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

[craig.topper]

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

[davide]

LGTM

llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp

for (unsigned i = 0; i != NumPHIValues; ++i) {
         InV = Builder->CreateFCmp(CI->getPredicate(), PN->getIncomingValue(i),
                                   C, "phitmp");
       NewPN->addIncoming(InV, PN->getIncomingBlock(i));
     }
   } else if (I.getNumOperands() == 2) {
     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 {
     CastInst *CI = cast<CastInst>(&I);
     Type *RetTy = CI->getType();
     for (unsigned i = 0; i != NumPHIValues; ++i) {
       Value *InV;
       if (Constant *InC = dyn_cast<Constant>(PN->getIncomingValue(i)))

llvm/trunk/test/Transforms/InstCombine/fast-math.ll

define fp128 @min4(fp128 %a, fp128 %b) {
   %c = call fast fp128 @fminl(fp128 %a, fp128 %b)
   ret fp128 %c
 
 ; CHECK-LABEL: min4(
 ; CHECK-NEXT:  fcmp fast olt fp128 %a, %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
+}