vendor/llvm/llvm-trunk-r301441

1 files changed, 30 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp
index 08eb95a1a3d3e..a0da81605a809 100644
--- a/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1289,6 +1289,36 @@ bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
   if (PredToDestList.empty())
     return false;
 
+  // If all the predecessors go to a single known successor, we want to fold,
+  // not thread. By doing so, we do not need to duplicate the current block and
+  // also miss potential opportunities in case we dont/cant duplicate.
+  if (OnlyDest && OnlyDest != MultipleDestSentinel) {
+    if (PredToDestList.size() ==
+        (size_t)std::distance(pred_begin(BB), pred_end(BB))) {
+      bool SeenFirstBranchToOnlyDest = false;
+      for (BasicBlock *SuccBB : successors(BB)) {
+        if (SuccBB == OnlyDest && !SeenFirstBranchToOnlyDest)
+          SeenFirstBranchToOnlyDest = true; // Don't modify the first branch.
+        else
+          SuccBB->removePredecessor(BB, true); // This is unreachable successor.
+      }
+
+      // Finally update the terminator.
+      TerminatorInst *Term = BB->getTerminator();
+      BranchInst::Create(OnlyDest, Term);
+      Term->eraseFromParent();
+
+      // If the condition is now dead due to the removal of the old terminator,
+      // erase it.
+      auto *CondInst = dyn_cast<Instruction>(Cond);
+      if (CondInst && CondInst->use_empty())
+        CondInst->eraseFromParent();
+      // FIXME: in case this instruction is defined in the current BB and it
+      // resolves to a single value from all predecessors, we can do RAUW.
+      return true;
+    }
+  }
+
   // Determine which is the most common successor.  If we have many inputs and
   // this block is a switch, we want to start by threading the batch that goes
   // to the most popular destination first.  If we only know about one