move MergeBasicBlockIntoOnlyPred to Transforms/Utils.

llvm-svn: 60162

1 files changed, 33 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index ed67c5eaaae..efb902267d9 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -200,3 +200,36 @@ bool llvm::dceInstruction(BasicBlock::iterator &BBI) {
   }
   return false;
 }
+
+//===----------------------------------------------------------------------===//
+//  Control Flow Graph Restructuring...
+//
+
+/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
+/// predecessor is known to have one successor (DestBB!).  Eliminate the edge
+/// between them, moving the instructions in the predecessor into DestBB and
+/// deleting the predecessor block.
+///
+void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) {
+  // If BB has single-entry PHI nodes, fold them.
+  while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
+    Value *NewVal = PN->getIncomingValue(0);
+    // Replace self referencing PHI with undef, it must be dead.
+    if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
+    PN->replaceAllUsesWith(NewVal);
+    PN->eraseFromParent();
+  }
+  
+  BasicBlock *PredBB = DestBB->getSinglePredecessor();
+  assert(PredBB && "Block doesn't have a single predecessor!");
+  
+  // Splice all the instructions from PredBB to DestBB.
+  PredBB->getTerminator()->eraseFromParent();
+  DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
+  
+  // Anything that branched to PredBB now branches to DestBB.
+  PredBB->replaceAllUsesWith(DestBB);
+  
+  // Nuke BB.
+  PredBB->eraseFromParent();
+}