[InstSimplify] fold 'fcmp nnan oge X, 0.0' when X is not negative

This re-raises some of the open questions about how to apply and use fast-math-flags in IR from PR38086:
https://bugs.llvm.org/show_bug.cgi?id=38086
...but given the current implementation (no FMF on casts), this is likely the only way to predicate the 
transform.

This is part of solving PR39475:
https://bugs.llvm.org/show_bug.cgi?id=39475

Differential Revision: https://reviews.llvm.org/D53874

llvm-svn: 345725

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index b1381932e7f..efe88507aef 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3612,6 +3612,10 @@ static Value *SimplifyFCmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
     if (C->isZero()) {
       switch (Pred) {
+      case FCmpInst::FCMP_OGE:
+        if (FMF.noNaNs() && CannotBeOrderedLessThanZero(LHS, Q.TLI))
+          return getTrue(RetTy);
+        break;
       case FCmpInst::FCMP_UGE:
         if (CannotBeOrderedLessThanZero(LHS, Q.TLI))
           return getTrue(RetTy);