[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, 3 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index d7af4b88a81..d3702a4885c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -5433,8 +5433,8 @@ Instruction *InstCombiner::visitFCmpInst(FCmpInst &I) {
         break;
       // fabs(x) < 0 --> false
       case FCmpInst::FCMP_OLT:
-        llvm_unreachable("handled by SimplifyFCmpInst");
-      // fabs(x) > 0 --> x != 0
+        llvm_unreachable("fcmp should have simplified");
+          // fabs(x) > 0 --> x != 0
       case FCmpInst::FCMP_OGT:
         return new FCmpInst(FCmpInst::FCMP_ONE, CI->getArgOperand(0), RHSC);
       // fabs(x) <= 0 --> x == 0
@@ -5442,6 +5442,7 @@ Instruction *InstCombiner::visitFCmpInst(FCmpInst &I) {
         return new FCmpInst(FCmpInst::FCMP_OEQ, CI->getArgOperand(0), RHSC);
       // fabs(x) >= 0 --> !isnan(x)
       case FCmpInst::FCMP_OGE:
+        assert(!I.hasNoNaNs() && "fcmp should have simplified");
         return new FCmpInst(FCmpInst::FCMP_ORD, CI->getArgOperand(0), RHSC);
       // fabs(x) == 0 --> x == 0
       // fabs(x) != 0 --> x != 0