[PATCH] D97982: [MC] Introduce NeverAlign fragment type

[Amir Ayupov via Phabricator via llvm-commits]

Amir added a subscriber: skan.
Amir added a comment.

We've considered two options to integrate this functionality into LLVM:

1. Similar to JCC errata/BoundaryAlign fragment:

Automatically insert NeverAlign into the section on cmp instruction, check if fuseable instruction follows it, and remove the fragment otherwise.

2. Similar to the current usage in BOLT: https://github.com/facebookincubator/BOLT/blob/68abc968b706b55585b1b8be315aef5d3bf90b1c/bolt/src/BinaryEmitter.cpp#L445

Check basic block instructions in advance, only emit NeverAlign fragment if fuseable sequence is found.

Pros and cons of each approach:

1. Pro is that macro-op fusion alignment will be applied to inline assembly as well as LLVM-produced MCs, the downside is that I've observed a lot of insertions and removals of NeverAlign fragment, which may hurt processing time.
2. Pro is that the check and insertion are happening once per basic block, the downside is that we need to integrate the logic for determining the insertion of this fragment higher up in the MCStreamer stack where we see the basic block.

Thoughts? 
+CC @skan


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