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     ||