[PATCH] Make SLPVectorizer cost model take credit for instructions that will be erased

[Arch D. Robison]

  Thanks for the pointers.  I've updated the test to use a check prefix.

Hi #llvm,

http://llvm-reviews.chandlerc.com/D3160

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D3160?vs=8058&id=8062#toc

Files:
  test/Transforms/SLPVectorizer/X86/insert-element-build-vector.ll
  lib/Transforms/Vectorize/SLPVectorizer.cpp

Index: test/Transforms/SLPVectorizer/X86/insert-element-build-vector.ll
===================================================================
--- test/Transforms/SLPVectorizer/X86/insert-element-build-vector.ll
+++ test/Transforms/SLPVectorizer/X86/insert-element-build-vector.ll
@@ -195,3 +195,29 @@
 }
 
 attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"="true" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+; RUN: opt -S -slp-vectorizer -slp-threshold=0 < %s | FileCheck %s -check-prefix=ZEROTHRESH
+
+; Check that cost model for vectorization takes credit for 
+; instructions that are erased.
+define <4 x float> @take_credit(<4 x float> %a, <4 x float> %b) {
+; ZEROTHRESH-LABEL: @take_credit(
+; ZEROTHRESH-CHECK: %1 = fadd <4 x float> %a, %b
+  %a0 = extractelement <4 x float> %a, i32 0
+  %b0 = extractelement <4 x float> %b, i32 0
+  %c0 = fadd float %a0, %b0
+  %a1 = extractelement <4 x float> %a, i32 1
+  %b1 = extractelement <4 x float> %b, i32 1
+  %c1 = fadd float %a1, %b1
+  %a2 = extractelement <4 x float> %a, i32 2
+  %b2 = extractelement <4 x float> %b, i32 2
+  %c2 = fadd float %a2, %b2
+  %a3 = extractelement <4 x float> %a, i32 3
+  %b3 = extractelement <4 x float> %b, i32 3
+  %c3 = fadd float %a3, %b3
+  %v0 = insertelement <4 x float> undef, float %c0, i32 0
+  %v1 = insertelement <4 x float> %v0, float %c1, i32 1
+  %v2 = insertelement <4 x float> %v1, float %c2, i32 2
+  %v3 = insertelement <4 x float> %v2, float %c3, i32 3
+  ret <4 x float> %v3
+}
Index: lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -1012,8 +1012,16 @@
       return 0;
     }
     case Instruction::ExtractElement: {
-      if (CanReuseExtract(VL))
-        return 0;
+      if (CanReuseExtract(VL)) {
+        int DeadCost = 0;
+        for (unsigned i = 0, e = VL.size(); i < e; ++i) {
+          ExtractElementInst *E = cast<ExtractElementInst>(VL[i]);
+          if (E->hasOneUse())
+            // Take credit for instruction that will become dead.
+            DeadCost += TTI->getVectorInstrCost(Instruction::ExtractElement, VecTy, i);
+        }
+        return -DeadCost;
+      }
       return getGatherCost(VecTy);
     }
     case Instruction::ZExt:
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