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common: add mp_log2()

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To be used in the next commit.

According to compiler explorer, __builtin_clz is very widely available,
and it barely makes sense to provide a fallback. clang also eats this
(and identifies at least as GCC 4). Actually, there's doubt that a fast
log2 implementation is needed at all (I guess UTF-8 parsing needs it,
but something UTF-8-specific would probably make it faster than using
log2). So the fallback is just something naive.
This commit is contained in:
wm4 2019-10-31 13:16:58 +01:00
parent 6d92e55502
commit 7510ed6f68
2 changed files with 20 additions and 0 deletions
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18
common/common.c
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@ -318,3 +318,21 @@ char **mp_dup_str_array(void *tctx, char **s)
MP_TARRAY_APPEND(tctx, r, num_r, NULL);
return r;
}
// Return rounded down integer log 2 of v, i.e. position of highest set bit.
// mp_log2(0) == 0
// mp_log2(1) == 0
// mp_log2(31) == 4
// mp_log2(32) == 5
unsigned int mp_log2(uint32_t v)
{
#if defined(__GNUC__) && __GNUC__ >= 4
return v ? 31 - __builtin_clz(v) : 0;
#else
for (int x = 31; x >= 0; x--) {
if (v & (((uint32_t)1) << x))
return x;
}
return 0;
#endif
}

2
common/common.h
View File

@ -88,6 +88,8 @@ bool mp_rect_intersection(struct mp_rect *rc, const struct mp_rect *rc2);
bool mp_rect_contains(struct mp_rect *rc, int x, int y);
bool mp_rect_equals(struct mp_rect *rc1, struct mp_rect *rc2);
unsigned int mp_log2(uint32_t v);
int mp_snprintf_cat(char *str, size_t size, const char *format, ...)
PRINTF_ATTRIBUTE(3, 4);