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

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 72fa468db600c212bf26e2b9cdc715156ab7c22d..09f4fb2e1393a461b5bff434420d4bec45cfe71d 100644 (file)
--- 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 (file)
index 0000000..329774e
--- /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
+}