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[LoopVectorize] Simplify scalar cost calculation in getInstructionCost

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This patch simplifies the calculation of certain costs in
getInstructionCost when isScalarAfterVectorization() returns a true value.
There are a few places where we multiply a cost by a number N, i.e.

  unsigned N = isScalarAfterVectorization(I, VF) ? VF.getKnownMinValue() : 1;
  return N * TTI.getArithmeticInstrCost(...

After some investigation it seems that there are only these cases that occur
in practice:

1. VF is a scalar, in which case N = 1.
2. VF is a vector. We can only get here if: a) the instruction is a
GEP/bitcast with scalar uses, or b) this is an update to an induction variable
that remains scalar.

I have changed the code so that N is assumed to always be 1. For GEPs
the cost is always 0, since this is calculated later on as part of the
load/store cost. For all other cases I have added an assert that none of the
users needs scalarising, which didn't fire in any unit tests.

Only one test required fixing and I believe the original cost for the scalar
add instruction to have been wrong, since only one copy remains after
vectorisation.

Differential Revision: https://reviews.llvm.org/D98512
This commit is contained in:
David Sherwood 2021-03-10 08:34:19 +00:00
parent 5e07382161
commit 4e4f3dfb9b
2 changed files with 38 additions and 27 deletions
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63
lib/Transforms/Vectorize/LoopVectorize.cpp
View File

@ -7253,10 +7253,36 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I, ElementCount VF,
Type *RetTy = I->getType();
if (canTruncateToMinimalBitwidth(I, VF))
RetTy = IntegerType::get(RetTy->getContext(), MinBWs[I]);
VectorTy = isScalarAfterVectorization(I, VF) ? RetTy : ToVectorTy(RetTy, VF);
auto SE = PSE.getSE();
TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
auto hasSingleCopyAfterVectorization = [this](Instruction *I,
ElementCount VF) -> bool {
if (VF.isScalar())
return true;
auto Scalarized = InstsToScalarize.find(VF);
assert(Scalarized != InstsToScalarize.end() &&
"VF not yet analyzed for scalarization profitability");
return !Scalarized->second.count(I) &&
llvm::all_of(I->users(), [&](User *U) {
auto *UI = cast<Instruction>(U);
return !Scalarized->second.count(UI);
});
};
if (isScalarAfterVectorization(I, VF)) {
VectorTy = RetTy;
// With the exception of GEPs, after scalarization there should only be one
// copy of the instruction generated in the loop. This is because the VF is
// either 1, or any instructions that need scalarizing have already been
// dealt with by the the time we get here. As a result, it means we don't
// have to multiply the instruction cost by VF.
assert(I->getOpcode() == Instruction::GetElementPtr ||
hasSingleCopyAfterVectorization(I, VF));
} else
VectorTy = ToVectorTy(RetTy, VF);
// TODO: We need to estimate the cost of intrinsic calls.
switch (I->getOpcode()) {
case Instruction::GetElementPtr:
@ -7384,21 +7410,16 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I, ElementCount VF,
Op2VK = TargetTransformInfo::OK_UniformValue;
SmallVector<const Value *, 4> Operands(I->operand_values());
unsigned N = isScalarAfterVectorization(I, VF) ? VF.getKnownMinValue() : 1;
return N * TTI.getArithmeticInstrCost(
I->getOpcode(), VectorTy, CostKind,
TargetTransformInfo::OK_AnyValue,
Op2VK, TargetTransformInfo::OP_None, Op2VP, Operands, I);
return TTI.getArithmeticInstrCost(
I->getOpcode(), VectorTy, CostKind, TargetTransformInfo::OK_AnyValue,
Op2VK, TargetTransformInfo::OP_None, Op2VP, Operands, I);
}
case Instruction::FNeg: {
assert(!VF.isScalable() && "VF is assumed to be non scalable.");
unsigned N = isScalarAfterVectorization(I, VF) ? VF.getKnownMinValue() : 1;
return N * TTI.getArithmeticInstrCost(
I->getOpcode(), VectorTy, CostKind,
TargetTransformInfo::OK_AnyValue,
TargetTransformInfo::OK_AnyValue,
TargetTransformInfo::OP_None, TargetTransformInfo::OP_None,
I->getOperand(0), I);
return TTI.getArithmeticInstrCost(
I->getOpcode(), VectorTy, CostKind, TargetTransformInfo::OK_AnyValue,
TargetTransformInfo::OK_AnyValue, TargetTransformInfo::OP_None,
TargetTransformInfo::OP_None, I->getOperand(0), I);
}
case Instruction::Select: {
SelectInst *SI = cast<SelectInst>(I);
@ -7522,14 +7543,7 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I, ElementCount VF,
}
}
unsigned N;
if (isScalarAfterVectorization(I, VF)) {
assert(!VF.isScalable() && "VF is assumed to be non scalable");
N = VF.getKnownMinValue();
} else
N = 1;
return N *
TTI.getCastInstrCost(Opcode, VectorTy, SrcVecTy, CCH, CostKind, I);
return TTI.getCastInstrCost(Opcode, VectorTy, SrcVecTy, CCH, CostKind, I);
}
case Instruction::Call: {
bool NeedToScalarize;
@ -7544,11 +7558,8 @@ LoopVectorizationCostModel::getInstructionCost(Instruction *I, ElementCount VF,
case Instruction::ExtractValue:
return TTI.getInstructionCost(I, TTI::TCK_RecipThroughput);
default:
// The cost of executing VF copies of the scalar instruction. This opcode
// is unknown. Assume that it is the same as 'mul'.
return VF.getKnownMinValue() * TTI.getArithmeticInstrCost(
Instruction::Mul, VectorTy, CostKind) +
getScalarizationOverhead(I, VF);
// This opcode is unknown. Assume that it is the same as 'mul'.
return TTI.getArithmeticInstrCost(Instruction::Mul, VectorTy, CostKind);
} // end of switch.
}

2
test/Transforms/LoopVectorize/AArch64/no_vector_instructions.ll
View File

@ -6,7 +6,7 @@ target triple = "aarch64--linux-gnu"
; CHECK-LABEL: all_scalar
; CHECK: LV: Found scalar instruction: %i.next = add nuw nsw i64 %i, 2
; CHECK: LV: Found an estimated cost of 2 for VF 2 For instruction: %i.next = add nuw nsw i64 %i, 2
; CHECK: LV: Found an estimated cost of 1 for VF 2 For instruction: %i.next = add nuw nsw i64 %i, 2
; CHECK: LV: Not considering vector loop of width 2 because it will not generate any vector instructions
;
define void @all_scalar(i64* %a, i64 %n) {