Don't infininitely thread branches when a threaded edge

goes back to the block, e.g.:

  Threading edge through bool from 'bb37.us.thread3829' to 'bb37.us' with cost: 1, across block:

bb37.us:		; preds = %bb37.us.thread3829, %bb37.us, %bb33
	%D1361.1.us = phi i32 [ %tmp36, %bb33 ], [ %D1361.1.us, %bb37.us ], [ 0, %bb37.us.thread3829 ]		; <i32> [#uses=2]
	%tmp39.us = icmp eq i32 %D1361.1.us, 0		; <i1> [#uses=1]
	br i1 %tmp39.us, label %bb37.us, label %bb42.us

llvm-svn: 50251

1 files changed, 22 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 92e40e66d43..be940254049 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -250,6 +250,13 @@ bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
     SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
   }
   
+  // If threading to the same block as we come from, we would infinite loop.
+  if (SuccBB == BB) {
+    DOUT << "  Not threading BB '" << BB->getNameStart()
+         << "' - would thread to self!\n";
+    return false;
+  }
+  
   // And finally, do it!
   DOUT << "  Threading edge from '" << PredBB->getNameStart() << "' to '"
        << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
@@ -319,6 +326,13 @@ bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
   // 'true' block.
   BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd);
   
+  // If threading to the same block as we come from, we would infinite loop.
+  if (SuccBB == BB) {
+    DOUT << "  Not threading BB '" << BB->getNameStart()
+    << "' - would thread to self!\n";
+    return false;
+  }
+  
   // And finally, do it!
   DOUT << "  Threading edge through bool from '" << PredBB->getNameStart()
        << "' to '" << SuccBB->getNameStart() << "' with cost: "
@@ -390,6 +404,14 @@ bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
   // Next, get our successor.
   BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
   
+  // If threading to the same block as we come from, we would infinite loop.
+  if (SuccBB == BB) {
+    DOUT << "  Not threading BB '" << BB->getNameStart()
+    << "' - would thread to self!\n";
+    return false;
+  }
+  
+  
   // And finally, do it!
   DOUT << "  Threading edge through bool from '" << PredBB->getNameStart()
        << "' to '" << SuccBB->getNameStart() << "' with cost: "