From 05bb789430bab7d8fae1e94fb9aa0bb21e679ebf Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Mon, 28 Jan 2008 00:32:30 +0000
Subject: [PATCH] Fix PR1938 by forcing the code that uses an undefined value
 to branch one way or the other.  Rewriting the code itself prevents
 subsequent analysis passes from making contradictory conclusions about the
 code that could cause an infeasible path to be made feasible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46427 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Scalar/SCCP.cpp                | 23 +++++++++---
 .../SCCP/2008-01-27-UndefCorrelate.ll         | 36 +++++++++++++++++++
 2 files changed, 55 insertions(+), 4 deletions(-)
 create mode 100644 test/Transforms/SCCP/2008-01-27-UndefCorrelate.ll

diff --git a/lib/Transforms/Scalar/SCCP.cpp b/lib/Transforms/Scalar/SCCP.cpp
index d5c98fc4955..07e70099769 100644
--- a/lib/Transforms/Scalar/SCCP.cpp
+++ b/lib/Transforms/Scalar/SCCP.cpp
@@ -1313,15 +1313,30 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
       continue;
     }
     
-    // If the edge to the first successor isn't thought to be feasible yet, mark
-    // it so now.
-    if (KnownFeasibleEdges.count(Edge(BB, TI->getSuccessor(0))))
+    // If the edge to the second successor isn't thought to be feasible yet,
+    // mark it so now.  We pick the second one so that this goes to some
+    // enumerated value in a switch instead of going to the default destination.
+    if (KnownFeasibleEdges.count(Edge(BB, TI->getSuccessor(1))))
       continue;
     
     // Otherwise, it isn't already thought to be feasible.  Mark it as such now
     // and return.  This will make other blocks reachable, which will allow new
     // values to be discovered and existing ones to be moved in the lattice.
-    markEdgeExecutable(BB, TI->getSuccessor(0));
+    markEdgeExecutable(BB, TI->getSuccessor(1));
+    
+    // This must be a conditional branch of switch on undef.  At this point,
+    // force the old terminator to branch to the first successor.  This is
+    // required because we are now influencing the dataflow of the function with
+    // the assumption that this edge is taken.  If we leave the branch condition
+    // as undef, then further analysis could think the undef went another way
+    // leading to an inconsistent set of conclusions.
+    if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
+      BI->setCondition(ConstantInt::getFalse());
+    } else {
+      SwitchInst *SI = cast<SwitchInst>(TI);
+      SI->setCondition(SI->getCaseValue(1));
+    }
+    
     return true;
   }
 
diff --git a/test/Transforms/SCCP/2008-01-27-UndefCorrelate.ll b/test/Transforms/SCCP/2008-01-27-UndefCorrelate.ll
new file mode 100644
index 00000000000..18ab5e8cdc5
--- /dev/null
+++ b/test/Transforms/SCCP/2008-01-27-UndefCorrelate.ll
@@ -0,0 +1,36 @@
+; RUN: llvm-as < %s | opt -sccp | llvm-dis | grep undef | count 1
+; PR1938
+
+define i32 @main() {
+entry:
+	br label %bb
+
+bb:
+	%indvar = phi i32 [ 0, %entry ], [ %k, %bb.backedge ]
+	%k = add i32 %indvar, 1
+	br i1 undef, label %cond_true, label %cond_false
+
+cond_true:
+	%tmp97 = icmp slt i32 %k, 10
+	br i1 %tmp97, label %bb.backedge, label %bb12
+
+bb.backedge:
+	br label %bb
+
+cond_false:
+	%tmp9 = icmp slt i32 %k, 10
+	br i1 %tmp9, label %bb.backedge, label %bb12
+
+bb12:
+	%tmp14 = icmp eq i32 %k, 10
+	br i1 %tmp14, label %cond_next18, label %cond_true17
+
+cond_true17:
+	tail call void @abort( )
+	unreachable
+
+cond_next18:
+	ret i32 0
+}
+
+declare void @abort()
-- 
2.34.1