JIT: Remove fallthrough quirk in fgFindInsertPoint

[amanasifkhalid]

Part of #95998.

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[AndyAyersMS]

Have you looked through diffs yet?

[amanasifkhalid]

I've looked at a few, though I'm thinking of adding the check for "fall through" for BBJ_ALWAYS blocks back in. Diffs are less extreme locally with that restriction. I'll update this soon...

[amanasifkhalid]

Diffs are smaller now, though still pretty big -- fgFindInsertPoint is called by fgNewBBinRegion, which is called in quite a few phases, hence the large churn. Those phases include importation, hence the MinOpts diffs. This change makes it harder for fgFindInsertPoint to decide to break up fallthrough behavior between two blocks by choosing to insert between them; previously, if a BBJ_ALWAYS to the next block did not have BBF_NONE_QUIRK set, we assumed the block did not initially jump to the next block, and flowgraph churn just happened to place it before its successor, so it's ok to break the fallthrough behavior. In my opinion, that's a tenuous conclusion, though perhaps so is the assumption that a BBJ_ALWAYS to the next block will continue to "fall through" (as in fgReorderBlocks won't break them up), and thus we should try to preserve the fallthrough here.

Once we have a new block layout algorithm in place, my hope is we can get rid of all of these attempts to prematurely preserve fallthrough behavior, and trust that block layout will do something reasonable (ideally, we'd be able to get rid of fgFindInsertPoint altogether at that point, and just append blocks to the end of the region as they're created, though maybe that might mess with other flowgraph passes?). For now, I'm not sure if we can reduce churn further if we want to get rid of BBF_NONE_QUIRK.

[AndyAyersMS]

Can you leave a todo here or a note in #93020 to revisit this after we have improved layout?

[amanasifkhalid]

Sure thing; I added a note to #93020.

[AndyAyersMS]

Improvements:

• [Perf] Linux/x64: 22 Improvements on 3/18/2024 6:21:23 PM perf-autofiling-issues#31798
• [Perf] Windows/x64: 4 Improvements on 3/18/2024 6:21:23 PM perf-autofiling-issues#31935