Fix SplitCriticalEdge to properly update LCSSA form when splitting a

loop exit edge -- new PHIs may be needed not only for the additional
splits that are made to preserve LoopSimplify form, but also for the
original split. Factor out the code that inserts new PHIs so that it
can be used for both. Remove LoopRotation.cpp's code for manually
updating LCSSA form, as it is now redundant. This fixes PR4934.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81363 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 51 insertions, 24 deletions

diff --git a/lib/Transforms/Utils/BreakCriticalEdges.cpp b/lib/Transforms/Utils/BreakCriticalEdges.cpp
index 5c0afbeece..849b2b5d5c 100644
--- a/lib/Transforms/Utils/BreakCriticalEdges.cpp
+++ b/lib/Transforms/Utils/BreakCriticalEdges.cpp
@@ -117,6 +117,38 @@ bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
   return false;
 }
 
+/// CreatePHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
+/// may require new PHIs in the new exit block. This function inserts the
+/// new PHIs, as needed.  Preds is a list of preds inside the loop, SplitBB
+/// is the new loop exit block, and DestBB is the old loop exit, now the
+/// successor of SplitBB.
+static void CreatePHIsForSplitLoopExit(SmallVectorImpl<BasicBlock *> &Preds,
+                                       BasicBlock *SplitBB,
+                                       BasicBlock *DestBB) {
+  // SplitBB shouldn't have anything non-trivial in it yet.
+  assert(SplitBB->getFirstNonPHI() == SplitBB->getTerminator() &&
+         "SplitBB has non-PHI nodes!");
+
+  // For each PHI in the destination block...
+  for (BasicBlock::iterator I = DestBB->begin();
+       PHINode *PN = dyn_cast<PHINode>(I); ++I) {
+    unsigned Idx = PN->getBasicBlockIndex(SplitBB);
+    Value *V = PN->getIncomingValue(Idx);
+    // If the input is a PHI which already satisfies LCSSA, don't create
+    // a new one.
+    if (const PHINode *VP = dyn_cast<PHINode>(V))
+      if (VP->getParent() == SplitBB)
+        continue;
+    // Otherwise a new PHI is needed. Create one and populate it.
+    PHINode *NewPN = PHINode::Create(PN->getType(), "split",
+                                     SplitBB->getTerminator());
+    for (unsigned i = 0, e = Preds.size(); i != e; ++i)
+      NewPN->addIncoming(V, Preds[i]);
+    // Update the original PHI.
+    PN->setIncomingValue(Idx, NewPN);
+  }
+}
+
 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
 /// split the critical edge.  This will update DominatorTree and
 /// DominatorFrontier  information if it is available, thus calling this pass
@@ -285,6 +317,16 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
       // the loop, to maintain a LoopSimplify guarantee.
       if (!TIL->contains(DestBB) &&
           P->mustPreserveAnalysisID(LoopSimplifyID)) {
+        assert(!TIL->contains(NewBB) &&
+               "Split point for loop exit is contained in loop!");
+
+        // Update LCSSA form in the newly created exit block.
+        if (P->mustPreserveAnalysisID(LCSSAID)) {
+          SmallVector<BasicBlock *, 1> OrigPred;
+          OrigPred.push_back(TIBB);
+          CreatePHIsForSplitLoopExit(OrigPred, NewBB, DestBB);
+        }
+
         // For each unique exit block...
         SmallVector<BasicBlock *, 4> ExitBlocks;
         TIL->getExitBlocks(ExitBlocks);
@@ -292,41 +334,26 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
           // Collect all the preds that are inside the loop, and note
           // whether there are any preds outside the loop.
           SmallVector<BasicBlock *, 4> Preds;
-          bool AllPredsInLoop = false;
+          bool HasPredOutsideOfLoop = false;
           BasicBlock *Exit = ExitBlocks[i];
           for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
                I != E; ++I)
             if (TIL->contains(*I))
               Preds.push_back(*I);
             else
-              AllPredsInLoop = true;
+              HasPredOutsideOfLoop = true;
           // If there are any preds not in the loop, we'll need to split
           // the edges. The Preds.empty() check is needed because a block
           // may appear multiple times in the list. We can't use
           // getUniqueExitBlocks above because that depends on LoopSimplify
           // form, which we're in the process of restoring!
-          if (Preds.empty() || !AllPredsInLoop) continue;
-          BasicBlock *NewBB = SplitBlockPredecessors(Exit,
-                                                     Preds.data(), Preds.size(),
-                                                     "split", P);
-          // Update LCSSA form by creating new PHIs in the new exit blocks
-          // as needed.
-          if (P->mustPreserveAnalysisID(LCSSAID))
-            for (BasicBlock::iterator I = Exit->begin();
-                 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
-              unsigned Idx = PN->getBasicBlockIndex(NewBB);
-              Value *V = PN->getIncomingValue(Idx);
-              // If the PHI is already suitable, don't create a new one.
-              if (PHINode *VP = dyn_cast<PHINode>(V))
-                if (VP->getParent() == NewBB)
-                  continue;
-              // A new PHI is needed. Create one and populate it.
-              PHINode *NewPN =
-                PHINode::Create(PN->getType(), "split", NewBB->getTerminator());
-              for (unsigned i = 0, e = Preds.size(); i != e; ++i)
-                NewPN->addIncoming(V, Preds[i]);
-              PN->setIncomingValue(Idx, NewPN);
-            }
+          if (!Preds.empty() && HasPredOutsideOfLoop) {
+            BasicBlock *NewExitBB =
+              SplitBlockPredecessors(Exit, Preds.data(), Preds.size(),
+                                     "split", P);
+            if (P->mustPreserveAnalysisID(LCSSAID))
+              CreatePHIsForSplitLoopExit(Preds, NewExitBB, Exit);
+          }
         }
       }
       // LCSSA form was updated above for the case where LoopSimplify is