[LoopUnswitch] Fix SCEV invalidation in unswitching

[mkazantsev]

Summary

Loop unswitching makes substantial changes to a loop that can also affect cached
SCEV info in its outer loops as well, but it only cares to invalidate SCEV cache for the
innermost loop in case of full unswitching and does not invalidate anything at all in
case of trivial unswitching. As result, we may end up with incorrect data in cache.

[mkazantsev]

This one appeared to be a real bug, at least we have a test that fails with assert for trivial unswitching. Thanks @uabelho for providing the test case.

[uabelho]

Thanks Max! This solves the problem I saw!

Not sure if I'm allowed to "LGTM" this but the change makes sense to me.

[mkazantsev]

Actually particularly with case of unswitching, I grow hesitant about one of our asserts that fails here. Maybe it is over-conservative. See comment https://reviews.llvm.org/D44676#1086144

I will check if its removal actually helps.

[mkazantsev]

Never mind, in this case the latch of outer loop changes, so we must clear cache.

[mkazantsev]

Ping?

[mzolotukhin]

Looks correct to me.

lib/Transforms/Scalar/LoopUnswitch.cpp

         << " blocks] in Function " << F->getName()
         << " when '" << *Val << "' == " << *LIC << "\n");
 
+  // We are going to make modifications to loop's block which can also affect
+  // cached invariants for its parent loops.
   if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
-    SEWP->getSE().forgetLoop(L);
+    SEWP->getSE().forgetTopmostLoop(L);
 
   LoopBlocks.clear();
   NewBlocks.clear();