CODEWITHSUNDEEP
cfunctionpointersmemory-alignment
Marco
Marco

Reputation: 9095

What does memory aligning a function to 32 do?

In HTSlib, Bonfield memory aligns a series of methods to 32. Why did he do this and what does it achieve?

So, for example here there's this code:

    if (type == 'c') {
        if (!(q = sam_parse_Bc_vals(b, q, &nused, &nalloc, &overflow)))
            return -1;
    } else if (type == 'C') {
        if (!(q = sam_parse_BC_vals(b, q, &nused, &nalloc, &overflow)))
            return -1;
    } else if (type == 's') {
        if (!(q = sam_parse_Bs_vals(b, q, &nused, &nalloc, &overflow)))
            return -1;
    } else if (type == 'S') {
    //... etc.
    }

And if you look at the function definitions, you see this:

HTS_ALIGN32
static char *sam_parse_Bc_vals(bam1_t *b, char *q, uint32_t *nused,
                               uint32_t *nalloc, int *overflow) {
    while (*q == ',') {
        if ((*nused)++ >= (*nalloc)) {
            if (grow_B_array(b, nalloc, 1) < 0)
                return NULL;
        }
        *(b->data + b->l_data) = hts_str2int(q + 1, &q, 8, overflow);
        b->l_data++;
    }
    return q;
}

That HTS_ALIGN32 is a macro for #define HTS_ALIGN32 __attribute__((aligned(32)))

What performance limitation does this overcome? (Or is there another reason that he did this?)

Upvotes: 2

Views: 112

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