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

[Rafael Espíndola via llvm-commits]

I would personally prefer to be precise by default and have a command line
option to switch it off.

Cheers,
Rafael
On Nov 3, 2015 12:13 PM, "Kuperstein, Michael M" <
michael.m.kuperstein at intel.com> wrote:

> 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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