Messages in this thread | | | Subject | Re: [PATCH] x86: Fix detection of GCC -mpreferred-stack-boundary support | From | Raymond Jennings <> | Date | Mon, 06 Jul 2015 21:01:12 -0700 |
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On Mon, 2015-07-06 at 10:59 -0700, Andy Lutomirski wrote: > On Mon, Jul 6, 2015 at 10:40 AM, Ingo Molnar <mingo@kernel.org> wrote: > > > > * Andy Lutomirski <luto@amacapital.net> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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