From f08c115e6c4b24b975ce376574f7daf6d5a92462 Mon Sep 17 00:00:00 2001
From: Andrew Trick <atrick@apple.com>
Date: Tue, 18 Sep 2012 17:51:33 +0000
Subject: LSR critical edge splitting fix for PR13756.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164147 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Scalar/LoopStrengthReduce.cpp       | 26 +++++++------
 .../LoopStrengthReduce/2011-10-03-CritEdgeMerge.ll | 44 ++++++++++++++++++++--
 2 files changed, 56 insertions(+), 14 deletions(-)

diff --git a/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index bcd27b132f..1d3d156887 100644
--- a/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -4464,17 +4464,21 @@ void LSRInstance::RewriteForPHI(PHINode *PN,
             SplitLandingPadPredecessors(Parent, BB, "", "", P, NewBBs);
             NewBB = NewBBs[0];
           }
-
-          // If PN is outside of the loop and BB is in the loop, we want to
-          // move the block to be immediately before the PHI block, not
-          // immediately after BB.
-          if (L->contains(BB) && !L->contains(PN))
-            NewBB->moveBefore(PN->getParent());
-
-          // Splitting the edge can reduce the number of PHI entries we have.
-          e = PN->getNumIncomingValues();
-          BB = NewBB;
-          i = PN->getBasicBlockIndex(BB);
+          // If NewBB==NULL, then SplitCriticalEdge refused to split because all
+          // phi predecessors are identical. The simple thing to do is skip
+          // splitting in this case rather than complicate the API.
+          if (NewBB) {
+            // If PN is outside of the loop and BB is in the loop, we want to
+            // move the block to be immediately before the PHI block, not
+            // immediately after BB.
+            if (L->contains(BB) && !L->contains(PN))
+              NewBB->moveBefore(PN->getParent());
+
+            // Splitting the edge can reduce the number of PHI entries we have.
+            e = PN->getNumIncomingValues();
+            BB = NewBB;
+            i = PN->getBasicBlockIndex(BB);
+          }
         }
       }
 
diff --git a/test/Transforms/LoopStrengthReduce/2011-10-03-CritEdgeMerge.ll b/test/Transforms/LoopStrengthReduce/2011-10-03-CritEdgeMerge.ll
index a6996a81fb..af3a53708b 100644
--- a/test/Transforms/LoopStrengthReduce/2011-10-03-CritEdgeMerge.ll
+++ b/test/Transforms/LoopStrengthReduce/2011-10-03-CritEdgeMerge.ll
@@ -1,15 +1,15 @@
 ; RUN: opt -loop-reduce -S < %s | FileCheck %s
 ;
 ; Test LSR's use of SplitCriticalEdge during phi rewriting.
-; Verify that identical edges are merged. rdar://problem/6453893
 
 target triple = "x86-apple-darwin"
 
-; CHECK: @test
+; Verify that identical edges are merged. rdar://problem/6453893
+; CHECK: @test1
 ; CHECK: bb89:
 ; CHECK: phi i8* [ %lsr.iv.next1, %bbA.bb89_crit_edge ], [ %lsr.iv.next1, %bbB.bb89_crit_edge ]{{$}}
 
-define i8* @test() {
+define i8* @test1() {
 entry:
   br label %loop
 
@@ -41,3 +41,41 @@ bb89:
 exit:
   ret i8* %tmp75phi
 }
+
+; Handle single-predecessor phis: PR13756
+; CHECK: @test2
+; CHECK: bb89:
+; CHECK: phi i8* [ %lsr.iv.next1, %bbA ], [ %lsr.iv.next1, %bbA ], [ %lsr.iv.next1, %bbA ]{{$}}
+define i8* @test2() {
+entry:
+  br label %loop
+
+loop:
+  %rec = phi i32 [ %next, %loop ], [ 0, %entry ]
+  %next = add i32 %rec, 1
+  %tmp75 = getelementptr i8* null, i32 %next
+  br i1 false, label %loop, label %loopexit
+
+loopexit:
+  br i1 false, label %bbA, label %bbB
+
+bbA:
+  switch i32 0, label %bb89 [
+    i32 47, label %bb89
+    i32 58, label %bb89
+  ]
+
+bbB:
+  switch i8 0, label %exit [
+    i8 47, label %exit
+    i8 58, label %exit
+  ]
+
+bb89:
+  %tmp75phi = phi i8* [ %tmp75, %bbA ], [ %tmp75, %bbA ], [ %tmp75, %bbA ]
+  br label %exit
+
+exit:
+  %result = phi i8* [ %tmp75phi, %bb89 ], [ %tmp75, %bbB ], [ %tmp75, %bbB ], [ %tmp75, %bbB ]
+  ret i8* %result
+}
-- 
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