[SimplifyCFG] propagate fast-math-flags (FMF) from phi to select

Similar to/extension of D70208 (rGee0882bdf866), but this one
may finally allow closing motivating bugs.

This is another step towards having FMF apply only to FP values
rather than those + fcmp. See PR38086 for one of the original
discussions/motivations:
https://bugs.llvm.org/show_bug.cgi?id=38086

And the test here is derived from PR39535:
https://bugs.llvm.org/show_bug.cgi?id=39535

Currently, we lose FMF when converting any phi to select in
SimplifyCFG. There are a small number of similar changes needed
to correct within SimplifyCFG, so it should be quick to patch
this pass up.

FMF was extended to select and phi with:
D61917
D67564

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index ab1a474db9a..79d4857c2c8 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2425,7 +2425,12 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
   if (IfBlock2)
     hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock2);
 
+  // Propagate fast-math-flags from phi nodes to replacement selects.
+  IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
   while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
+    if (isa<FPMathOperator>(PN))
+      Builder.setFastMathFlags(PN->getFastMathFlags());
+
     // Change the PHI node into a select instruction.
     Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
     Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);