Re-apply r124518 with fix. Watch out for invalidated iterator.

llvm-svn: 124526

2 files changed, 35 insertions, 21 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 5d3af3759a0..acaea195e71 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -509,7 +509,32 @@ void llvm::FindFunctionBackedges(const Function &F,
       // Go up one level.
       InStack.erase(VisitStack.pop_back_val().first);
     }
-  } while (!VisitStack.empty());
-  
-  
+  } while (!VisitStack.empty()); 
+}
+
+/// FoldReturnIntoUncondBranch - This method duplicates the specified return
+/// instruction into a predecessor which ends in an unconditional branch. If
+/// the return instruction returns a value defined by a PHI, propagate the
+/// right value into the return. It returns the new return instruction in the
+/// predecessor.
+ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
+                                             BasicBlock *Pred) {
+  Instruction *UncondBranch = Pred->getTerminator();
+  // Clone the return and add it to the end of the predecessor.
+  Instruction *NewRet = RI->clone();
+  Pred->getInstList().push_back(NewRet);
+      
+  // If the return instruction returns a value, and if the value was a
+  // PHI node in "BB", propagate the right value into the return.
+  for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
+       i != e; ++i)
+    if (PHINode *PN = dyn_cast<PHINode>(*i))
+      if (PN->getParent() == BB)
+        *i = PN->getIncomingValueForBlock(Pred);
+      
+  // Update any PHI nodes in the returning block to realize that we no
+  // longer branch to them.
+  BB->removePredecessor(Pred);
+  UncondBranch->eraseFromParent();
+  return cast<ReturnInst>(NewRet);
 }
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index f6d7d76dbf6..b9432c2e6e1 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -28,6 +28,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/CFG.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ConstantRange.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -36,6 +37,10 @@
 #include <map>
 using namespace llvm;
 
+static cl::opt<bool>
+DupRet("simplifycfg-dup-ret", cl::Hidden, cl::init(false),
+       cl::desc("Duplicate return instructions into unconditional branches"));
+
 STATISTIC(NumSpeculations, "Number of speculative executed instructions");
 
 namespace {
@@ -2027,28 +2032,12 @@ bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI) {
   }
   
   // If we found some, do the transformation!
-  if (!UncondBranchPreds.empty()) {
+  if (!UncondBranchPreds.empty() && DupRet) {
     while (!UncondBranchPreds.empty()) {
       BasicBlock *Pred = UncondBranchPreds.pop_back_val();
       DEBUG(dbgs() << "FOLDING: " << *BB
             << "INTO UNCOND BRANCH PRED: " << *Pred);
-      Instruction *UncondBranch = Pred->getTerminator();
-      // Clone the return and add it to the end of the predecessor.
-      Instruction *NewRet = RI->clone();
-      Pred->getInstList().push_back(NewRet);
-      
-      // If the return instruction returns a value, and if the value was a
-      // PHI node in "BB", propagate the right value into the return.
-      for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
-           i != e; ++i)
-        if (PHINode *PN = dyn_cast<PHINode>(*i))
-          if (PN->getParent() == BB)
-            *i = PN->getIncomingValueForBlock(Pred);
-      
-      // Update any PHI nodes in the returning block to realize that we no
-      // longer branch to them.
-      BB->removePredecessor(Pred);
-      UncondBranch->eraseFromParent();
+      (void)FoldReturnIntoUncondBranch(RI, BB, Pred);
     }
     
     // If we eliminated all predecessors of the block, delete the block now.