[SimplifyCFG] ConstantFoldTerminator(): handle matching destinations of condbr earlier

We need to handle this case before dealing with the case of constant
branch condition, because if the destinations match, latter fold
would try to remove the DomTree edge that would still be present.

This allows to make that particular DomTree update non-permissive

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index e3bdfba..107929e 100644 (file)
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -134,27 +134,10 @@ bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
   // Branch - See if we are conditional jumping on constant
   if (auto *BI = dyn_cast<BranchInst>(T)) {
     if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
+
     BasicBlock *Dest1 = BI->getSuccessor(0);
     BasicBlock *Dest2 = BI->getSuccessor(1);
 
-    if (auto *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
-      // Are we branching on constant?
-      // YES.  Change to unconditional branch...
-      BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
-      BasicBlock *OldDest     = Cond->getZExtValue() ? Dest2 : Dest1;
-
-      // Let the basic block know that we are letting go of it.  Based on this,
-      // it will adjust it's PHI nodes.
-      OldDest->removePredecessor(BB);
-
-      // Replace the conditional branch with an unconditional one.
-      Builder.CreateBr(Destination);
-      BI->eraseFromParent();
-      if (DTU)
-        DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, OldDest}});
-      return true;
-    }
-
     if (Dest2 == Dest1) {       // Conditional branch to same location?
       // This branch matches something like this:
       //     br bool %cond, label %Dest, label %Dest
@@ -172,6 +155,25 @@ bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
         RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
       return true;
     }
+
+    if (auto *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
+      // Are we branching on constant?
+      // YES.  Change to unconditional branch...
+      BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
+      BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1;
+
+      // Let the basic block know that we are letting go of it.  Based on this,
+      // it will adjust it's PHI nodes.
+      OldDest->removePredecessor(BB);
+
+      // Replace the conditional branch with an unconditional one.
+      Builder.CreateBr(Destination);
+      BI->eraseFromParent();
+      if (DTU)
+        DTU->applyUpdates({{DominatorTree::Delete, BB, OldDest}});
+      return true;
+    }
+
     return false;
   }