Costmodel: Add support for horizontal vector reductions

Upcoming SLP vectorization improvements will want to be able to estimate costs
of horizontal reductions. Add infrastructure to support this.

We model reductions as a series of (shufflevector,add) tuples ultimately
followed by an extractelement. For example, for an add-reduction of <4 x float>
we could generate the following sequence:

 (v0, v1, v2, v3)
   \   \  /  /
     \  \  /
       +  +

 (v0+v2, v1+v3, undef, undef)
    \      /
 ((v0+v2) + (v1+v3), undef, undef)

 %rdx.shuf = shufflevector <4 x float> %rdx, <4 x float> undef,
                           <4 x i32> <i32 2, i32 3, i32 undef, i32 undef>
 %bin.rdx = fadd <4 x float> %rdx, %rdx.shuf
 %rdx.shuf7 = shufflevector <4 x float> %bin.rdx, <4 x float> undef,
                          <4 x i32> <i32 1, i32 undef, i32 undef, i32 undef>
 %bin.rdx8 = fadd <4 x float> %bin.rdx, %rdx.shuf7
 %r = extractelement <4 x float> %bin.rdx8, i32 0

This commit adds a cost model interface "getReductionCost(Opcode, Ty, Pairwise)"
that will allow clients to ask for the cost of such a reduction (as backends
might generate more efficient code than the cost of the individual instructions
summed up). This interface is excercised by the CostModel analysis pass which
looks for reduction patterns like the one above - starting at extractelements -
and if it sees a matching sequence will call the cost model interface.

We will also support a second form of pairwise reduction that is well supported
on common architectures (haddps, vpadd, faddp).

 (v0, v1, v2, v3)
  \   /    \  /
 (v0+v1, v2+v3, undef, undef)
    \     /
 ((v0+v1)+(v2+v3), undef, undef, undef)

  %rdx.shuf.0.0 = shufflevector <4 x float> %rdx, <4 x float> undef,
        <4 x i32> <i32 0, i32 2 , i32 undef, i32 undef>
  %rdx.shuf.0.1 = shufflevector <4 x float> %rdx, <4 x float> undef,
        <4 x i32> <i32 1, i32 3, i32 undef, i32 undef>
  %bin.rdx.0 = fadd <4 x float> %rdx.shuf.0.0, %rdx.shuf.0.1
  %rdx.shuf.1.0 = shufflevector <4 x float> %bin.rdx.0, <4 x float> undef,
        <4 x i32> <i32 0, i32 undef, i32 undef, i32 undef>
  %rdx.shuf.1.1 = shufflevector <4 x float> %bin.rdx.0, <4 x float> undef,
        <4 x i32> <i32 1, i32 undef, i32 undef, i32 undef>
  %bin.rdx.1 = fadd <4 x float> %rdx.shuf.1.0, %rdx.shuf.1.1
  %r = extractelement <4 x float> %bin.rdx.1, i32 0

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190876 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/CodeGen/BasicTargetTransformInfo.cpp b/lib/CodeGen/BasicTargetTransformInfo.cpp
index 9c4b49aa7c..24aa1abffa 100644
--- a/lib/CodeGen/BasicTargetTransformInfo.cpp
+++ b/lib/CodeGen/BasicTargetTransformInfo.cpp
@@ -113,6 +113,7 @@ public:
                                          ArrayRef<Type*> Tys) const;
   virtual unsigned getNumberOfParts(Type *Tp) const;
   virtual unsigned getAddressComputationCost(Type *Ty, bool IsComplex) const;
+  virtual unsigned getReductionCost(unsigned Opcode, Type *Ty, bool IsPairwise) const;
 
   /// @}
 };
@@ -510,3 +511,17 @@ unsigned BasicTTI::getNumberOfParts(Type *Tp) const {
 unsigned BasicTTI::getAddressComputationCost(Type *Ty, bool IsComplex) const {
   return 0;
 }
+
+unsigned BasicTTI::getReductionCost(unsigned Opcode, Type *Ty,
+                                    bool IsPairwise) const {
+  assert(Ty->isVectorTy() && "Expect a vector type");
+  unsigned NumVecElts = Ty->getVectorNumElements();
+  unsigned NumReduxLevels = Log2_32(NumVecElts);
+  unsigned ArithCost = NumReduxLevels *
+    TopTTI->getArithmeticInstrCost(Opcode, Ty);
+  // Assume the pairwise shuffles add a cost.
+  unsigned ShuffleCost =
+      NumReduxLevels * (IsPairwise + 1) *
+      TopTTI->getShuffleCost(SK_ExtractSubvector, Ty, NumVecElts / 2, Ty);
+  return ShuffleCost + ArithCost + getScalarizationOverhead(Ty, false, true);
+}