Before this patch, we explicitly modify rsp, which isn't necessarily
universally acceptable, since the space under the stack pointer might
be modified in things like signal handlers. Therefore, use an explicit
register to hold the stack pointer relative to the bottom of the stack
(i.e. rsp). This will also clear out valgrind errors about the use of
uninitialized data that started occurring after the idct16x16/ssse3
optimizations were first merged.
Runtime of the full 32x32 idct goes from 2446 to 2441 cycles (intra) or
from 1425 to 1306 cycles (inter). Overall runtime is not significantly
affected.
Runtime of all IDCTs together goes from 3327 to 2473 cycles (intra, i.e.
~35% faster) or from 2312 to 1448 cycles (inter, i.e. ~60% faster). Total
decode time of ped1080p.webm goes from 8.086sec to 7.974sec (1.4% faster).
Sub-IDCTs will follow later. ped1080.webm goes from 9.295s to 8.191s
(13.5% faster). The IDCT itself goes from 4372 (intra) or 4337 (inter)
to 403 (intra) or 329 (inter) cycles for the DC-only form, 23755 (intra)
or 23723 (inter) to 3497 (intra) or 3607 (inter) cycles for the no-DC
form, which averages from 23393 (intra) or 16612 (inter) to 3449 (intra)
or 2392 (inter) for all 32x32s together, i.e. about ~7x faster (all
tests done on ped1080p.webm).
Sub8x8 speed (w/o dc-only case) goes from ~750 cycles (inter) or ~735
cycles (intra) to ~415 cycles (inter) or ~430 cycles (intra). Average
overall 16x16 idct speed goes from ~635 cycles (inter) or ~720 cycles
(intra) to ~415 cycles (inter) or ~545 (intra) - all measurements done
using ped1080p.webm.
Currently only dc-only and full 16x16. Other subforms will follow in the
near future. Total decoding time of ped1080p.webm goes from 9.7 to 9.3
seconds. DC-only goes from 957 -> 131 cycles, and the full IDCT goes
from ~4050 to ~745 cycles.
