[src/trunk]: src/sys/dev/audio Improve slinear16-to-mulaw conversion calculat...

[isaki <isaki%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/6118ea720936
branches:  trunk
changeset: 938598:6118ea720936
user:      isaki <isaki%NetBSD.org@localhost>
date:      Sat Sep 12 06:09:16 2020 +0000

description:
Improve slinear16-to-mulaw conversion calculation.
It's about 2~3 times faster on my amd64 and x68k(68030).

diffstat:

 sys/dev/audio/mulaw.c |  39 +++++++++++++++++++++++----------------
 1 files changed, 23 insertions(+), 16 deletions(-)

diffs (81 lines):

diff -r 0bce4a2c20fc -r 6118ea720936 sys/dev/audio/mulaw.c
--- a/sys/dev/audio/mulaw.c     Sat Sep 12 05:19:15 2020 +0000
+++ b/sys/dev/audio/mulaw.c     Sat Sep 12 06:09:16 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: mulaw.c,v 1.3 2020/01/11 04:06:13 isaki Exp $  */
+/*     $NetBSD: mulaw.c,v 1.4 2020/09/12 06:09:16 isaki Exp $  */
 
 /*
  * Copyright (C) 2017 Tetsuya Isaki. All rights reserved.
@@ -27,7 +27,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: mulaw.c,v 1.3 2020/01/11 04:06:13 isaki Exp $");
+__KERNEL_RCSID(0, "$NetBSD: mulaw.c,v 1.4 2020/09/12 06:09:16 isaki Exp $");
 
 #include <sys/types.h>
 #include <sys/systm.h>
@@ -45,7 +45,7 @@
  *
  * 2. Calculation (default)
  *  It calculates mu-law with full spec and its precision is 14bit.
- *  It's about 10 times slower but the size is less than a half (on m68k,
+ *  It's about 3 times slower but the size is less than a half (on m68k,
  *  for example).
  *
  * mu-law is no longer a popular format.  I think size-optimized is better.
@@ -247,33 +247,40 @@
                m = slinear8_to_mulaw[val];
 #else
                /* 14bit (fullspec, slow but small) encoder */
-               int16_t val;
+               uint16_t val;
                int c;
 
-               val = (int16_t)(*s++ >> (AUDIO_INTERNAL_BITS - 16));
-               if (val < 0) {
+               val = *s++ >> (AUDIO_INTERNAL_BITS - 16);
+               if ((int16_t)val < 0) {
                        m = 0;
                } else {
                        val = ~val;
                        m = 0x80;
                }
                /* limit */
-               if (val < -8158 * 4)
+               if ((int16_t)val < -8158 * 4)
                        val = -8158 * 4;
                val -= 33 * 4;  /* bias */
 
-               val <<= 1;
-               for (c = 0; c < 7; c++) {
-                       if (val >= 0) {
-                               break;
-                       }
+               // Before(1)         Before(2)         Before(3)
+               // S0MMMMxx_xxxxxxxx 0MMMMxxx_xxxxxxx0 c=0,v=0MMMMxxx_xxxxxxx0
+               // S10MMMMx_xxxxxxxx 10MMMMxx_xxxxxxx0 c=1,v=0MMMMxxx_xxxxxx00
+               // S110MMMM_xxxxxxxx 110MMMMx_xxxxxxx0 c=2,v=0MMMMxxx_xxxxx000
+               // :                 :                 :
+               // S1111110_MMMMxxxx 1111110M_MMMxxxx0 c=6,v=0MMMMxxx_x0000000
 
-                       m += (1 << 4);  /* exponent */
-                       val <<= 1;
-               }
+               // (1) Push out sign bit
                val <<= 1;
 
-               m += (val >> 12) & 0x0f; /* mantissa */
+               // (2) Find first zero (and align val to left)
+               c = 0;
+               if (val >= 0xf000) c += 4, val <<= 4;
+               if (val >= 0xc000) c += 2, val <<= 2;
+               if (val >= 0x8000) c += 1, val <<= 1;
+
+               // (3)
+               m += (c << 4);
+               m += (val >> 11) & 0x0f;
 #endif
 
 #if defined(MULAW32)