RewriteLoopBodyWithConditionConstant can end up rewriting the

condition we're unswitching on.  In this case, don't try to
simplify the second copy of the loop which may be dead or not,
but is probably a constant now.  This fixes PR6879

llvm-svn: 101870

1 files changed, 14 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
index 3918738cc8b..de124d10aaa 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -677,15 +677,22 @@ void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
   LoopProcessWorklist.push_back(NewLoop);
   redoLoop = true;
 
+  // Keep a WeakVH holding onto LIC.  If the first call to RewriteLoopBody
+  // deletes the instruction (for example by simplifying a PHI that feeds into
+  // the condition that we're unswitching on), we don't rewrite the second
+  // iteration.
+  WeakVH LICHandle(LIC);
+  
   // Now we rewrite the original code to know that the condition is true and the
   // new code to know that the condition is false.
   RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
-  
-  // It's possible that simplifying one loop could cause the other to be
-  // deleted.  If so, don't simplify it.
-  if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
-    RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
 
+  // It's possible that simplifying one loop could cause the other to be
+  // changed to another value or a constant.  If its a constant, don't simplify
+  // it.
+  if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
+      LICHandle && !isa<Constant>(LICHandle))
+    RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
 }
 
 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
@@ -981,6 +988,8 @@ void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
       continue;
     }
     
+    // FIXME: Change this to use instruction simplify interfaces!
+    
     // Special case hacks that appear commonly in unswitched code.
     switch (I->getOpcode()) {
     case Instruction::Select: