[LoopInterchange] Do not change LI for BBs in child loops.

[fhahn]

Summary

If a loop with child loops becomes our new inner loop after
interchanging, we only need to update LoopInfo for the blocks defined in
the old outer loop. BBs in child loops will stay there.

[fhahn]

LoopInterchange did that wrong and this additional assertion catches that. The fix for LoopInterchange is at D45970

[efriedma]

Needs testcase.

LoopInterchange did that wrong and this additional assertion catches that. The fix for LoopInterchange is at D45970

Did you mean to post this on a different patch?

[fhahn]

In D45970#1075792, @efriedma wrote:

Needs testcase.

tests/Transform/LoopInterchange/phi-ordering.ll has a loop nest with 3 levels deep that gets interchanged twice and exposes the problem. With D45971, -verify-loop-info on the test case should fail (and I think there is another test case that also fails with D45971). I am happy to add a separate test case, in case you think that's valuable.

LoopInterchange did that wrong and this additional assertion catches that. The fix for LoopInterchange is at D45970

Did you mean to post this on a different patch?

Yes, I meant to post this on D45971...

[efriedma]

LGTM

I'm not really happy with LoopInterchangeTransform::restructureLoops overall; it seems like complicated, fragile code. But I don't see a better alternative.

[fhahn]

In D45970#1075854, @efriedma wrote:

LGTM

I'm not really happy with LoopInterchangeTransform::restructureLoops overall; it seems like complicated, fragile code. But I don't see a better alternative.

Thanks Eli. It is not ideal.... Maybe there's potential for a more generic API to update LoopInfo, but the things we need to update here seem quite specific to interchanging.

lib/Transforms/Scalar/LoopInterchange.cpp

for (BasicBlock *BB : NewInner->blocks())
     if (LI->getLoopFor(BB) == NewInner)
       NewOuter->addBlockEntry(BB);
 
   // Now remove inner loop header and latch from the new inner loop and move
   // other BBs (the loop body) to the new inner loop.
   BasicBlock *OuterHeader = NewOuter->getHeader();
   BasicBlock *OuterLatch = NewOuter->getLoopLatch();
   for (BasicBlock *BB : OrigInnerBBs) {
+    // Nothing will change for BBs in child loops.
+    if (LI->getLoopFor(BB) != NewOuter)
+      continue;
     // Remove the new outer loop header and latch from the new inner loop.
     if (BB == OuterHeader || BB == OuterLatch)
       NewInner->removeBlockFromLoop(BB);
     else
       LI->changeLoopFor(BB, NewInner);
   }
 
   // The preheader of the original outer loop becomes part of the new