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[TTI] `BasicTTIImplBase::getInterleavedMemoryOpCost()`: fix load discounting 

The math here is:
Cost of 1 load = cost of n loads / n
Cost of live loads = num live loads * Cost of 1 load
Cost of live loads = num live loads * (cost of n loads / n)
Cost of live loads = cost of n loads * (num live loads / n)

But, all the variables here are integers,
and integer division rounds down,
but this calculation clearly expects float semantics.

Instead multiply upfront, and then perform round-up-division.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112302
This commit is contained in:
Roman Lebedev 2021-10-22 14:04:38 +03:00
parent 8ae83a1baf
commit 2eaef53023
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GPG Key ID: 083C3EBB4A1689E0
2 changed files with 4 additions and 3 deletions
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5
llvm/include/llvm/CodeGen/BasicTTIImpl.h
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@ -1214,7 +1214,7 @@ public:
// //
// TODO: Note that legalization can turn masked loads/stores into unmasked // TODO: Note that legalization can turn masked loads/stores into unmasked
// (legalized) loads/stores. This can be reflected in the cost. // (legalized) loads/stores. This can be reflected in the cost.
if (VecTySize > VecTyLTSize) { if (Cost.isValid() && VecTySize > VecTyLTSize) {
// The number of loads of a legal type it will take to represent a load // The number of loads of a legal type it will take to represent a load
// of the unlegalized vector type. // of the unlegalized vector type.
unsigned NumLegalInsts = divideCeil(VecTySize, VecTyLTSize); unsigned NumLegalInsts = divideCeil(VecTySize, VecTyLTSize);
@ -1231,7 +1231,8 @@ public:
// Scale the cost of the load by the fraction of legal instructions that // Scale the cost of the load by the fraction of legal instructions that
// will be used. // will be used.
Cost *= UsedInsts.count() / NumLegalInsts; Cost = divideCeil(UsedInsts.count() * Cost.getValue().getValue(),
NumLegalInsts);
} }
// Then plus the cost of interleave operation. // Then plus the cost of interleave operation.

2
llvm/test/Transforms/LoopVectorize/AArch64/interleaved_cost.ll
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@ -168,7 +168,7 @@ entry:
; gaps. ; gaps.
; ;
; VF_2-LABEL: Checking a loop in "i64_factor_8" ; VF_2-LABEL: Checking a loop in "i64_factor_8"
; VF_2: Found an estimated cost of 6 for VF 2 For instruction: %tmp2 = load i64, i64* %tmp0, align 8 ; VF_2: Found an estimated cost of 10 for VF 2 For instruction: %tmp2 = load i64, i64* %tmp0, align 8
; VF_2-NEXT: Found an estimated cost of 0 for VF 2 For instruction: %tmp3 = load i64, i64* %tmp1, align 8 ; VF_2-NEXT: Found an estimated cost of 0 for VF 2 For instruction: %tmp3 = load i64, i64* %tmp1, align 8
; VF_2-NEXT: Found an estimated cost of 7 for VF 2 For instruction: store i64 0, i64* %tmp0, align 8 ; VF_2-NEXT: Found an estimated cost of 7 for VF 2 For instruction: store i64 0, i64* %tmp0, align 8
; VF_2-NEXT: Found an estimated cost of 7 for VF 2 For instruction: store i64 0, i64* %tmp1, align 8 ; VF_2-NEXT: Found an estimated cost of 7 for VF 2 For instruction: store i64 0, i64* %tmp1, align 8