[PATCH] D92882: [MBP] Prevent rotating a chain contains entry block

[Guozhi Wei via Phabricator via llvm-commits]

Carrot added a comment.

In D92882#2444285 <https://reviews.llvm.org/D92882#2444285>, @davidxl wrote:

> It is possible to synthesize a test case that rotates the entry without the fix, no? The expected output is that the entry is not rotated (no need to reproduce crash).

The key is to generate an entry block and make the following condition in buildCFGChains() evaluated as false:

  if (!TII->analyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
    break;

It means the entry block must end with an unanalyzable branch and it must also fall through to following block, so the entry block will be pre-merged with the following block, the following block is a loop header, so later it can be rotated by rotateLoop.

I failed to construct such a test case after a whole day's experiments :(


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