On Thu, 20 Apr 2000, Olaf Petzold wrote:
: > The perfomance difference not so significant, rewriting critical paths in
: > asm is more effective usually. It should be noted that using
: > -malign-double option can lead to different struct fields aligning, even
: > in the same app.
:
: Oh dear... Can I avoid this by using
:
: typedef struct {
: flag_t inuse;
: flag_t valid;
: double um[65536]
: } __attribute__ ((aligned)) shm_slut_t;
:
: ??? Is it guarantee that all structures (inside shm, inside local static copy,
: inside kernel space code produced by Gnu-C and user space program compiled by
: Gnu-C++) have the same aligment - even if compiled with the Option
: -align-double? Or could I use an explizit __attribute__ ((aligned (size))) .
: Which is a good size for those structures ??? __attribute__((packed)) is the
: safest way at moment, because they didn't have any aligment (I have have right
: read the gcc-info) but, it's a performance killer and I live from loock up
: tables (LUT) and performance (it's a controller task for magnetic bearing).
If you don't use ugly hacks like :
...
shm_slut_t t;
unsigned char *p = &t;
double *um = (double *)(p + sizeof(flag_t) * 2);
um[0] = 1.0;
...
all must be ok.
:
Rus
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