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[x86] avoid code explosion from LoopVectorizer for gather loop (PR27826)

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By making pointer extraction from a vector more expensive in the cost model,
we avoid the vectorization of a loop that is very likely to be memory-bound:
https://llvm.org/bugs/show_bug.cgi?id=27826

There are still bugs related to this, so we may need a more general solution
to avoid vectorizing obviously memory-bound loops when we don't have HW gather
support.

Differential Revision: http://reviews.llvm.org/D20601



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270729 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjay Patel 2016-05-25 17:27:54 +00:00
parent c863690403
commit cab076f44c
2 changed files with 51 additions and 2 deletions
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12
lib/Target/X86/X86TargetTransformInfo.cpp
View File

@ -963,6 +963,8 @@ int X86TTIImpl::getIntrinsicInstrCost(Intrinsic::ID IID, Type *RetTy,
int X86TTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val, unsigned Index) {
assert(Val->isVectorTy() && "This must be a vector type");
Type *ScalarType = Val->getScalarType();
if (Index != -1U) {
// Legalize the type.
std::pair<int, MVT> LT = TLI->getTypeLegalizationCost(DL, Val);
@ -976,11 +978,17 @@ int X86TTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val, unsigned Index) {
Index = Index % Width;
// Floating point scalars are already located in index #0.
if (Val->getScalarType()->isFloatingPointTy() && Index == 0)
if (ScalarType->isFloatingPointTy() && Index == 0)
return 0;
}
return BaseT::getVectorInstrCost(Opcode, Val, Index);
// Add to the base cost if we know that the extracted element of a vector is
// destined to be moved to and used in the integer register file.
int RegisterFileMoveCost = 0;
if (Opcode == Instruction::ExtractElement && ScalarType->isPointerTy())
RegisterFileMoveCost = 1;
return BaseT::getVectorInstrCost(Opcode, Val, Index) + RegisterFileMoveCost;
}
int X86TTIImpl::getScalarizationOverhead(Type *Ty, bool Insert, bool Extract) {

41
test/Transforms/LoopVectorize/X86/cost-model.ll
View File

@ -39,3 +39,44 @@ for.body: ; preds = %for.body, %entry
for.end: ; preds = %for.body
ret void
}
; This function uses a stride that is generally too big to benefit from vectorization without
; really good support for a gather load. We were not computing an accurate cost for the
; vectorization and subsequent scalarization of the pointer induction variables.
define float @PR27826(float* nocapture readonly %a, float* nocapture readonly %b, i32 %n) {
; CHECK-LABEL: @PR27826(
; CHECK-NOT: <4 x float>
; CHECK-NOT: <8 x float>
; CHECK: ret float %s.0.lcssa
entry:
%cmp = icmp sgt i32 %n, 0
br i1 %cmp, label %preheader, label %for.end
preheader:
%t0 = sext i32 %n to i64
br label %for
for:
%indvars.iv = phi i64 [ 0, %preheader ], [ %indvars.iv.next, %for ]
%s.02 = phi float [ 0.0, %preheader ], [ %add4, %for ]
%arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
%t1 = load float, float* %arrayidx, align 4
%arrayidx3 = getelementptr inbounds float, float* %b, i64 %indvars.iv
%t2 = load float, float* %arrayidx3, align 4
%add = fadd fast float %t1, %s.02
%add4 = fadd fast float %add, %t2
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 8
%cmp1 = icmp slt i64 %indvars.iv.next, %t0
br i1 %cmp1, label %for, label %loopexit
loopexit:
%add4.lcssa = phi float [ %add4, %for ]
br label %for.end
for.end:
%s.0.lcssa = phi float [ 0.0, %entry ], [ %add4.lcssa, %loopexit ]
ret float %s.0.lcssa
}