Teach InstCombine that (fmul X, -1.0) can be simplified to (fneg X), which LLVM expresses as (fsub -0.0, X).

llvm-svn: 199420

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -426,6 +426,16 @@ Instruction *InstCombiner::visitFMul(BinaryOperator &I) {
         return NV;
 
     ConstantFP *C = dyn_cast<ConstantFP>(Op1);
+
+    // (fmul X, -1.0) --> (fsub -0.0, X)
+    if (C && C->isExactlyValue(-1.0)) {
+      Instruction *RI = BinaryOperator::CreateFSub(
+        ConstantFP::getNegativeZero(C->getType()),
+        Op0);
+      RI->copyFastMathFlags(&I);
+      return RI;
+    }
+
     if (C && AllowReassociate && C->getValueAPF().isFiniteNonZero()) {
       // Let MDC denote an expression in one of these forms:
       // X * C, C/X, X/C, where C is a constant.

llvm/test/Transforms/InstCombine/fmul.ll

@@ -93,3 +93,15 @@ for.body:                                         ; preds = %for.cond
 for.end:                                          ; preds = %for.cond
   ret void
 }
+
+; X * -1.0 => -0.0 - X
+define float @test9(float %x) {
+  %mul = fmul float %x, -1.0
+  ret float %mul
+
+; CHECK-LABEL: @test9(
+; CHECK-NOT: fmul
+; CHECK: fsub
+}
+
+