[InstSimplify] use FMF to improve fcmp+select fold

This is part of a series of patches needed to solve PR39535:
https://bugs.llvm.org/show_bug.cgi?id=39535

1 files changed, 10 insertions, 7 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 11a12ab84e9..83b767a8fd1 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3909,18 +3909,21 @@ static Value *simplifySelectWithICmpCond(Value *CondVal, Value *TrueVal,
 
 /// Try to simplify a select instruction when its condition operand is a
 /// floating-point comparison.
-static Value *simplifySelectWithFCmp(Value *Cond, Value *T, Value *F) {
+static Value *simplifySelectWithFCmp(Value *Cond, Value *T, Value *F,
+                                     const SimplifyQuery &Q) {
   FCmpInst::Predicate Pred;
   if (!match(Cond, m_FCmp(Pred, m_Specific(T), m_Specific(F))) &&
       !match(Cond, m_FCmp(Pred, m_Specific(F), m_Specific(T))))
     return nullptr;
 
-  // TODO: The transform may not be valid with -0.0. An incomplete way of
-  // testing for that possibility is to check if at least one operand is a
-  // non-zero constant.
+  // This transform is safe if we do not have (do not care about) -0.0 or if
+  // at least one operand is known to not be -0.0. Otherwise, the select can
+  // change the sign of a zero operand.
+  bool HasNoSignedZeros = Q.CxtI && isa<FPMathOperator>(Q.CxtI) &&
+                          Q.CxtI->hasNoSignedZeros();
   const APFloat *C;
-  if ((match(T, m_APFloat(C)) && C->isNonZero()) ||
-      (match(F, m_APFloat(C)) && C->isNonZero())) {
+  if (HasNoSignedZeros || (match(T, m_APFloat(C)) && C->isNonZero()) ||
+                          (match(F, m_APFloat(C)) && C->isNonZero())) {
     // (T == F) ? T : F --> F
     // (F == T) ? T : F --> F
     if (Pred == FCmpInst::FCMP_OEQ)
@@ -3971,7 +3974,7 @@ static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
           simplifySelectWithICmpCond(Cond, TrueVal, FalseVal, Q, MaxRecurse))
     return V;
 
-  if (Value *V = simplifySelectWithFCmp(Cond, TrueVal, FalseVal))
+  if (Value *V = simplifySelectWithFCmp(Cond, TrueVal, FalseVal, Q))
     return V;
 
   if (Value *V = foldSelectWithBinaryOp(Cond, TrueVal, FalseVal))