[LV] Fix maximum legal VF calculation

This patch fixes pr34283, which exposed that the computation of
maximum legal width for vectorization was wrong, because it relied
on MaxInterleaveFactor to obtain the maximum stride used in the loop,
however not all strided accesses in the loop have an interleave-group
associated with them.
Instead of recording the maximum stride in the loop, which can be over
conservative (e.g. if the access with the maximum stride is not involved
in the dependence limitation), this patch tracks the actual maximum legal
width imposed by accesses that are involved in dependencies.

Differential Revision: https://reviews.llvm.org/D37507

llvm-svn: 313237

1 files changed, 51 insertions, 0 deletions

diff --git a/llvm/test/Transforms/LoopVectorize/memdep.ll b/llvm/test/Transforms/LoopVectorize/memdep.ll
index fb608836dfe..bea9cc39ab4 100644
--- a/llvm/test/Transforms/LoopVectorize/memdep.ll
+++ b/llvm/test/Transforms/LoopVectorize/memdep.ll
@@ -1,5 +1,7 @@
 ; RUN: opt < %s -loop-vectorize -force-vector-width=2 -force-vector-interleave=1 -S | FileCheck %s
 ; RUN: opt < %s -loop-vectorize -force-vector-width=4 -force-vector-interleave=1 -S | FileCheck %s -check-prefix=WIDTH
+; RUN: opt -S -loop-vectorize -force-vector-width=4 < %s | FileCheck %s -check-prefix=RIGHTVF
+; RUN: opt -S -loop-vectorize -force-vector-width=8 < %s | FileCheck %s -check-prefix=WRONGVF
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
@@ -220,3 +222,52 @@ for.body:
 for.end:
   ret void
 }
+
+
+;Check the new calculation of the maximum safe distance in bits which can be vectorized.
+;The previous behavior did not take account that the stride was 2.
+;Therefore the maxVF was computed as 8 instead of 4, as the dependence distance here is 6 iterations, given by |N-(N-12)|/2.  
+
+;#define M 32
+;#define N 2 * M
+;unsigned int a [N];
+;void pr34283(){
+;	unsigned int j=0;
+;   for (j = 0; j < M - 6; ++j)
+;    {
+;        a[N - 2 * j] = 69;
+;        a[N - 12 - 2 * j] = 7;
+;    }
+;
+;}
+
+; RIGHTVF-LABEL: @pr34283
+; RIGHTVF: <4 x i64>
+
+; WRONGVF-LABLE: @pr34283
+; WRONGVF-NOT: <8 x i64>
+
+@a = common local_unnamed_addr global [64 x i32] zeroinitializer, align 16
+
+; Function Attrs: norecurse nounwind uwtable
+define void @pr34283() local_unnamed_addr {
+entry:
+  br label %for.body
+
+for.body:
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %0 = shl i64 %indvars.iv, 1
+  %1 = sub nuw nsw i64 64, %0
+  %arrayidx = getelementptr inbounds [64 x i32], [64 x i32]* @a, i64 0, i64 %1
+  store i32 69, i32* %arrayidx, align 8
+  %2 = sub nuw nsw i64 52, %0
+  %arrayidx4 = getelementptr inbounds [64 x i32], [64 x i32]* @a, i64 0, i64 %2
+  store i32 7, i32* %arrayidx4, align 8
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 26
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret void
+}
+