[llvm] 8daaea2 - [InstCombine] Use +0.0 instead of -0.0 as the FP identity for some folds
David Sherwood via llvm-commits
llvm-commits at lists.llvm.org
Fri Jun 10 04:42:40 PDT 2022
Author: David Sherwood
Date: 2022-06-10T12:42:34+01:00
New Revision: 8daaea206b5905858bdbdd881be4fe9bdcc54516
URL: https://github.com/llvm/llvm-project/commit/8daaea206b5905858bdbdd881be4fe9bdcc54516
DIFF: https://github.com/llvm/llvm-project/commit/8daaea206b5905858bdbdd881be4fe9bdcc54516.diff
LOG: [InstCombine] Use +0.0 instead of -0.0 as the FP identity for some folds
In foldSelectIntoOp we sometimes transform a select of a fadd into a
fadd of a select, where we select between data and an identity value.
For both fadd and fsub the identity is always -0.0, but if the nsz
flag is set on the select instruction we can use +0.0 instead. Doing
so then triggers other optimisations, such as when folding the select
of masked load into a new masked load.
Differential Revision: https://reviews.llvm.org/D126774
Added:
Modified:
llvm/include/llvm/IR/Constants.h
llvm/lib/IR/Constants.cpp
llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
Removed:
################################################################################
diff --git a/llvm/include/llvm/IR/Constants.h b/llvm/include/llvm/IR/Constants.h
index 6c655407cb5e..0caee4cb8a50 100644
--- a/llvm/include/llvm/IR/Constants.h
+++ b/llvm/include/llvm/IR/Constants.h
@@ -289,7 +289,8 @@ class ConstantFP final : public ConstantData {
APInt *Payload = nullptr);
static Constant *getSNaN(Type *Ty, bool Negative = false,
APInt *Payload = nullptr);
- static Constant *getNegativeZero(Type *Ty);
+ static Constant *getZero(Type *Ty, bool Negative = false);
+ static Constant *getNegativeZero(Type *Ty) { return getZero(Ty, true); }
static Constant *getInfinity(Type *Ty, bool Negative = false);
/// Return true if Ty is big enough to represent V.
@@ -1120,9 +1121,12 @@ class ConstantExpr : public Constant {
/// commutative, callers can acquire the operand 1 identity constant by
/// setting AllowRHSConstant to true. For example, any shift has a zero
/// identity constant for operand 1: X shift 0 = X.
+ /// If this is a fadd/fsub operation and we don't care about signed zeros,
+ /// then setting NSZ to true returns the identity +0.0 instead of -0.0.
/// Return nullptr if the operator does not have an identity constant.
static Constant *getBinOpIdentity(unsigned Opcode, Type *Ty,
- bool AllowRHSConstant = false);
+ bool AllowRHSConstant = false,
+ bool NSZ = false);
/// Return the absorbing element for the given binary
/// operation, i.e. a constant C such that X op C = C and C op X = C for
diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index 26da7fae391b..965012e7f5ed 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -1037,9 +1037,9 @@ Constant *ConstantFP::getSNaN(Type *Ty, bool Negative, APInt *Payload) {
return C;
}
-Constant *ConstantFP::getNegativeZero(Type *Ty) {
+Constant *ConstantFP::getZero(Type *Ty, bool Negative) {
const fltSemantics &Semantics = Ty->getScalarType()->getFltSemantics();
- APFloat NegZero = APFloat::getZero(Semantics, /*Negative=*/true);
+ APFloat NegZero = APFloat::getZero(Semantics, Negative);
Constant *C = get(Ty->getContext(), NegZero);
if (VectorType *VTy = dyn_cast<VectorType>(Ty))
@@ -1048,7 +1048,6 @@ Constant *ConstantFP::getNegativeZero(Type *Ty) {
return C;
}
-
Constant *ConstantFP::getZeroValueForNegation(Type *Ty) {
if (Ty->isFPOrFPVectorTy())
return getNegativeZero(Ty);
@@ -2835,7 +2834,7 @@ Constant *ConstantExpr::getExactLogBase2(Constant *C) {
}
Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty,
- bool AllowRHSConstant) {
+ bool AllowRHSConstant, bool NSZ) {
assert(Instruction::isBinaryOp(Opcode) && "Only binops allowed");
// Commutative opcodes: it does not matter if AllowRHSConstant is set.
@@ -2850,8 +2849,7 @@ Constant *ConstantExpr::getBinOpIdentity(unsigned Opcode, Type *Ty,
case Instruction::And: // X & -1 = X
return Constant::getAllOnesValue(Ty);
case Instruction::FAdd: // X + -0.0 = X
- // TODO: If the fadd has 'nsz', should we return +0.0?
- return ConstantFP::getNegativeZero(Ty);
+ return ConstantFP::getZero(Ty, !NSZ);
case Instruction::FMul: // X * 1.0 = X
return ConstantFP::get(Ty, 1.0);
default:
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 96db638bd773..c18e8f856b2f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -450,8 +450,11 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
}
if (OpToFold) {
- Constant *C = ConstantExpr::getBinOpIdentity(TVI->getOpcode(),
- TVI->getType(), true);
+ FastMathFlags FMF;
+ if (isa<FPMathOperator>(&SI))
+ FMF = SI.getFastMathFlags();
+ Constant *C = ConstantExpr::getBinOpIdentity(
+ TVI->getOpcode(), TVI->getType(), true, FMF.noSignedZeros());
Value *OOp = TVI->getOperand(2-OpToFold);
// Avoid creating select between 2 constants unless it's selecting
// between 0, 1 and -1.
@@ -460,6 +463,8 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
if (!isa<Constant>(OOp) ||
(OOpIsAPInt && isSelect01(C->getUniqueInteger(), *OOpC))) {
Value *NewSel = Builder.CreateSelect(SI.getCondition(), OOp, C);
+ if (isa<FPMathOperator>(&SI))
+ cast<Instruction>(NewSel)->setFastMathFlags(FMF);
NewSel->takeName(TVI);
BinaryOperator *BO = BinaryOperator::Create(TVI->getOpcode(),
FalseVal, NewSel);
@@ -482,8 +487,11 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
}
if (OpToFold) {
- Constant *C = ConstantExpr::getBinOpIdentity(FVI->getOpcode(),
- FVI->getType(), true);
+ FastMathFlags FMF;
+ if (isa<FPMathOperator>(&SI))
+ FMF = SI.getFastMathFlags();
+ Constant *C = ConstantExpr::getBinOpIdentity(
+ FVI->getOpcode(), FVI->getType(), true, FMF.noSignedZeros());
Value *OOp = FVI->getOperand(2-OpToFold);
// Avoid creating select between 2 constants unless it's selecting
// between 0, 1 and -1.
@@ -492,6 +500,8 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
if (!isa<Constant>(OOp) ||
(OOpIsAPInt && isSelect01(C->getUniqueInteger(), *OOpC))) {
Value *NewSel = Builder.CreateSelect(SI.getCondition(), C, OOp);
+ if (isa<FPMathOperator>(&SI))
+ cast<Instruction>(NewSel)->setFastMathFlags(FMF);
NewSel->takeName(FVI);
BinaryOperator *BO = BinaryOperator::Create(FVI->getOpcode(),
TrueVal, NewSel);
diff --git a/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll b/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
index 00360b29b453..496854c7d731 100644
--- a/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
+++ b/llvm/test/Transforms/InstCombine/select-binop-foldable-floating-point.ll
@@ -45,6 +45,39 @@ define float @select_fadd_swapped_fast_math(i1 %cond, float %A, float %B) {
ret float %D
}
+define <4 x float> @select_nsz_fadd_v4f32(<4 x i1> %cond, <4 x float> %A, <4 x float> %B) {
+; CHECK-LABEL: @select_nsz_fadd_v4f32(
+; CHECK-NEXT: [[C:%.*]] = select nnan nsz <4 x i1> [[COND:%.*]], <4 x float> [[B:%.*]], <4 x float> zeroinitializer
+; CHECK-NEXT: [[D:%.*]] = fadd nnan nsz <4 x float> [[C]], [[A:%.*]]
+; CHECK-NEXT: ret <4 x float> [[D]]
+;
+ %C = fadd nsz nnan <4 x float> %A, %B
+ %D = select nsz nnan <4 x i1> %cond, <4 x float> %C, <4 x float> %A
+ ret <4 x float> %D
+}
+
+define <vscale x 4 x float> @select_nsz_fadd_nxv4f32(<vscale x 4 x i1> %cond, <vscale x 4 x float> %A, <vscale x 4 x float> %B) {
+; CHECK-LABEL: @select_nsz_fadd_nxv4f32(
+; CHECK-NEXT: [[C:%.*]] = select nnan nsz <vscale x 4 x i1> [[COND:%.*]], <vscale x 4 x float> [[B:%.*]], <vscale x 4 x float> zeroinitializer
+; CHECK-NEXT: [[D:%.*]] = fadd nnan nsz <vscale x 4 x float> [[C]], [[A:%.*]]
+; CHECK-NEXT: ret <vscale x 4 x float> [[D]]
+;
+ %C = fadd nsz nnan <vscale x 4 x float> %A, %B
+ %D = select nsz nnan <vscale x 4 x i1> %cond, <vscale x 4 x float> %C, <vscale x 4 x float> %A
+ ret <vscale x 4 x float> %D
+}
+
+define <vscale x 4 x float> @select_nsz_fadd_nxv4f32_swapops(<vscale x 4 x i1> %cond, <vscale x 4 x float> %A, <vscale x 4 x float> %B) {
+; CHECK-LABEL: @select_nsz_fadd_nxv4f32_swapops(
+; CHECK-NEXT: [[C:%.*]] = select fast <vscale x 4 x i1> [[COND:%.*]], <vscale x 4 x float> zeroinitializer, <vscale x 4 x float> [[B:%.*]]
+; CHECK-NEXT: [[D:%.*]] = fadd fast <vscale x 4 x float> [[C]], [[A:%.*]]
+; CHECK-NEXT: ret <vscale x 4 x float> [[D]]
+;
+ %C = fadd fast <vscale x 4 x float> %A, %B
+ %D = select fast <vscale x 4 x i1> %cond, <vscale x 4 x float> %A, <vscale x 4 x float> %C
+ ret <vscale x 4 x float> %D
+}
+
define float @select_fmul(i1 %cond, float %A, float %B) {
; CHECK-LABEL: @select_fmul(
; CHECK-NEXT: [[C:%.*]] = select i1 [[COND:%.*]], float [[B:%.*]], float 1.000000e+00
@@ -135,7 +168,7 @@ define float @select_fsub_swapped_fast_math(i1 %cond, float %A, float %B) {
define <4 x float> @select_nsz_fsub_v4f32(<4 x i1> %cond, <4 x float> %A, <4 x float> %B) {
; CHECK-LABEL: @select_nsz_fsub_v4f32(
-; CHECK-NEXT: [[C:%.*]] = select <4 x i1> [[COND:%.*]], <4 x float> [[B:%.*]], <4 x float> zeroinitializer
+; CHECK-NEXT: [[C:%.*]] = select nsz <4 x i1> [[COND:%.*]], <4 x float> [[B:%.*]], <4 x float> zeroinitializer
; CHECK-NEXT: [[D:%.*]] = fsub <4 x float> [[A:%.*]], [[C]]
; CHECK-NEXT: ret <4 x float> [[D]]
;
@@ -146,7 +179,7 @@ define <4 x float> @select_nsz_fsub_v4f32(<4 x i1> %cond, <4 x float> %A, <4 x f
define <vscale x 4 x float> @select_nsz_fsub_nxv4f32(<vscale x 4 x i1> %cond, <vscale x 4 x float> %A, <vscale x 4 x float> %B) {
; CHECK-LABEL: @select_nsz_fsub_nxv4f32(
-; CHECK-NEXT: [[C:%.*]] = select <vscale x 4 x i1> [[COND:%.*]], <vscale x 4 x float> [[B:%.*]], <vscale x 4 x float> zeroinitializer
+; CHECK-NEXT: [[C:%.*]] = select nsz <vscale x 4 x i1> [[COND:%.*]], <vscale x 4 x float> [[B:%.*]], <vscale x 4 x float> zeroinitializer
; CHECK-NEXT: [[D:%.*]] = fsub <vscale x 4 x float> [[A:%.*]], [[C]]
; CHECK-NEXT: ret <vscale x 4 x float> [[D]]
;
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