This speeds up aac_tablegen to a ludicruous degree (~97%), i.e to the point
where it can be argued that runtime initialization can always be done instead of
hard-coded tables. The only cost is essentially a trivial increase in
the stack size.
Even if one does not care about this, the patch also improves accuracy
as detailed below.
Performance:
Benchmark obtained by looping 10^4 times over ff_aac_tableinit.
Sample benchmark (x86-64, Haswell, GNU/Linux):
old:
1295292 decicycles in ff_aac_tableinit, 512 runs, 0 skips
1275981 decicycles in ff_aac_tableinit, 1024 runs, 0 skips
1272932 decicycles in ff_aac_tableinit, 2048 runs, 0 skips
1262164 decicycles in ff_aac_tableinit, 4096 runs, 0 skips
1256720 decicycles in ff_aac_tableinit, 8192 runs, 0 skips
new:
25691 decicycles in ff_aac_tableinit, 505 runs, 7 skips
25130 decicycles in ff_aac_tableinit, 1016 runs, 8 skips
25973 decicycles in ff_aac_tableinit, 2036 runs, 12 skips
25911 decicycles in ff_aac_tableinit, 4078 runs, 18 skips
25816 decicycles in ff_aac_tableinit, 8154 runs, 38 skips
Accuracy:
The previous code was resulting in needless loss of
accuracy due to the pow being called in succession. As an illustration
of this:
ff_aac_pow34sf_tab[3]
old : 0.000000000007598092294225
new : 0.000000000007598091426864
real: 0.000000000007598091778545
truncated to float
old : 0.000000000007598092294225
new : 0.000000000007598091426864
real: 0.000000000007598091426864
showing that the old value was not correctly rounded. This affects a
large number of elements of the array.
Patch tested with FATE.
Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com>
---
libavcodec/aac_tablegen.h | 38 ++++++++++++++++++++++++++++++++++++--
1 file changed, 36 insertions(+), 2 deletions(-)
diff --git a/libavcodec/aac_tablegen.h b/libavcodec/aac_tablegen.h
index 8b223f9..255723b 100644
--- a/libavcodec/aac_tablegen.h
+++ b/libavcodec/aac_tablegen.h
@@ -35,9 +35,43 @@ float ff_aac_pow34sf_tab[428];
av_cold void ff_aac_tableinit(void)
{
int i;
+
+ /* 2^(i/16) for 0 <= i <= 15 */
+ const double exp2_lut[] = {
+ 1.00000000000000000000,
+ 1.04427378242741384032,
+ 1.09050773266525765921,
+ 1.13878863475669165370,
+ 1.18920711500272106672,
+ 1.24185781207348404859,
+ 1.29683955465100966593,
+ 1.35425554693689272830,
+ 1.41421356237309504880,
+ 1.47682614593949931139,
+ 1.54221082540794082361,
+ 1.61049033194925430818,
+ 1.68179283050742908606,
+ 1.75625216037329948311,
+ 1.83400808640934246349,
+ 1.91520656139714729387,
+ };
+ double t1 = 8.8817841970012523233890533447265625e-16; // 2^(-50)
+ double t2 = 3.63797880709171295166015625e-12; // 2^(-38)
+ int t1_inc_cur, t2_inc_cur;
+ int t1_inc_prev = 0;
+ int t2_inc_prev = 8;
+
for (i = 0; i < 428; i++) {
- ff_aac_pow2sf_tab[i] = pow(2, (i - POW_SF2_ZERO) / 4.0);
- ff_aac_pow34sf_tab[i] = pow(ff_aac_pow2sf_tab[i], 3.0/4.0);
+ t1_inc_cur = 4 * (i % 4);
+ t2_inc_cur = (8 + 3*i) % 16;
+ if (t1_inc_cur < t1_inc_prev)
+ t1 *= 2;
+ if (t2_inc_cur < t2_inc_prev)
+ t2 *= 2;
+ ff_aac_pow2sf_tab[i] = t1 * exp2_lut[t1_inc_cur];
+ ff_aac_pow34sf_tab[i] = t2 * exp2_lut[t2_inc_cur];
+ t1_inc_prev = t1_inc_cur;
+ t2_inc_prev = t2_inc_cur;
}
}
#endif /* CONFIG_HARDCODED_TABLES */
--
2.6.2
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