[InstCombine] Extend saturating idempotent atomicrmw transform to FP

I'm assuming that the nan propogation logic for InstructonSimplify's handling of fadd and fsub is correct, and applying the same to atomicrmw.

Differential Revision: https://reviews.llvm.org/D58836

llvm-svn: 355222

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
index d3a7d32ec75..5f37a00f56c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
@@ -57,15 +57,23 @@ bool isIdempotentRMW(AtomicRMWInst& RMWI) {
 }
 
 /// Return true if the given instruction always produces a value in memory
-/// equivelent to its value operand.
+/// equivalent to its value operand.
 bool isSaturating(AtomicRMWInst& RMWI) {
+  if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
+    switch(RMWI.getOperation()) {
+    case AtomicRMWInst::FAdd:
+    case AtomicRMWInst::FSub:
+      return CF->isNaN();
+    default:
+      return false;
+    };
+
   auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
   if(!C)
     return false;
 
   switch(RMWI.getOperation()) {
   default:
-    // TODO: fadd, fsub w/Nan
     return false;
   case AtomicRMWInst::Xchg:
     return true;