[PATCH] D20601: [x86] make pointer extractions from a vector more expensive (PR27826)

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: hfinkel, delena, nadav.
spatel added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

Among possible other problems, we have a vector of induction variable pointers and no hardware gather support in PR27826:
https://llvm.org/bugs/show_bug.cgi?id=27826

We should adjust the cost model for pointer extracts from a vector because those values require a move to a GPR in order to be used as part of a load/store address. 

Alternatively, we could add a penalty to getAddressComputationCost()? There is already a check for a vector operand there in the x86 override, but the penalty is only applied if the computation is specified with "IsComplex" by the caller. 

When I tried adjusting that function, I got several regression test failures, so this patch is the path of least resistance. The PPC cost model does something similar to its getVectorInstrCost() to simulate the LHS penalty when moving values from FP/Int <--> VMX.

http://reviews.llvm.org/D20601

Files:
  lib/Target/X86/X86TargetTransformInfo.cpp
  test/Transforms/LoopVectorize/X86/cost-model.ll

Index: test/Transforms/LoopVectorize/X86/cost-model.ll
===================================================================
--- test/Transforms/LoopVectorize/X86/cost-model.ll
+++ test/Transforms/LoopVectorize/X86/cost-model.ll
@@ -39,3 +39,44 @@
 for.end:                                          ; preds = %for.body
   ret void
 }
+
+; This function uses a stride that is generally too big to benefit from vectorization without
+; really good support for a gather load. We were not computing an accurate cost for the 
+; vectorization and subsequent scalarization of the pointer induction variables.
+
+define float @PR27826(float* nocapture readonly %a, float* nocapture readonly %b, i32 %n) {
+; CHECK-LABEL: @PR27826(
+; CHECK-NOT:   <4 x float> 
+; CHECK-NOT:   <8 x float> 
+; CHECK:       ret float %s.0.lcssa
+
+entry:
+  %cmp = icmp sgt i32 %n, 0
+  br i1 %cmp, label %preheader, label %for.end
+
+preheader:
+  %t0 = sext i32 %n to i64
+  br label %for
+
+for:
+  %indvars.iv = phi i64 [ 0, %preheader ], [ %indvars.iv.next, %for ]
+  %s.02 = phi float [ 0.0, %preheader ], [ %add4, %for ]
+  %arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
+  %t1 = load float, float* %arrayidx, align 4
+  %arrayidx3 = getelementptr inbounds float, float* %b, i64 %indvars.iv
+  %t2 = load float, float* %arrayidx3, align 4
+  %add = fadd fast float %t1, %s.02
+  %add4 = fadd fast float %add, %t2
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 8
+  %cmp1 = icmp slt i64 %indvars.iv.next, %t0
+  br i1 %cmp1, label %for, label %loopexit
+
+loopexit:
+  %add4.lcssa = phi float [ %add4, %for ]
+  br label %for.end
+
+for.end:
+  %s.0.lcssa = phi float [ 0.0, %entry ], [ %add4.lcssa, %loopexit ]
+  ret float %s.0.lcssa
+}
+
Index: lib/Target/X86/X86TargetTransformInfo.cpp
===================================================================
--- lib/Target/X86/X86TargetTransformInfo.cpp
+++ lib/Target/X86/X86TargetTransformInfo.cpp
@@ -963,6 +963,8 @@
 int X86TTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val, unsigned Index) {
   assert(Val->isVectorTy() && "This must be a vector type");
 
+  Type *ScalarType = Val->getScalarType();
+
   if (Index != -1U) {
     // Legalize the type.
     std::pair<int, MVT> LT = TLI->getTypeLegalizationCost(DL, Val);
@@ -976,11 +978,17 @@
     Index = Index % Width;
 
     // Floating point scalars are already located in index #0.
-    if (Val->getScalarType()->isFloatingPointTy() && Index == 0)
+    if (ScalarType->isFloatingPointTy() && Index == 0)
       return 0;
   }
 
-  return BaseT::getVectorInstrCost(Opcode, Val, Index);
+  // Add to the base cost if we know that the extracted element of a vector is
+  // destined to be moved to and used in the integer register file.
+  int RegisterFileMoveCost = 0;
+  if (Opcode == Instruction::ExtractElement && ScalarType->isPointerTy())
+    RegisterFileMoveCost = 1;
+
+  return BaseT::getVectorInstrCost(Opcode, Val, Index) + RegisterFileMoveCost;
 }
 
 int X86TTIImpl::getScalarizationOverhead(Type *Ty, bool Insert, bool Extract) {


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