Neal Becker wrote:
> Martin C. Martin wrote:
>
>>
>> Robert Bradshaw wrote:
>>> On Apr 9, 2008, at 8:25 AM, Martin C. Martin wrote:
>>>> Hi,
>>>>
>>>> There's a potential optimization I mentioned on the Lisp inspired
>>>> transforms page, where you could reorder bitfields in order to pack them
>>>> most efficiently. Eerily, someone at my job just committed something
>>>> that did just that. We have a custom defstruct, called defstruct-bv,
>>>> which allows you to specify bit fields. (Lisp doesn't come with
>>>> bitfields.) Here's the checkin message:
>>>>
>> ...
>>> I believe in Cython we would just rely on the C compiler to make
>>> optimizations of this nature.
>> That's the point, C prescribes that it *can't* make such optimizations.
>> C fields (including bitfields) must appear in memory in the order they
>> appear in the source code.
>>
>> For bit fields, this is an advantage when you're, say, parsing the
>> header of some binary file, or writing a device driver. But if you just
>> want a struct that stores some data compactly, the C compiler can't help
>> you out in that case.
>>
>> Best,
>
> What about gcc's 'packed' attribute?
>
> `packed'
> The `packed' attribute specifies that a variable or structure field
> should have the smallest possible alignment--one byte for a
> variable, and one bit for a field, unless you specify a larger
> value with the `aligned' attribute.
>
> Here is a structure in which the field `x' is packed, so that it
> immediately follows `a':
>
> struct foo
> {
> char a;
> int x[2] __attribute__ ((packed));
> };
Access to non-word aligned ints can be very slow, that's why packed
isn't the default.
The use case is more like:
struct foo {
int a;
void *p;
int b;
}
On a 64 bit machine with 32 bit ints, that structure takes up 3 words
(24 bytes). Instead, if you wrote it as:
struct foo {
int a;
int b;
void *p;
}
Everything is properly aligned, and it only takes 2 words (16 bytes).
Best,
Martin
_______________________________________________
Cython-dev mailing list
[email protected]
http://codespeak.net/mailman/listinfo/cython-dev
