modulus optimization

modulus optimization

Summary:
Test optimization for `x % y` when y is `2^n`.
Reuse the hhmeasure for microbenchmarking.
I'm not sure if this is the right way to do microbenchmarking. If not, please let me know.

Some benchmark result:
* Run with `/home/ylo/fbsource/fbcode/buck-out/gen/hphp/hhvm/hhvm`
For y = 2048 | 4096, skip optimization. Using the same binary to compare the performance result between y = 2048|4096 and y = 1024|8192.

For 10000 iterations,
Operation Name                                 Median (ms)      Mean            Max              Min
run_modulus_1                                  10.235521       23.030888      136.837838        9.501931
run_modulus_optimization_1              7.621622       20.401544      135.416988        7.575290
run_modulus_2                                   9.629344       23.462162      145.359073        9.474903
run_modulus_optimization_2              7.664093       20.894595      136.158301        7.555985

In conclusion, we make 1 modulus op into 3 logical op (`(x & (y - 1)) | (-y & (x >> 63))`).
We can still see some performance improvement.

Reviewed By: paulbiss

Differential Revision: D10117485

fbshipit-source-id: a7d94b4f21f8e748722bf189ae1b3acead31096e