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svolume: add some comments

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This commit is contained in:
Wim Taymans 2009-08-14 13:12:30 +02:00
parent e396fe67fb
commit dcae9a3113
1 changed files with 18 additions and 0 deletions
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18
src/pulsecore/svolume_mmx.c
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@ -101,6 +101,22 @@ pa_volume_ulaw_mmx (uint8_t *samples, int32_t *volumes, unsigned channels, unsig
}
#endif
/* in s: 2 int16_t samples
* in v: 2 int32_t volumes, fixed point 16:16
* out s: contains scaled and clamped int16_t samples.
*
* We calculate the high 32 bits of a 32x16 multiply which we then
* clamp to 16 bits. The calulcation is:
*
* vl = (v & 0xffff)
* vh = (v >> 16)
* s = ((s * vl) >> 16) + (s * vh);
*
* For the first multiply we have to do a sign correction as we need to
* multiply a signed int with an unsigned int. Hacker's delight 8-3 gives a
* simple formula to correct the sign of the high word after the signed
* multiply.
*/
#define VOLUME_32x16(s,v) /* .. | vh | vl | */ \
" pxor %%mm4, %%mm4 \n\t" /* .. | 0 | 0 | */ \
" punpcklwd %%mm4, "#s" \n\t" /* .. | 0 | p0 | */ \
@ -116,6 +132,8 @@ pa_volume_ulaw_mmx (uint8_t *samples, int32_t *volumes, unsigned channels, unsig
" pmaddwd "#s", "#v" \n\t" /* .. | p0 * v0 | */ \
" packssdw "#v", "#v" \n\t" /* .. | p1*v1 | p0*v0 | */
/* approximately advances %3 = (%3 + a) % b. This function requires that
* a <= b. */
#define MOD_ADD(a,b) \
" add "#a", %3 \n\t" \
" mov %3, %4 \n\t" \