Index: llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -937,11 +937,15 @@
     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 {
Index: llvm/trunk/test/Transforms/InstCombine/fast-math.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/fast-math.ll
+++ llvm/trunk/test/Transforms/InstCombine/fast-math.ll
@@ -831,3 +831,26 @@
 ; 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
+}