linux-kernel - Re: [PATCH] Optimize relay_alloc_page_array() slightly by using vzalloc rather than vmalloc and memset

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Message-ID: <20101101133819.GA28138@Krystal>
Date:	Mon, 1 Nov 2010 09:38:19 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jesper Juhl <jj@...osbits.net>, linux-kernel@...r.kernel.org,
	Tom Zanussi <zanussi@...ibm.com>,
	Karim Yaghmour <karim@...rsys.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] Optimize relay_alloc_page_array() slightly by using
	vzalloc rather than vmalloc and memset

* Andrew Morton (akpm@...ux-foundation.org) wrote:
> On Sun, 31 Oct 2010 14:39:14 -0400 Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> 
> > * Jesper Juhl (jj@...osbits.net) wrote:
> > > On Sat, 30 Oct 2010, Mathieu Desnoyers wrote:
> > > 
> > > > * Jesper Juhl (jj@...osbits.net) wrote:
[...]
> > Which looks to me like a misunderstanding of the C99 standard. What you
> > do is:
> > 
> > static struct page **relay_alloc_page_array(unsigned int n_pages)
> > {
> > 	const size_t pa_size = n_pages * sizeof(struct page *);
> > 	...
> > }
> > 
> > So the compiler has no choice but to emit code that will fill in the
> > value of pa_size at runtime, because it depends on "n_pages", a
> > parameter received by the function. So pa_size is everything but
> > constant.
> > 
> > The C99 standard, section 6.7.3 (Type qualifiers) states:
> > 
> > "The implementation may place a const object that is not volatile in a
> > read-only region of storage. Moreover, the implementation need not
> > allocate storage for such an object if its address is never used."
> > 
> > So maybe gcc is kind here and it just removes this const specifier
> > without complaining, but a different compiler might be more strict and
> > fail to compile because you would be dynamically assigning a value to a
> > variable placed in read-only storage.

Actually, "object" in the C99 standard refers to global variables, not
local variables. The misunderstanding was on my part.

Sorry about that,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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