Re: [PATCH] x86: Fix detection of GCC -mpreferred-stack-boundary support

[Raymond Jennings]

On Mon, 2015-07-06 at 10:59 -0700, Andy Lutomirski wrote:
> On Mon, Jul 6, 2015 at 10:40 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> >> > My reasoning: on modern uarchs there's no penalty for 32-bit misalignment of
> >> > 64-bit variables, only if they cross 64-byte cache lines, which should be rare
> >> > with a chance of 1:16. This small penalty (of at most +1 cycle in some
> >> > circumstances IIRC) should be more than counterbalanced by the compression of
> >> > the stack by 5% on average.
> >>
> >> I'll counter with: what's the benefit?  There are no operations that will
> >> naturally change RSP by anything that isn't a multiple of 8 (there's no pushl in
> >> 64-bit mode, or at least not on AMD chips -- the Intel manual is a bit vague on
> >> this point), so we'll end up with RSP being a multiple of 8 regardless.  Even if
> >> we somehow shaved 4 bytes off in asm, that still wouldn't buy us anything, as a
> >> dangling 4 bytes at the bottom of the stack isn't useful for anything.
> >
> > Yeah, so it might be utilized in frame-pointer less builds (which we might be able
> > to utilize in the future if sane Dwarf code comes around), which does not use
> > push/pop to manage the stack but often has patterns like:
> >
> > ffffffff8102aa90 <SyS_getpriority>:
> > ffffffff8102aa90:       48 83 ec 18             sub    $0x18,%rsp
> >
> > and uses MOVs to manage the stack. Those kinds of stack frames could be 4-byte
> > granular as well.
> >
> > But yeah ... it's pretty marginal.
> 
> To get even that, we'd need an additional ABI-changing GCC flag to
> change GCC's idea of the alignment of long from 8 to 4.  (I just
> checked: g++ thinks that alignof(long) == 8.  I was too lazy to look
> up how to ask the equivalent question in C.)

I just want to point out that long itself is 8 bytes on 64-bit x86, but
only 4 bytes on 32-bit x86.

Perhaps we should keep in mind sizeof(long) and not just alignof(long)?

My opinion btw, is that if long is 8 bytes wide, it should also be 8
bytes aligned.

> --Andy
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