[llvm] r244649 - SelectionDAG: Prefer to combine multiplication with less uses for fma

[Jingyue Wu via llvm-commits]

Author: jingyue
Date: Tue Aug 11 14:21:46 2015
New Revision: 244649

URL: http://llvm.org/viewvc/llvm-project?rev=244649&view=rev
Log:
SelectionDAG: Prefer to combine multiplication with less uses for fma

Summary:
For example:

  s6 = s0*s5;
  s2 = s6*s6 + s6;
  ...
  s4 = s6*s3;

We notice that it is possible for s2 is folded to fma (s0, s5, fmul (s6 s6)).
This only happens when Aggressive is true, otherwise hasOneUse() check
already prevents from folding the multiplication with more uses.

Test Plan: test/CodeGen/NVPTX/fma-assoc.ll

Patch by Xuetian Weng

Reviewers: hfinkel, apazos, jingyue, ohsallen, arsenm

Subscribers: arsenm, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D11855

Modified:
    llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    llvm/trunk/test/CodeGen/NVPTX/fma-assoc.ll

Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=244649&r1=244648&r2=244649&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Tue Aug 11 14:21:46 2015
@@ -7426,6 +7426,14 @@ SDValue DAGCombiner::visitFADDForFMAComb
   bool Aggressive = TLI.enableAggressiveFMAFusion(VT);
   bool LookThroughFPExt = TLI.isFPExtFree(VT);
 
+  // If we have two choices trying to fold (fadd (fmul u, v), (fmul x, y)),
+  // prefer to fold the multiply with fewer uses.
+  if (Aggressive && N0.getOpcode() == ISD::FMUL &&
+      N1.getOpcode() == ISD::FMUL) {
+    if (N0.getNode()->use_size() > N1.getNode()->use_size())
+      std::swap(N0, N1);
+  }
+
   // fold (fadd (fmul x, y), z) -> (fma x, y, z)
   if (N0.getOpcode() == ISD::FMUL &&
       (Aggressive || N0->hasOneUse())) {

Modified: llvm/trunk/test/CodeGen/NVPTX/fma-assoc.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/NVPTX/fma-assoc.ll?rev=244649&r1=244648&r2=244649&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/NVPTX/fma-assoc.ll (original)
+++ llvm/trunk/test/CodeGen/NVPTX/fma-assoc.ll Tue Aug 11 14:21:46 2015
@@ -23,3 +23,16 @@ define ptx_device double @t1_f64(double
   %d = fadd double %c, %z
   ret double %d
 }
+
+define double @two_choices(double %val1, double %val2) {
+; CHECK-LABEL: two_choices(
+; CHECK: mul.f64
+; CHECK-NOT: mul.f64
+; CHECK: fma.rn.f64
+  %1 = fmul double %val1, %val2
+  %2 = fmul double %1, %1
+  %3 = fadd double %1, %2
+
+  ret double %3
+}
+