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    SubjectRe: [PATCH] Optimize relay_alloc_page_array() slightly by using vzalloc rather than vmalloc and memset
    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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