[PATCH] [x86] Implement combineRepeatedFPDivisors

[Sanjay Patel]

Hi chandlerc, RKSimon, andreadb, qcolombet,

This is a trivial patch, but I want to make sure that I'm not being too aggressive for any existing chips.

I've set the transform bar at 2 divisions because the fastest x86 FP divider circuit that I know of is in SandyBridge / Haswell at 10 cycle latency (best case) relative to a 5 cycle multiplier. So that's the worst case for this transform (no latency win), but multiplies are obviously pipelined while divisions are not, so there's still a big throughput win which we would expect to show up in typical FP code.

These are the sequences I'm comparing:
        divss   %xmm2, %xmm0
        mulss   %xmm1, %xmm0
        divss   %xmm2, %xmm0

Becomes:
        movss   LCPI0_0(%rip), %xmm3    ## xmm3 = mem[0],zero,zero,zero
        divss   %xmm2, %xmm3
        mulss   %xmm3, %xmm0
        mulss   %xmm1, %xmm0
        mulss   %xmm3, %xmm0


[Ignore for the moment that we don't optimize the chain of 3 multiplies into 2 independent fmuls followed by 1 dependent fmul...this is the DAG version of: https://llvm.org/bugs/show_bug.cgi?id=21768 ...if we fix that, then the transform becomes even more profitable on all targets.]

http://reviews.llvm.org/D8941

Files:
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/fdiv-combine.ll

Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -12817,6 +12817,16 @@
   return SDValue();
 }
 
+/// If we have at least two divisions that use the same divisor, convert to
+/// multplication by a reciprocal. This may need to be adjusted for a given
+/// CPU if a division's cost is not at least twice the cost of a multiplication.
+/// This is because we still need one division to calculate the reciprocal and
+/// then we need two multiplies by that reciprocal as replacements for the
+/// original divisions.
+bool X86TargetLowering::combineRepeatedFPDivisors(unsigned NumUsers) const {
+  return NumUsers > 1;
+}
+
 static bool isAllOnes(SDValue V) {
   ConstantSDNode *C = dyn_cast<ConstantSDNode>(V);
   return C && C->isAllOnesValue();
Index: lib/Target/X86/X86ISelLowering.h
===================================================================
--- lib/Target/X86/X86ISelLowering.h
+++ lib/Target/X86/X86ISelLowering.h
@@ -1072,6 +1072,9 @@
     /// Use rcp* to speed up fdiv calculations.
     SDValue getRecipEstimate(SDValue Operand, DAGCombinerInfo &DCI,
                              unsigned &RefinementSteps) const override;
+
+    /// Reassociate floating point divisions into multiply by reciprocal.
+    bool combineRepeatedFPDivisors(unsigned NumUsers) const override;
   };
 
   namespace X86 {
Index: test/CodeGen/X86/fdiv-combine.ll
===================================================================
--- test/CodeGen/X86/fdiv-combine.ll
+++ test/CodeGen/X86/fdiv-combine.ll
@@ -0,0 +1,31 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
+
+; Anything more than one division using a single divisor operand
+; should be converted into a reciprocal and multiplication.
+
+define float @div1_arcp(float %x, float %y, float %z) #0 {
+; CHECK-LABEL: div1_arcp:
+; CHECK:       # BB#0:
+; CHECK-NEXT:    divss %xmm1, %xmm0
+; CHECK-NEXT:    retq
+  %div1 = fdiv arcp float %x, %y
+  ret float %div1
+}
+
+define float @div2_arcp(float %x, float %y, float %z) #0 {
+; CHECK-LABEL: div2_arcp:
+; CHECK:       # BB#0:
+; CHECK-NEXT:    movss {{.*#+}} xmm3 = mem[0],zero,zero,zero
+; CHECK-NEXT:    divss %xmm2, %xmm3
+; CHECK-NEXT:    mulss %xmm3, %xmm0
+; CHECK-NEXT:    mulss %xmm1, %xmm0
+; CHECK-NEXT:    mulss %xmm3, %xmm0
+; CHECK-NEXT:    retq
+  %div1 = fdiv arcp float %x, %z
+  %mul = fmul arcp float %div1, %y
+  %div2 = fdiv arcp float %mul, %z
+  ret float %div2
+}
+
+; FIXME: If the backend understands 'arcp', then this attribute is unnecessary.
+attributes #0 = { "unsafe-fp-math"="true" }

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D8941.23542.patch
Type: text/x-patch
Size: 2680 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150409/94c19b47/attachment.bin>