[SLP vectorizer]: Choose the best consecutive candidate to pair with a store instruction.

The patch changes the SLPVectorizer::vectorizeStores to choose the immediate
succeeding or preceding candidate for a store instruction when it has multiple
consecutive candidates. In this way it has better chance to find more slp
vectorization opportunities.

Differential Revision: http://reviews.llvm.org/D10445


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243666 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SLPVectorizer/X86/pr23510.ll

@@ -0,0 +1,38 @@
+; PR23510
+; RUN: opt < %s -basicaa -slp-vectorizer -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @_Z3fooPml(
+; CHECK: lshr <2 x i64>
+; CHECK: lshr <2 x i64>
+
+@total = global i64 0, align 8
+
+define void @_Z3fooPml(i64* nocapture %a, i64 %i) {
+entry:
+  %tmp = load i64, i64* %a, align 8
+  %shr = lshr i64 %tmp, 4
+  store i64 %shr, i64* %a, align 8
+  %arrayidx1 = getelementptr inbounds i64, i64* %a, i64 1
+  %tmp1 = load i64, i64* %arrayidx1, align 8
+  %shr2 = lshr i64 %tmp1, 4
+  store i64 %shr2, i64* %arrayidx1, align 8
+  %arrayidx3 = getelementptr inbounds i64, i64* %a, i64 %i
+  %tmp2 = load i64, i64* %arrayidx3, align 8
+  %tmp3 = load i64, i64* @total, align 8
+  %add = add i64 %tmp3, %tmp2
+  store i64 %add, i64* @total, align 8
+  %tmp4 = load i64, i64* %a, align 8
+  %shr5 = lshr i64 %tmp4, 4
+  store i64 %shr5, i64* %a, align 8
+  %tmp5 = load i64, i64* %arrayidx1, align 8
+  %shr7 = lshr i64 %tmp5, 4
+  store i64 %shr7, i64* %arrayidx1, align 8
+  %tmp6 = load i64, i64* %arrayidx3, align 8
+  %tmp7 = load i64, i64* @total, align 8
+  %add9 = add i64 %tmp7, %tmp6
+  store i64 %add9, i64* @total, align 8
+  ret void
+}