[PATCH] D22630: Loop rotation

[Aditya Kumar via llvm-commits]

hiraditya added inline comments.

================
Comment at: llvm/lib/Transforms/Scalar/LoopRotation.cpp:68-95
@@ -51,4 +67,30 @@
 
-namespace {
+static void insertBetween(BasicBlock *NewBB, BasicBlock *PredBefore,
+                          BasicBlock *Succ) {
+  BranchInst *NewBI = BranchInst::Create(Succ, NewBB);
+  NewBI->setDebugLoc(PredBefore->getTerminator()->getDebugLoc());
+
+  BranchInst *BLI = dyn_cast<BranchInst>(PredBefore->getTerminator());
+  for (unsigned I = 0, E = BLI->getNumSuccessors(); I < E; ++I)
+    if (BLI->getSuccessor(I) == Succ) {
+      BLI->setSuccessor(I, NewBB);
+      break;
+    }
+  // Move NewBB physically from the end of the block list.
+  Function *F = Succ->getParent();
+  F->getBasicBlockList().splice(Succ->getIterator(), F->getBasicBlockList(),
+                                NewBB);
+}
+
+// Remove the arguments of all phi nodes in PhiBB coming from block From.
+static void discardIncomingValues(BasicBlock *PhiBB, BasicBlock *From) {
+  for (Instruction &I : *PhiBB) {
+    PHINode *PN = dyn_cast<PHINode>(&I);
+    if (!PN)
+      break;
+    PN->removeIncomingValue(PN->getBasicBlockIndex(From));
+  }
+}
+
 /// A simple loop rotation transformation.
 class LoopRotate {
----------------
Using SplitEdge results in ICE because SplitEdge (for critical edges) tries to update PHI in a strange way. I think there is a bug in SplitEdge PHI update for single-entry PHIs. Moreover, loop-rotation needs to do its own PHI-update later anyways.

Specifically, BreakCriticalEdges:174, PN->getBasicBlockIndex may return -1, and threre is no check for that here.
      if (PN->getIncomingBlock(BBIdx) != TIBB)
        BBIdx = PN->getBasicBlockIndex(TIBB);
      PN->setIncomingBlock(BBIdx, NewBB);



https://reviews.llvm.org/D22630