[Fast-math] Disable "(C1/X)*C2 => (C1*C2)/X" if C1/X has multiple uses.

  If "C1/X" were having multiple uses, the only benefit of this
transformation is to potentially shorten critical path. But it is at the
cost of instroducing additional div.

  The additional div may or may not incur cost depending on how div is
implemented. If it is implemented using Newton–Raphson iteration, it dosen't
seem to incur any cost (FIXME). However, if the div blocks the entire
pipeline, that sounds to be pretty expensive. Let CodeGen to take care
this transformation.

  This patch sees 6% on a benchmark.

rdar://15032743

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191037 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index cc6a3015408551263e06918320c9cab7b0e325b7..9c310f04dcdd0e1f191fc1a8798362ae9e0b7d85 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -374,9 +374,12 @@ Value *InstCombiner::foldFMulConst(Instruction *FMulOrDiv, ConstantFP *C,
   } else {
     if (C0) {
       // (C0 / X) * C => (C0 * C) / X
-      ConstantFP *F = cast<ConstantFP>(ConstantExpr::getFMul(C0, C));
-      if (isNormalFp(F))
-        R = BinaryOperator::CreateFDiv(F, Opnd1);
+      if (FMulOrDiv->hasOneUse()) {
+        // It would otherwise introduce another div.
+        ConstantFP *F = cast<ConstantFP>(ConstantExpr::getFMul(C0, C));
+        if (isNormalFp(F))
+          R = BinaryOperator::CreateFDiv(F, Opnd1);
+      }
     } else {
       // (X / C1) * C => X * (C/C1) if C/C1 is not a denormal
       ConstantFP *F = cast<ConstantFP>(ConstantExpr::getFDiv(C, C1));

diff --git a/test/Transforms/InstCombine/fast-math.ll b/test/Transforms/InstCombine/fast-math.ll
index a9a7015f62d0bbab93833fcef67f2e59a10c560a..d8ba2a59ff5e44af109b782402aa2fe43718e1f4 100644 (file)
--- a/test/Transforms/InstCombine/fast-math.ll
+++ b/test/Transforms/InstCombine/fast-math.ll
@@ -202,6 +202,18 @@ define float @fmul2(float %f1) {
 ; CHECK: fdiv fast float 1.200000e+07, %f1
 }
 
+; X/C1 * C2 => X * (C2/C1) is disabled if X/C1 has multiple uses
+@fmul2_external = external global float
+define float @fmul2_disable(float %f1) {
+  %div = fdiv fast float 1.000000e+00, %f1 
+  store float %div, float* @fmul2_external
+  %mul = fmul fast float %div, 2.000000e+00
+  ret float %mul
+; CHECK-LABEL: @fmul2_disable
+; CHECK: store
+; CHECK: fmul fast
+}
+
 ; X/C1 * C2 => X * (C2/C1) (if C2/C1 is normal Fp)
 define float @fmul3(float %f1, float %f2) {
   %t1 = fdiv float %f1, 2.0e+3