[PATCH] D18712: [LoopUnroll] Fix the way we update DT after complete unrolling.

[Sanjoy Das via llvm-commits]

sanjoy added a comment.

Hi Michael,

I don't quite grok this statement "That set is a superset of
exit-blocks set.".  What if we have

  maybe_goto exit0;
  
  for (...)
    if (cond) {
      // block a
      maybe_goto exit0;
    } else {
      maybe_goto exit1;
    }
  }
  
  exit0:
   return
  
  exit1:
    return

The set of dominance children of exiting blocks of the above loop is
{exit1}, but the set of exit blocks is {exit0, exit1} (so the set of
dominance children of exiting blocks is not a superset of the exit
blocks of the loop).  Is there some additional invariant that applies
to unrolled loops that I'm missing here?


================
Comment at: lib/Transforms/Utils/LoopUnroll.cpp:559
@@ -556,6 +558,3 @@
   if (DT && Count > 1) {
-    for (auto Exit : ExitBlocks) {
-      BasicBlock *PrevIDom = DT->getNode(Exit)->getIDom()->getBlock();
-      BasicBlock *NewIDom =
-          DT->findNearestCommonDominator(PrevIDom, Latches[0]);
-      DT->changeImmediateDominator(Exit, NewIDom);
+    for (auto BB : ExitingBlocks) {
+      auto *BBDomNode = DT->getNode(BB);
----------------
Use `auto *` here and below (to make it obvious that you're binding to a pointer).


http://reviews.llvm.org/D18712