Messages in this thread | | | Date | Mon, 1 Nov 2010 09:38:19 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] Optimize relay_alloc_page_array() slightly by using vzalloc rather than vmalloc and memset |
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* Andrew Morton (akpm@linux-foundation.org) wrote: > On Sun, 31 Oct 2010 14:39:14 -0400 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote: > > > * Jesper Juhl (jj@chaosbits.net) wrote: > > > On Sat, 30 Oct 2010, Mathieu Desnoyers wrote: > > > > > > > * Jesper Juhl (jj@chaosbits.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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