[InstCombine] Make sure we preserve fast math flags when folding fp instructions into phi nodes

Summary: I noticed in the select folding code that we copied fast math flags, but did not do the same for the similar handling in phi nodes. This patch fixes that to do the same thing as select

Reviewers: spatel, davide, majnemer, hfinkel

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31690

llvm-svn: 299838

2 files changed, 29 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index bc168dd5776..398b49748ec 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -937,11 +937,15 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
     Constant *C = cast<Constant>(I.getOperand(1));
     for (unsigned i = 0; i != NumPHIValues; ++i) {
       Value *InV = nullptr;
-      if (Constant *InC = dyn_cast<Constant>(PN->getIncomingValue(i)))
+      if (Constant *InC = dyn_cast<Constant>(PN->getIncomingValue(i))) {
         InV = ConstantExpr::get(I.getOpcode(), InC, C);
-      else
+      } else {
         InV = Builder->CreateBinOp(cast<BinaryOperator>(I).getOpcode(),
                                    PN->getIncomingValue(i), C, "phitmp");
+        auto *FPInst = dyn_cast<Instruction>(InV);
+        if (FPInst && isa<FPMathOperator>(FPInst))
+          FPInst->copyFastMathFlags(&I);
+      }
       NewPN->addIncoming(InV, PN->getIncomingBlock(i));
     }
   } else {
diff --git a/llvm/test/Transforms/InstCombine/fast-math.ll b/llvm/test/Transforms/InstCombine/fast-math.ll
index ad8a9247e4e..6ddf3a58529 100644
--- a/llvm/test/Transforms/InstCombine/fast-math.ll
+++ b/llvm/test/Transforms/InstCombine/fast-math.ll
@@ -831,3 +831,26 @@ define fp128 @min4(fp128 %a, fp128 %b) {
 ; CHECK-NEXT:  select {{.*}} fp128 %a, fp128 %b 
 ; CHECK-NEXT:  ret
 }
+
+define float @test55(i1 %which, float %a) {
+; CHECK-LABEL: @test55(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[WHICH:%.*]], label [[FINAL:%.*]], label [[DELAY:%.*]]
+; CHECK:       delay:
+; CHECK-NEXT:    [[PHITMP:%.*]] = fadd fast float [[A:%.*]], 1.000000e+00
+; CHECK-NEXT:    br label [[FINAL]]
+; CHECK:       final:
+; CHECK-NEXT:    [[A:%.*]] = phi float [ 3.000000e+00, [[ENTRY:%.*]] ], [ [[PHITMP]], [[DELAY]] ]
+; CHECK-NEXT:    ret float [[A]]
+;
+entry:
+  br i1 %which, label %final, label %delay
+
+delay:
+  br label %final
+
+final:
+  %A = phi float [ 2.0, %entry ], [ %a, %delay ]
+  %value = fadd fast float %A, 1.0
+  ret float %value
+}