commit b1c5760ed8bf605496e17f48d9e8d7ec39b2f6b5
author Timothy B. Terriberry <tterriberry@mozilla.com> Thu Feb 16 10:53:39 2017 -0800
committer Tim Terriberry <tterriberry@mozilla.com> Wed Apr 12 20:17:54 2017 +0000
tree 011480d2bc6c0065209130cd3202a33f24aaf8f5
parent 7e32a4cc341399debf99401b463be3b674754360

Add ec_smallmul experiment.

This reduces the multiplier width of daala_ec from 16x15->31 to
8x15->23, which reduces hardware latency by an estimated 20% (and
area for this module by an estimated 40%).

These are the smallest logical changes required to achieve this,
but the approach will be optimized significantly in subsequent
commits.

When enabled:

ec_smallmul1c_base@2017-03-08T00:49:01.830Z ->
 ec_smallmul1c@2017-03-08T00:49:45.091Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0203 |  0.0203 |  0.0204 |   0.0203 | 0.0203 |  0.0203 |     0.0202

Change-Id: Idbbd3743e9189146cb519d5b984bdabd69e3f4c0

aom_dsp/entdec.c

diff --git a/aom_dsp/entdec.c b/aom_dsp/entdec.c
index 28d1388..621c600 100644
--- a/aom_dsp/entdec.c
+++ b/aom_dsp/entdec.c
@@ -202,7 +202,11 @@
   r = dec->rng;
   OD_ASSERT(dif >> (OD_EC_WINDOW_SIZE - 16) < r);
   OD_ASSERT(32768U <= r);
+#if CONFIG_EC_SMALLMUL
+  v = r - ((r >> 8) * (uint32_t)(32768U - fz) >> 7);
+#else
   v = fz * (uint32_t)r >> 15;
+#endif
   vw = (od_ec_window)v << (OD_EC_WINDOW_SIZE - 16);
   ret = 0;
   r_new = v;
@@ -381,7 +385,11 @@
   ret = -1;
   do {
     u = v;
+#if CONFIG_EC_SMALLMUL
+    v = r - ((r >> 8) * (uint32_t)(32768U - cdf[++ret]) >> 7);
+#else
     v = cdf[++ret] * (uint32_t)r >> 15;
+#endif
   } while (v <= c);
   OD_ASSERT(v <= r);
   r = v - u;