From cce7c24af13376e460a217a2bad01568de8fd6fb Mon Sep 17 00:00:00 2001
From: Anna Thomas <anna@azul.com>
Date: Mon, 13 Aug 2018 19:55:25 +0000
Subject: [PATCH] NFC: Add a test to LV showing that reduction is not possible
 when reduction var is reset in the loop

Added a test case to reduction showing where it's illegal to identify
vectorize a loop.
Resetting the reduction var during loop iterations disallows us from
widening the dependency cycle to VF, thereby making it illegal to
vectorize the loop.

llvm-svn: 339605
---
 .../Transforms/LoopVectorize/reduction.ll     | 39 +++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/llvm/test/Transforms/LoopVectorize/reduction.ll b/llvm/test/Transforms/LoopVectorize/reduction.ll
index f521b623fad2..994ea03e39ae 100644
--- a/llvm/test/Transforms/LoopVectorize/reduction.ll
+++ b/llvm/test/Transforms/LoopVectorize/reduction.ll
@@ -539,3 +539,42 @@ end:
   %final = add i32 %f1, %f2
   ret i32 %final
 }
+
+; This looks like a predicated reduction, but it is a reset of the reduction
+; variable. We cannot vectorize this.
+; CHECK-LABEL: reduction_reset(
+; CHECK-NOT: <4 x i32>
+define void @reduction_reset(i32 %N, i32* nocapture readonly %arrayA, i32* nocapture %arrayB) { 
+entry:
+  %c4 = icmp sgt i32 %N, 0
+  br i1 %c4, label %.lr.ph.preheader, label %._crit_edge
+
+.lr.ph.preheader:                                 ; preds = %entry
+  %c5 = add i32 %N, -1
+  %wide.trip.count = zext i32 %N to i64
+  br label %.lr.ph
+
+.lr.ph:                                           ; preds = %.lr.ph, %.lr.ph.preheader
+  %indvars.iv = phi i64 [ 0, %.lr.ph.preheader ], [ %indvars.iv.next, %.lr.ph ]
+  %.017 = phi i32 [ 100, %.lr.ph.preheader ], [ %csel, %.lr.ph ]
+  %c6 = getelementptr inbounds i32, i32* %arrayA, i64 %indvars.iv
+  %c7 = load i32, i32* %c6, align 4
+  %c8 = icmp sgt i32 %c7, 0
+  %c9 = add nsw i32 %c7, %.017
+  %csel = select i1 %c8, i32 %c9, i32 0
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, %wide.trip.count
+  br i1 %exitcond, label %._crit_edge.loopexit, label %.lr.ph
+
+._crit_edge.loopexit:                             ; preds = %.lr.ph
+  %csel.lcssa = phi i32 [ %csel, %.lr.ph ]
+  %phitmp19 = sext i32 %c5 to i64
+  br label %._crit_edge
+
+._crit_edge:                                      ; preds = %._crit_edge.loopexit, %entry
+  %.015.lcssa = phi i64 [ -1, %entry ], [ %phitmp19, %._crit_edge.loopexit ]
+  %.0.lcssa = phi i32 [ 100, %entry ], [ %csel.lcssa, %._crit_edge.loopexit ]
+  %c10 = getelementptr inbounds i32, i32* %arrayB, i64 %.015.lcssa
+  store i32 %.0.lcssa, i32* %c10, align 4
+  ret void
+}