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[InstCombine] fold fdiv with pow divisor (PR49147)

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This is unusual in the general (non-reciprocal) case because we need
an extra instruction, but that should be better for general FP
reassociation and codegen. We conservatively check for "arcp" FMF
here as we do with existing fdiv folds, but it is not strictly
necessary to have that.

This is part of solving:
https://llvm.org/PR49147
(The powi variant potentially has a different constraint.)

Differential Revision: https://reviews.llvm.org/D96648
This commit is contained in:
Sanjay Patel
2021-02-14 08:06:15 -05:00
parent 2c5c7d5feb
commit b40fde062c
2 changed files with 25 additions and 5 deletions
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11
llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@@ -1325,6 +1325,17 @@ Instruction *InstCombinerImpl::visitFDiv(BinaryOperator &I) {
// replaced by a multiplication.
if (match(Op1, m_FDiv(m_SpecificFP(1.0), m_Value(Y))))
return BinaryOperator::CreateFMulFMF(Y, Op0, &I);
// Negate the exponent of pow to fold division-by-pow() into multiply:
// Z / pow(X, Y) --> Z * pow(X, -Y)
// In the general case, this creates an extra instruction, but fmul allows
// for better canonicalization and optimization than fdiv.
if (match(Op1,
m_OneUse(m_Intrinsic<Intrinsic::pow>(m_Value(X), m_Value(Y))))) {
Value *NegY = Builder.CreateFNegFMF(Y, &I);
Value *Pow = Builder.CreateBinaryIntrinsic(Intrinsic::pow, X, NegY, &I);
return BinaryOperator::CreateFMulFMF(Op0, Pow, &I);
}
}
if (I.hasAllowReassoc() && Op0->hasOneUse() && Op1->hasOneUse()) {

19
llvm/test/Transforms/InstCombine/fdiv.ll
View File

@@ -661,8 +661,9 @@ define float @fabs_fabs_extra_use3(float %x, float %y) {
define float @pow_divisor(float %x, float %y, float %z) {
; CHECK-LABEL: @pow_divisor(
; CHECK-NEXT: [[P:%.*]] = call float @llvm.pow.f32(float [[X:%.*]], float [[Y:%.*]])
; CHECK-NEXT: [[R:%.*]] = fdiv reassoc arcp float [[Z:%.*]], [[P]]
; CHECK-NEXT: [[TMP1:%.*]] = fneg reassoc arcp float [[Y:%.*]]
; CHECK-NEXT: [[TMP2:%.*]] = call reassoc arcp float @llvm.pow.f32(float [[X:%.*]], float [[TMP1]])
; CHECK-NEXT: [[R:%.*]] = fmul reassoc arcp float [[TMP2]], [[Z:%.*]]
; CHECK-NEXT: ret float [[R]]
;
%p = call float @llvm.pow.f32(float %x, float %y)
@@ -670,6 +671,8 @@ define float @pow_divisor(float %x, float %y, float %z) {
ret float %r
}
; Negative test - don't create an extra pow
define float @pow_divisor_extra_use(float %x, float %y, float %z) {
; CHECK-LABEL: @pow_divisor_extra_use(
; CHECK-NEXT: [[P:%.*]] = call float @llvm.pow.f32(float [[X:%.*]], float [[Y:%.*]])
@@ -683,6 +686,8 @@ define float @pow_divisor_extra_use(float %x, float %y, float %z) {
ret float %r
}
; Negative test - must have reassoc+arcp
define float @pow_divisor_not_enough_fmf(float %x, float %y, float %z) {
; CHECK-LABEL: @pow_divisor_not_enough_fmf(
; CHECK-NEXT: [[P:%.*]] = call fast float @llvm.pow.f32(float [[X:%.*]], float [[Y:%.*]])
@@ -694,6 +699,8 @@ define float @pow_divisor_not_enough_fmf(float %x, float %y, float %z) {
ret float %r
}
; Negative test - must have reassoc+arcp
define float @pow_divisor_not_enough_fmf2(float %x, float %y, float %z) {
; CHECK-LABEL: @pow_divisor_not_enough_fmf2(
; CHECK-NEXT: [[P:%.*]] = call fast float @llvm.pow.f32(float [[X:%.*]], float [[Y:%.*]])
@@ -705,11 +712,13 @@ define float @pow_divisor_not_enough_fmf2(float %x, float %y, float %z) {
ret float %r
}
; Special-case - reciprocal does not require extra fmul
define <2 x half> @pow_recip(<2 x half> %x, <2 x half> %y) {
; CHECK-LABEL: @pow_recip(
; CHECK-NEXT: [[P:%.*]] = call <2 x half> @llvm.pow.v2f16(<2 x half> [[X:%.*]], <2 x half> [[Y:%.*]])
; CHECK-NEXT: [[R:%.*]] = fdiv reassoc ninf arcp <2 x half> <half 0xH3C00, half 0xH3C00>, [[P]]
; CHECK-NEXT: ret <2 x half> [[R]]
; CHECK-NEXT: [[TMP1:%.*]] = fneg reassoc ninf arcp <2 x half> [[Y:%.*]]
; CHECK-NEXT: [[TMP2:%.*]] = call reassoc ninf arcp <2 x half> @llvm.pow.v2f16(<2 x half> [[X:%.*]], <2 x half> [[TMP1]])
; CHECK-NEXT: ret <2 x half> [[TMP2]]
;
%p = call <2 x half> @llvm.pow.v2f16(<2 x half> %x, <2 x half> %y)
%r = fdiv reassoc arcp ninf <2 x half> <half 1.0, half 1.0>, %p