Use rep_movsb for copying one-byte strings in substring intrinsic.

Use rep_movsb for copying one-byte strings in substring intrinsic.
Committed: https://code.google.com/p/dart/source/detail?r=21293

[Florian Schneider, 2013-04-11 09:04:02]

LGTM.

[srdjan, 2013-04-11 16:39:43]

Committed patchset #3 manually as r21293 (presubmit successful).

[srdjan, 2013-04-11 18:35:03]

Unfortunately  this CL has a >10% regression for Json benchmarks (it did not
have it on my Mac).  I will either revert to loop or will specialize for
lengths/architectures. Any suggestions?

[Florian Schneider, 2013-04-12 08:14:45]

On 2013/04/11 18:35:03, srdjan wrote:
> Unfortunately  this CL has a >10% regression for Json benchmarks (it did not
> have it on my Mac).  I will either revert to loop or will specialize for
> lengths/architectures. Any suggestions?

Ok, that is surprising. It would be interesting to see what role the length of
strings plays. The ramp-up cost of rep movsb may be too high for small trip
counts. Maybe let's leave the normal loop for now.