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[LoopVectorize] use IR fast-math-flags exclusively (not FP function attributes)

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I am trying to untangle the fast-math-flags propagation logic
in the vectorizers (see a6f022127 for SLP).

The loop vectorizer has a mix of checking FP function attributes,
IR-level FMF, and just wrong assumptions.

I am trying to avoid regressions while fixing this, and I think
the IR-level logic is good enough for that, but it's hard to say
for sure. This would be the 1st step in the clean-up.

The existing test that I changed to include 'fast' actually shows
a miscompile: the function only had the equivalent of nnan, but we
created new instructions that had fast (all FMF set). This is
similar to the example in https://llvm.org/PR35538

Differential Revision: https://reviews.llvm.org/D95452
This commit is contained in:
Sanjay Patel 2021-01-27 14:11:22 -05:00
parent 8ef46f8e36
commit 2ae45edb62
6 changed files with 10 additions and 26 deletions
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6
include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
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@ -344,9 +344,6 @@ public:
unsigned getNumStores() const { return LAI->getNumStores(); }
unsigned getNumLoads() const { return LAI->getNumLoads(); }
// Returns true if the NoNaN attribute is set on the function.
bool hasFunNoNaNAttr() const { return HasFunNoNaNAttr; }
/// Returns all assume calls in predicated blocks. They need to be dropped
/// when flattening the CFG.
const SmallPtrSetImpl<Instruction *> &getConditionalAssumes() const {
@ -495,9 +492,6 @@ private:
/// outside the loop.
SmallPtrSet<Value *, 4> AllowedExit;
/// Can we assume the absence of NaNs.
bool HasFunNoNaNAttr = false;
/// Vectorization requirements that will go through late-evaluation.
LoopVectorizationRequirements *Requirements;

7
lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
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@ -605,11 +605,6 @@ static bool isTLIScalarize(const TargetLibraryInfo &TLI, const CallInst &CI) {
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.
@ -673,7 +668,7 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
InductionDescriptor ID;
if (InductionDescriptor::isInductionPHI(Phi, TheLoop, PSE, ID)) {
addInductionPhi(Phi, ID, AllowedExit);
if (ID.hasUnsafeAlgebra() && !HasFunNoNaNAttr)
if (ID.hasUnsafeAlgebra())
Requirements->addUnsafeAlgebraInst(ID.getUnsafeAlgebraInst());
continue;
}

2
lib/Transforms/Vectorize/LoopVectorize.cpp
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@ -8908,7 +8908,7 @@ void LoopVectorizationPlanner::adjustRecipesForInLoopReductions(
? RecipeBuilder.createBlockInMask(R->getParent(), Plan)
: nullptr;
VPReductionRecipe *RedRecipe = new VPReductionRecipe(
&RdxDesc, R, ChainOp, VecOp, CondOp, Legal->hasFunNoNaNAttr(), TTI);
&RdxDesc, R, ChainOp, VecOp, CondOp, TTI);
WidenRecipe->getVPValue()->replaceAllUsesWith(RedRecipe);
Plan->removeVPValueFor(R);
Plan->addVPValue(R, RedRecipe);

6
lib/Transforms/Vectorize/VPlan.h
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@ -1130,17 +1130,15 @@ public:
class VPReductionRecipe : public VPRecipeBase, public VPUser, public VPValue {
/// The recurrence decriptor for the reduction in question.
RecurrenceDescriptor *RdxDesc;
/// Fast math flags to use for the resulting reduction operation.
bool NoNaN;
/// Pointer to the TTI, needed to create the target reduction
const TargetTransformInfo *TTI;
public:
VPReductionRecipe(RecurrenceDescriptor *R, Instruction *I, VPValue *ChainOp,
VPValue *VecOp, VPValue *CondOp, bool NoNaN,
VPValue *VecOp, VPValue *CondOp,
const TargetTransformInfo *TTI)
: VPRecipeBase(VPRecipeBase::VPReductionSC), VPUser({ChainOp, VecOp}),
VPValue(VPValue::VPVReductionSC, I, this), RdxDesc(R), NoNaN(NoNaN),
VPValue(VPValue::VPVReductionSC, I, this), RdxDesc(R),
TTI(TTI) {
if (CondOp)
addOperand(CondOp);

13
test/Transforms/LoopVectorize/X86/float-induction-x86.ll
View File

@ -4,7 +4,7 @@
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
; This test checks auto-vectorization with FP induction variable.
; The FP operation is not "fast" and requires "fast-math" function attribute.
; FMF is required on the IR instructions.
;void fp_iv_loop1(float * __restrict__ A, int N) {
; float x = 1.0;
@ -149,7 +149,7 @@ define void @fp_iv_loop1(float* noalias nocapture %A, i32 %N) #0 {
; AUTO_VEC-NEXT: [[X_06:%.*]] = phi float [ [[CONV1:%.*]], [[FOR_BODY]] ], [ 1.000000e+00, [[FOR_BODY_PREHEADER]] ], [ [[IND_END]], [[MIDDLE_BLOCK]] ]
; AUTO_VEC-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds float, float* [[A]], i64 [[INDVARS_IV]]
; AUTO_VEC-NEXT: store float [[X_06]], float* [[ARRAYIDX]], align 4
; AUTO_VEC-NEXT: [[CONV1]] = fadd float [[X_06]], 5.000000e-01
; AUTO_VEC-NEXT: [[CONV1]] = fadd fast float [[X_06]], 5.000000e-01
; AUTO_VEC-NEXT: [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
; AUTO_VEC-NEXT: [[TMP45:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT]], [[ZEXT]]
; AUTO_VEC-NEXT: br i1 [[TMP45]], label [[FOR_END]], label [[FOR_BODY]], [[LOOP4:!llvm.loop !.*]]
@ -168,7 +168,7 @@ for.body: ; preds = %for.body.preheader,
%x.06 = phi float [ %conv1, %for.body ], [ 1.000000e+00, %for.body.preheader ]
%arrayidx = getelementptr inbounds float, float* %A, i64 %indvars.iv
store float %x.06, float* %arrayidx, align 4
%conv1 = fadd float %x.06, 5.000000e-01
%conv1 = fadd fast float %x.06, 5.000000e-01
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%lftr.wideiv = trunc i64 %indvars.iv.next to i32
%exitcond = icmp eq i32 %lftr.wideiv, %N
@ -181,7 +181,7 @@ for.end: ; preds = %for.end.loopexit, %
ret void
}
; The same as the previous, FP operation is not fast, different function attribute
; The same as the previous, but FP operation has no FMF.
; Vectorization should be rejected.
;void fp_iv_loop2(float * __restrict__ A, int N) {
; float x = 1.0;
@ -191,7 +191,7 @@ for.end: ; preds = %for.end.loopexit, %
; }
;}
define void @fp_iv_loop2(float* noalias nocapture %A, i32 %N) #1 {
define void @fp_iv_loop2(float* noalias nocapture %A, i32 %N) {
; AUTO_VEC-LABEL: @fp_iv_loop2(
; AUTO_VEC-NEXT: entry:
; AUTO_VEC-NEXT: [[CMP4:%.*]] = icmp sgt i32 [[N:%.*]], 0
@ -538,6 +538,3 @@ for.end:
%t1 = phi double [ %j, %for.body ]
ret double %t1
}
attributes #0 = { "no-nans-fp-math"="true" }
attributes #1 = { "no-nans-fp-math"="false" }

2
unittests/Transforms/Vectorize/VPlanTest.cpp
View File

@ -643,7 +643,7 @@ TEST(VPRecipeTest, CastVPReductionRecipeToVPUser) {
VPValue ChainOp;
VPValue VecOp;
VPValue CondOp;
VPReductionRecipe Recipe(nullptr, nullptr, &ChainOp, &CondOp, &VecOp, false,
VPReductionRecipe Recipe(nullptr, nullptr, &ChainOp, &CondOp, &VecOp,
nullptr);
EXPECT_TRUE(isa<VPUser>(&Recipe));
VPRecipeBase *BaseR = &Recipe;