The code for scale factors calculation with joint stereo support has
been moved to a separate function. It can get platform-specific
SIMD optimizations later for best possible performance.
But even this change in C code improves performance because of the
use of __builtin_clz() instead of loops similar to what was done
to sbc_calc_scalefactors earlier. Also technically it does loop
unrolling by processing two channels at once, which might be either
good or bad for performance (if the registers pressure is increased
and more data is spilled to memory). But the benchmark from 32-bit
x86 system (pentium-m) shows that it got clearly faster:
$ time ./sbcenc.old -b53 -s8 -j test.au > /dev/null
real 0m1.868s
user 0m1.808s
sys 0m0.048s
$ time ./sbcenc.new -b53 -s8 -j test.au > /dev/null
real 0m1.742s
user 0m1.668s
sys 0m0.064s
This prevents overflows and audible artefacts for the audio files which
originally had loudness maximized. Music from audio CD disks is an
example of such files, see http://en.wikipedia.org/wiki/Loudness_war
Buffer position in X array was not always 16-bytes aligned.
Strict 16-byte alignment is strictly required for powerpc altivec
simd optimizations because altivec does not have support for
unaligned vector loads at all.
