Improves performance for calls from Crankshaft code to certain string and...

Improves performance for calls from Crankshaft code to certain string and
regexp JS functions (exec, replace, split, match) when the return value is
unused. New versions of these functions are introduced which execute the
regexp, reproducing all side effects, but do not create the result.
Calls to the original functions with no uses are replaced with calls to
the simplified versions.
BUG=none
TEST=none

[danno, 2012-10-12 15:49:47]

lgtm

Yang, can you please take a look?

[danno, 2012-10-12 15:51:15]

Whoops, hit the wrong button. This doesn't look good to me on this patch quite
yet,

[Yang, 2012-10-22 09:28:16]

On 2012/10/12 15:51:15, danno wrote:
> Whoops, hit the wrong button. This doesn't look good to me on this patch quite
> yet,

A quick test shows that this patch causes the V8 benchmark bundled in the V8
repository (benchmark/run.js) to crash with segfaults (RayTrace and RegExp).

This patch seems to attempt to improve V8's performance for string and regexp
operations when the result is not used. I expect this to be very rare in actual
javascript applications (why would you run an expensive string/regexp operation
when you don't use its result?).
However, V8's RegExp benchmark contains a lot of those operations with unused
results. This has been a known fact and there are plans (see 10913269) to change
that so that this benchmark tests what it's intended to test
(https://chromiumcodereview.appspot.com/10913269/). That's why I think this
patch is taking the wrong direction.