[llvm] r251904 - [X86] Generate .cfi_adjust_cfa_offset correctly when pushing arguments

[Kuperstein, Michael M via llvm-commits]

Sorry, I wasn't clear - when I said "always", I meant both -O2 and -Os -fomit-frame-pointers.
But "-Os -fomit-frame-pointers" isn't a configuration that's normally used, so I'm not entirely sure it was a conscious decision.

Anyway, if there's consensus that we need to always be precise, the code is already in place, we just need to flip the switch - but I'm afraid of the size impact.

-----Original Message-----
From: Rafael Espíndola [mailto:rafael.espindola at gmail.com] 
Sent: Tuesday, November 03, 2015 18:09
To: Kuperstein, Michael M
Cc: llvm-commits
Subject: Re: [llvm] r251904 - [X86] Generate .cfi_adjust_cfa_offset correctly when pushing arguments

On 3 November 2015 at 10:55, Kuperstein, Michael M <michael.m.kuperstein at intel.com> wrote:
> GCC is always precise, but that seems unnecessary when we only care about performing unwinding for synchronous EH (since in this case, we only need to be right at call sites), and inflates the module size.
>
> It's actually a bit hard to compare apples-to-apples here, since for -O2, there's no definitely harm in emitting extra CFI, but clang doesn't use pushes, and for -Os, GCC defaults to -no-omit-frame-pointer, so the issue does not arise.

-Os -fomit-frame-pointers?

I think it is incorrect to say that we don't care about asynchronous unwinding (not exceptions). The .eh_frame is part of the ABI and any tool wanting to unwind can depend on it being precise.

Cheers,
Rafael
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