() << "\n:::: Transforming " << Worklist.size()
<< " instructions ::::\n");
for (TransformJob Job : Worklist) {
// Transform the EVL parameter.
switch (Job.Strategy.EVLParamStrategy) {
case VPLegalization::Legal:
case VPLegalization::Discard:
case VPLegalization::Convert:
if (foldEVLIntoMask(*Job.PI))
Job.Strategy.EVLParamStrategy = VPLegalization::Legal;
// Replace with a non-predicated operation.
switch (Job.Strategy.OpStrategy) {
case VPLegalization::Legal:
case VPLegalization::Discard:
llvm_unreachable("Invalid strategy for operators.");
case VPLegalization::Convert:
Job.Strategy.OpStrategy = VPLegalization::Legal;
assert(Job.isDone() && "incomplete transformation");
return true;
class ExpandVectorPredication : public FunctionPass {
static char ID;
ExpandVectorPredication() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
const auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
CachingVPExpander VPExpander(F, *TTI);
return VPExpander.expandVectorPredication();
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // namespace
char ExpandVectorPredication::ID;
INITIALIZE_PASS_BEGIN(ExpandVectorPredication, "expandvp",
"Expand vector predication intrinsics", false, false)
INITIALIZE_PASS_END(ExpandVectorPredication, "expandvp",
"Expand vector predication intrinsics", false, false)
FunctionPass *llvm::createExpandVectorPredicationPass() {
return new ExpandVectorPredication();
ExpandVectorPredicationPass::run(Function &F, FunctionAnalysisManager &AM) {
const auto &TTI = AM.getResult<TargetIRAnalysis>(F);
CachingVPExpander VPExpander(F, TTI);
if (!VPExpander.expandVectorPredication())
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;