[all-commits] [llvm/llvm-project] 7263e8: [LoopUnroll][NFC] Clean up remainder followup meta...

[Joel E. Denny via All-commits]

  Branch: refs/heads/users/jdenny-ornl/unroll-cleanup-remainder-followup
  Home:   https://github.com/llvm/llvm-project
  Commit: 7263e8e065e66d2abbc53c4737ff41476533047a
      https://github.com/llvm/llvm-project/commit/7263e8e065e66d2abbc53c4737ff41476533047a
  Author: Joel E. Denny <jdenny.ornl at gmail.com>
  Date:   2025-10-27 (Mon, 27 Oct 2025)

  Changed paths:
    M llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
    M llvm/test/Transforms/LoopUnroll/followup.ll

  Log Message:
  -----------
  [LoopUnroll][NFC] Clean up remainder followup metadata handling

Followup metadata for remainder loops is handled by two
implementations, both added by 7244852557ca6:

1. `tryToUnrollLoop` in `LoopUnrollPass.cpp`'.
2. `CloneLoopBlocks` in `LoopUnrollRuntime.cpp`.

As far as I can tell, 2 is useless: I added `assert(!NewLoopID)` for
the `NewLoopID` returned by the `makeFollowupLoopID` call, and it
never fails throughout check-all for my build.

Moreover, if 2 were useful, it appears it would have a bug caused by
7cd826a321d9.  That commit skips adding loop metadata to a new
remainder loop if the remainder loop itself is to be completely
unrolled because it will then no longer be a loop.  However, that
commit incorrectly assumes that `UnrollRemainder` dictates complete
unrolling of a remainder loop, and thus it skips adding loop metadata
even if the remainder loop will be only partially unrolled.

To avoid further confusion here, this patch removes 2.  check-all
continues to pass for my build.  If 2 actually is useful, please
advise so we can create a test that covers that usage.

Near 2, this patch retains the `UnrollRemainder` guard on the
`setLoopAlreadyUnrolled` call, which adds `llvm.loop.unroll.disable`
to the remainder loop.  That behavior exists both before and after
7cd826a321d9.  The logic appears to be that remainder loop unrolling
(whether complete or partial) is opt-in.  That is, unless
`UnrollRemainder` is true, `UnrollRuntimeLoopRemainder` skips running
remainder loop unrolling, and `llvm.loop.unroll.disable` suppresses
any later attempt at it.

This patch also extends testing of remainder loop followup metadata to
be sure remainder loop partial unrolling is handled correctly by 1.



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