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by haberman 1886 days ago
Wow thanks for the pointer! That seems unfortunate, I wonder if there is a way to evaluate whether the extra jump is actually worth it, and whether this optimization could be allowed.
1 comments
chandlerc1024 1886 days ago
At the very least, can check when the target isn't a basic block and thus it's a clear win. Will fix your case.

I'm dubious about the whole thing though. Seems like it may day from when branching "down" vs. "up" mattered to branch prediction.

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pyler 1886 days ago
It seems that that code handles just the case when target is function.

https://github.com/llvm/llvm-project/blob/535df472b04208d6ae...

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ndesaulniers 1885 days ago
commit a468601e0ead ("[X86] Re-enable conditional tail calls and fix PR31257.")

https://reviews.llvm.org/D29856

added the comment.

If I remove the check for `OptForSize` and `PredTBB == MBB`, we can optimize OP's report, but seem to regress basic tests like llvm/test/CodeGen/X86/conditional-tailcall.ll, literally flipping the branches incorrectly IIUC.

Filed: https://bugs.llvm.org/show_bug.cgi?id=50130

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