[PATCH] D31690: [InstCombine] Make sure we preserve fast math flags when folding fp instructions into phi nodes

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

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


https://reviews.llvm.org/D31690

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/fast-math.ll


Index: test/Transforms/InstCombine/fast-math.ll
===================================================================
--- test/Transforms/InstCombine/fast-math.ll
+++ 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
+}
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ 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 {


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