From: Jingyue Wu <jingyue@google.com>
Date: Tue, 11 Aug 2015 19:21:46 +0000 (+0000)
Subject: SelectionDAG: Prefer to combine multiplication with less uses for fma
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=d1ff8a7f6e7ff783a5096ef40f11397ebcf3a8f9;p=oota-llvm.git

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244649 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2c87d120cb7..c4cbeb00766 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7426,6 +7426,14 @@ SDValue DAGCombiner::visitFADDForFMACombine(SDNode *N) {
   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())) {
diff --git a/test/CodeGen/NVPTX/fma-assoc.ll b/test/CodeGen/NVPTX/fma-assoc.ll
index fc04c61dd69..80a08a86316 100644
--- a/test/CodeGen/NVPTX/fma-assoc.ll
+++ b/test/CodeGen/NVPTX/fma-assoc.ll
@@ -23,3 +23,16 @@ define ptx_device double @t1_f64(double %x, double %y, double %z,
   %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
+}
+