Use a combination of table lookups and pshufb to convert coefficients
to zero run/level format. Two 16-entry lookup tables are used for a
total of 192 bytes worth of tables. (The existing SSE2 version uses a
table of size 2048 bytes.)
Speedup is ~1.5x-3x as compared with the SSE2 version on Haswell (the
speedup is greater for input with many trailing zeros).
The use of popcnt makes it require SSE4.2. This can be replaced with
a small LUT and accumulation which would reduce the requirement to
SSSE3.
Previously the assembly sources had mixed indentation consisting
of both spaces and tabs, making it quite hard to read unless
the right tab size was used in the editor.
Tabs have been interpreted as 4 spaces in most cases, matching
the surrounding code.
