[LV] Avoid vectorizing first order recurrence when phi uses are outside loop

In the vectorization of first order recurrence, we vectorize such
that the last element in the vector will be the one extracted to pass into the
scalar remainder loop. However, this is not true when there is a phi (other
than the primary induction variable) is used outside the loop.
In such a case, we need the value from the second last iteration (i.e.
the phi value), not the last iteration (which would be the phi update).
I've added a test case for this. Also see PR32396.

A follow up patch would generate the correct code gen for such cases,
and turn this vectorization on.

Differential Revision: https://reviews.llvm.org/D31910

Reviewers: mssimpso
llvm-svn: 299985

2 files changed, 28 insertions, 3 deletions

diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll b/llvm/test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll
index c7ced757581..e090ddf1d1a 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/loop-vectorization-factors.ll
@@ -235,10 +235,13 @@ for.body:                                         ; preds = %entry, %for.body
 }
 
 ; CHECK-LABEL: @add_phifail2(
-; CHECK: load <16 x i8>, <16 x i8>*
-; CHECK: add nuw nsw <16 x i32>
-; CHECK: store <16 x i8>
+; CHECK-NOT: load <16 x i8>, <16 x i8>*
+; CHECK-NOT: add nuw nsw <16 x i32>
+; CHECK-NOT: store <16 x i8>
 ; Function Attrs: nounwind
+; FIXME: Currently, if we vectorize this loop, we will generate incorrect code
+; if %len evenly divides VF. Vectorized loop code gen returns a_phi = p[len -1],
+; whereas it should be the previous value a_phi = p[len -2]
 define i8 @add_phifail2(i8* noalias nocapture readonly %p, i8* noalias nocapture %q, i32 %len) #0 {
 entry:
   br label %for.body
diff --git a/llvm/test/Transforms/LoopVectorize/first-order-recurrence.ll b/llvm/test/Transforms/LoopVectorize/first-order-recurrence.ll
index 7803e74679f..9155820216b 100644
--- a/llvm/test/Transforms/LoopVectorize/first-order-recurrence.ll
+++ b/llvm/test/Transforms/LoopVectorize/first-order-recurrence.ll
@@ -349,3 +349,25 @@ scalar.body:
 for.end:
   ret void
 }
+
+; FIXME: we can vectorize this first order recurrence, by generating two
+; extracts - one for the phi `val.phi` and other for the phi update `addx`.
+; val.phi at end of loop is 94 + x.
+; CHECK-LABEL: extract_second_last_iteration
+; CHECK-NOT: vector.body
+define i32 @extract_second_last_iteration(i32* %cval, i32 %x)  {
+entry:
+  br label %for.body
+
+for.body:
+  %inc.phi = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %val.phi = phi i32 [ 0, %entry ], [ %addx, %for.body ]
+  %inc = add i32 %inc.phi, 1
+  %bc = zext i32 %inc.phi to i64
+  %addx = add i32 %inc.phi, %x
+  %cmp = icmp eq i32 %inc.phi, 95
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret i32 %val.phi
+}