linux-kernel - Re: [PATCH] reduce large do_mount stack usage with noinlines

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Date:	Wed, 06 Feb 2008 17:01:32 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] reduce large do_mount stack usage with noinlines

Arjan van de Ven wrote:
> On Wed, 6 Feb 2008 14:34:57 -0800
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
>> Does the patch actually help?  I mean, if a() calls b() and both use N
>> bytes of locals, our worst-case stack usage remains ~2N whether or
>> not b() was inlined in a()?  In fact, uninlining makes things a
>> little worse due to callframe stuff.
> 
> it gets interesting at the three-way..
> if a() calls b() and then calls c(), and they all use N,
> the total usage is now 3N not 2N.

*nod*

It'd be nice if it could be max of (a,b,c) though.  Or maybe compilers
don't work that way.  :)

> (although current gcc is already somewhat smarter about this, and 3N might actually be 2N for some cases)

on x86, gcc 4.1.2, do_mount goes from 360 to 112 bytes w/ the patch I sent.

with gcc 4.3, it goes from 364 to 104.

-ERic
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