Faster unorm_to_unorm for wide processing.
Optimizing the unorm_to_unorm functions allows a speedup from:
src_8888_2x10 = L1: 62.08 L2: 60.73 M: 59.61 ( 4.30%) HT: 46.81
VT: 42.17 R: 43.18 RT: 26.01 (325Kops/s)
to:
src_8888_2x10 = L1: 76.94 L2: 78.43 M: 75.87 ( 5.59%) HT: 56.73
VT: 52.39 R: 53.00 RT: 29.29 (363Kops/s)
on a i7 Q720 -based laptop.
The key of the patch is the observation that unorm_to_unorm's work can
more easily be done with a simple multiplication and shift, when the
function is applied repeatedly and the parameters are not compile-time
constants. For instance, converting from 0xfe to 0xfefe (expanding
from 8 bits to 16 bits) can be done by calculating
c = c * 0x101
However, sometimes the result is not a neat replication of all the
bits. For instance, going from 10 bits to 16 bits can be done by
calculating
c = c * 0x401UL >> 4
where the intermediate result is 20 bit wide repetition of the 10-bit
pattern followed by shifting off the unnecessary lowest bits.
The patch has the algorithm to calculate the factor and the shift, and
converts the code to use it.