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

diff --git a/lib/Transforms/Scalar/SCCP.cpp b/lib/Transforms/Scalar/SCCP.cpp
index d5c98fc4955d0ad6efb6d4042e765fb5a9bb4bfe..07e70099769bfeb15bb90b5f6c6f1e89a5760ce4 100644 (file)
--- 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 (file)
index 0000000..18ab5e8
--- /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()