[x86] Implement combineRepeatedFPDivisors

Set the transform bar at 2 divisions because the fastest current
x86 FP divider circuit 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.]

Differential Revision: http://reviews.llvm.org/D8941

llvm-svn: 235012

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -12818,6 +12818,16 @@ SDValue X86TargetLowering::getRecipEstimate(SDValue Op,
   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();

llvm/lib/Target/X86/X86ISelLowering.h

@@ -1072,6 +1072,9 @@ namespace llvm {
     /// 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 {

llvm/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" }