The first routine I worked on was memcpy().

Comparison of stock memcpy() of a single page on FreeBSD vs Linux on a Westmere (values are TSC deltas):

x fbsd/westmere/builtin
+ linux/builtin
    N           Min           Max        Median           Avg        Stddev
x 1000           336         18444           340       361.628     573.11483
+ 1000           276          9996           280       288.924     307.34136
Difference at 95.0% confidence
        -72.704 +/- 40.3074
        -20.1046% +/- 11.1461%
        (Student's t, pooled s = 459.847)

Idea	Westmere	Sandy Bridge	Ivy Bridge
Replace dec with sub	none	none	none
Use movsd instead of movsq	slightly slower	slightly slower	6% faster
Simple movdqa loop	138% slower	58% slower	46% slower
movdqa 32 at a time (old)	27% slower	14% faster	17% faster
movdqa 32 at a time (new)	27% slower	15% faster	18% faster
movdqa 64 at a time	224% slower	131% slower	116% slower