linux-kernel - Re: [PATCH] X86-32: Let gcc decide whether to inline memcpy was Re: New x86 warning

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Message-ID: <alpine.LFD.2.00.0904221417110.3101@localhost.localdomain>
Date:	Wed, 22 Apr 2009 14:19:00 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Ingo Molnar <mingo@...e.hu>, Jeff Garzik <jeff@...zik.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH] X86-32: Let gcc decide whether to inline memcpy was Re:
 New x86 warning



On Wed, 22 Apr 2009, Andi Kleen wrote:
> 
> AFAIK it's all true on 3.2+ when it can figure out the alignment
> (but some gcc versions had problems passing the alignment around e.g.
> through inlining), under the assumption that out of line can do
> a better job with unaligned data. That's not true with my patch,
> but could be true in theory.

Maybe it was the unaligned case that I remember.

Because it's definitely not true that out-of-line code can do any better 
with unaligned data, at least not for small constants. And the case I 
remember was for some silly 8-byte case or similar.

> Quick test here:

How about you just compile the kernel with gcc-3.2 and compare the number 
of calls to memcpy before-and-after instead? That's the real test.

		Linus
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