[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

1 files changed, 4 insertions, 6 deletions

diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index 26da7fa..965012e 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: