Messages in this thread | | | Date | Mon, 1 Nov 2010 20:22:37 +0800 | From | Américo Wang <> | Subject | Re: [PATCH] Optimize relay_alloc_page_array() slightly by using vzalloc rather than vmalloc and memset |
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On Sun, Oct 31, 2010 at 02:39:14PM -0400, Mathieu Desnoyers wrote: >* Jesper Juhl (jj@chaosbits.net) wrote: >> On Sat, 30 Oct 2010, Mathieu Desnoyers wrote: >> >> > * Jesper Juhl (jj@chaosbits.net) wrote: >> > > Hi, >> > > >> > > We can optimize kernel/relay.c::relay_alloc_page_array() slightly by using >> > > vzalloc. The patch makes these changes: >> > > >> > > - use vzalloc instead of vmalloc+memset. >> > > - remove redundant local variable 'array'. >> > > - declare local 'pa_size' as const. >> > >> > Hrm ? How does declaring a local variable as const helps the compiler in >> > any way ? >> > >> >> Hmm, probably not very much in this case (but it doesn't hurt either ;) - >> actually, removing the const yielded the exact same result, so it's >> "not at all" in this case). >> That bit came from my "build-in" tendency to declare stuff const when it >> obviously doesn't change/nor should. It's a habbit.. > >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." >
This is not enforced by C99. This is C, not C++. :)
>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. >
That compiler would be broken if it exists. Also, I doubt linux kernel could be compiled with other compilers than gcc (except icc?).
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