From 2adc5b6a17268834b08fda444b1a84550e8c5ae8 Mon Sep 17 00:00:00 2001
From: Eli Friedman <eli.friedman@gmail.com>
Date: Wed, 9 Mar 2011 00:48:33 +0000
Subject: [PATCH] PR9420; an instruction before an unreachable is guaranteed
 not to have any reachable uses, but there still might be uses in dead blocks.
  Use the standard solution of replacing all the uses with undef.  This is a
 rare case because it's very sensitive to phase ordering in SimplifyCFG.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127299 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Utils/SimplifyCFG.cpp          |  4 ++-
 .../SimplifyCFG/2011-03-08-UnreachableUse.ll  | 31 +++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 test/Transforms/SimplifyCFG/2011-03-08-UnreachableUse.ll

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 72fa468db60..09f4fb2e139 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2168,7 +2168,9 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
       if (LI->isVolatile())
         break;
     
-    // Delete this instruction
+    // Delete this instruction (any uses are guaranteed to be dead)
+    if (!BBI->use_empty())
+      BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
     BBI->eraseFromParent();
     Changed = true;
   }
diff --git a/test/Transforms/SimplifyCFG/2011-03-08-UnreachableUse.ll b/test/Transforms/SimplifyCFG/2011-03-08-UnreachableUse.ll
new file mode 100644
index 00000000000..329774e2242
--- /dev/null
+++ b/test/Transforms/SimplifyCFG/2011-03-08-UnreachableUse.ll
@@ -0,0 +1,31 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+; PR9420
+
+; Note that the crash in PR9420 test is sensitive to the ordering of
+; the transformations done by SimplifyCFG, so this test is likely to rot
+; quickly.
+
+define noalias i8* @func_29() nounwind {
+; CHECK: entry:
+; CHECK-NEXT: unreachable
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc38, %entry
+  %p_34.addr.0 = phi i16 [ 1, %entry ], [ %conv40, %for.inc38 ]
+  br label %for.cond1
+
+for.cond1:                                        ; preds = %for.inc29, %for.cond
+  %p_32.addr.0 = phi i1 [ true, %for.cond ], [ true, %for.inc29 ]
+  br i1 %p_32.addr.0, label %for.body8, label %for.inc38
+
+for.body8:                                        ; preds = %for.cond1
+  unreachable
+
+for.inc29:                                        ; preds = %for.cond17
+  br label %for.cond1
+
+for.inc38:                                        ; preds = %for.end32
+  %conv40 = add i16 %p_34.addr.0, 1
+  br label %for.cond
+}