fix an invisible bug when combining repeated FP divisors

This patch fixes bugs that were exposed by the addition of fast-math-flags in the DAG:
r237046 ( http://reviews.llvm.org/rL237046 ):

1. When replacing a division node, it's not enough to RAUW.
   We should call CombineTo() to delete dead nodes and combine again.
2. Because we are changing the DAG, we can't return an empty SDValue
   after the transform. As the code comments say:

    Visitation implementation - Implement dag node combining for different node types.
    The semantics are as follows: Return Value:
      SDValue.getNode() == 0 - No change was made
      SDValue.getNode() == N - N was replaced, is dead and has been handled.
      otherwise - N should be replaced by the returned Operand.

The new test case shows no difference with or without this patch, but it will crash if
we re-apply r237046 or enable FMF via the current -enable-fmf-dag cl::opt.

Differential Revision: http://reviews.llvm.org/D9893

llvm-svn: 241826

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/fdiv-combine.ll b/llvm/test/CodeGen/X86/fdiv-combine.ll
index 8adfde7f3f3..34eac62e367 100644
--- a/llvm/test/CodeGen/X86/fdiv-combine.ll
+++ b/llvm/test/CodeGen/X86/fdiv-combine.ll
@@ -27,5 +27,22 @@ define float @div2_arcp(float %x, float %y, float %z) #0 {
   ret float %div2
 }
 
+; If the reciprocal is already calculated, we should not
+; generate an extra multiplication by 1.0. 
+
+define double @div3_arcp(double %x, double %y, double %z) #0 {
+; CHECK-LABEL: div3_arcp:
+; CHECK:       # BB#0:
+; CHECK-NEXT:    movsd{{.*#+}} xmm2 = mem[0],zero
+; CHECK-NEXT:    divsd %xmm1, %xmm2
+; CHECK-NEXT:    mulsd %xmm2, %xmm0
+; CHECK-NEXT:    addsd %xmm2, %xmm0
+; CHECK-NEXT:    retq
+  %div1 = fdiv fast double 1.0, %y
+  %div2 = fdiv fast double %x, %y
+  %ret = fadd fast double %div2, %div1
+  ret double %ret
+}
+
 ; FIXME: If the backend understands 'arcp', then this attribute is unnecessary.
 attributes #0 = { "unsafe-fp-math"="true" }