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

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Date:	Mon, 6 Jul 2015 10:10:27 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Jan Kara <jack@...e.cz>, Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH] x86: Fix detection of GCC -mpreferred-stack-boundary support

On Mon, Jul 6, 2015 at 6:44 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> So looking at this I question the choice of -mpreferred-stack-boundary=3. Why not
> do -mpreferred-stack-boundary=2?

It wouldn't make sense anyway - it would only make code worse (if it
worked) and not any better.

The reason the "=3" value is good is because 8-byte alignment is the
"natural" alignment - it's what you get with a normal call sequence,
simply because the return address is 8 bytes in size.

That means that with "=3" you don't get extra code to align the stack
for the simple functions that don't need a frame.

Anything smaller than 3 wouldn't help even if it worked, because none
of the normal stack operations (pushing/popping registers to
save/restore them) would be any smaller anyway.

But bigger values than 3 result in the compiler having to generate
extra stack adjustments just to align the stack after a call that very
naturally mis-aligned it. And it doesn't help anyway, since in the
kernel we don't put stuff on the stack that needs bigger alignment
(of, the fxsave buffer is a counter-example, but it's a very odd one
that we _shouldn't_ have put on the stack).

                Linus
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