#77856 causes a minor performance regression in stage2 clang builds

[dianqk]

FWIW, it seems like this causes a minor performance regression in stage2 clang builds (https://llvm-compile-time-tracker.com/compare.php?from=5aec9392674572fa5a06283173a6a739742d261d&to=5932fcc47855fdd209784f38820422d2369b84b2&stat=instructions:u). Though code size does reduce as well.

Originally posted by @nikic in #77856 (comment)

[dianqk]

Reopen, as the compilation time hasn't been restored, and there are some test cases failing simultaneously.

[dianqk]

@nikic I can't reproduce the issue by reverting the relevant code based on the current main branch right now.

This is the revert mentioned in the PR: https://llvm-compile-time-tracker.com/compare.php?from=f3c5278efa3b783ada9e7a34b751cf4c5b864535&to=58622ef6755a02f97e5127bea29ed5b8812fe25e&stat=instructions:u. I can see the expected outcome: a reduction in stage2-O3.

I can see similar result before merging the PR: https://llvm-compile-time-tracker.com/compare.php?from=4f88c2311130791cf69da34b743b1b3ba7584a7b&to=98b32349eb3fdda673a415681524ae3e288ecd26&stat=instructions:u.

But I can't see the expected result after merging: https://llvm-compile-time-tracker.com/compare.php?from=de9b386f84b58ad0ffc12e221bc6d9161ca5b62d&to=882814edd33cab853859f07b1dd4c4fa1393e0ea&stat=instructions:u.

I performed a revert based on the new commit and still couldn't achieve the expected result: https://llvm-compile-time-tracker.com/compare.php?from=1241e7692a466ceb420be2780f1c3e8bbab7d469&to=e983d001859e4d4f1c8c0587dec34dc6fa6fad80&stat=instructions:u.

I speculate that among those 1800 commits, the issue was somehow addressed: https://llvm-compile-time-tracker.com/compare.php?from=4f88c2311130791cf69da34b743b1b3ba7584a7b&to=de9b386f84b58ad0ffc12e221bc6d9161ca5b62d&stat=instructions%3Au. I should be able to close this issue now.

Based on the improvements mentioned by @dtcxzyw on #85160 (comment), I believe I only need to address the test failures on the CI and reland it.

[nikic]

@dianqk Thanks for looking into it! Agree that we can go ahead and close this issue.