[InstCombine] loosen FP 0.0 constraint for fcmp+select substitution

It looks like we correctly removed edge cases with 0.0 from D50714,
but we were a bit conservative because getBinOpIdentity() doesn't
distinguish between +0.0 and -0.0 and 'nsz' is effectively always
true for fcmp (see discussion in:
https://bugs.llvm.org/show_bug.cgi?id=38086

Without this change, we would get regressions by canonicalizing
to +0.0 in all fcmp, and that's a step towards solving:
https://bugs.llvm.org/show_bug.cgi?id=39475

llvm-svn: 346143

1 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 00dcacccb40..724662f0128 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -75,13 +75,22 @@ static Instruction *foldSelectBinOpIdentity(SelectInst &Sel,
   else
     return nullptr;
 
-  // A select operand must be a binop, and the compare constant must be the
-  // identity constant for that binop.
+  // A select operand must be a binop.
   BinaryOperator *BO;
-  if (!match(Sel.getOperand(IsEq ? 1 : 2), m_BinOp(BO)) ||
-      ConstantExpr::getBinOpIdentity(BO->getOpcode(), BO->getType(), true) != C)
+  if (!match(Sel.getOperand(IsEq ? 1 : 2), m_BinOp(BO)))
     return nullptr;
 
+  // The compare constant must be the identity constant for that binop.
+  // If this a floating-point compare with 0.0, any zero constant will do.
+  Type *Ty = BO->getType();
+  Constant *IdC = ConstantExpr::getBinOpIdentity(BO->getOpcode(), Ty, true);
+  if (IdC != C) {
+    if (!IdC || !CmpInst::isFPPredicate(Pred))
+      return nullptr;
+    if (!match(IdC, m_AnyZeroFP()) || !match(C, m_AnyZeroFP()))
+      return nullptr;
+  }
+
   // Last, match the compare variable operand with a binop operand.
   Value *Y;
   if (!BO->isCommutative() && !match(BO, m_BinOp(m_Value(Y), m_Specific(X))))