Speed up repeating the separator between elements in sparse array .join

Speed up repeating the separator between elements in sparse array .join

BUG=

Committed: https://crrev.com/a20f793ff4394470aa781fc91b1e9a5770123e5a
Cr-Commit-Position: refs/heads/master@{#34778}

[Toon Verwaest, 2016-03-15 09:00:16]

verwaest@chromium.org changed reviewers:
+ yangguo@chromium.org

[Toon Verwaest, 2016-03-15 09:00:17]

ptal

[Yang, 2016-03-15 09:23:10]

lgtm

https://codereview.chromium.org/1806443002/diff/1/src/runtime/runtime-strings.cc
File src/runtime/runtime-strings.cc (right):

https://codereview.chromium.org/1806443002/diff/1/src/runtime/runtime-strings...
src/runtime/runtime-strings.cc:598: CopyChars(buffer, start, block_chars);
I wonder whether it makes sense to, in case of one-byte strings and
one-character separator (which is probably often the case), just use memset.

[Toon Verwaest, 2016-03-15 09:54:50]

The CQ bit was checked by verwaest@chromium.org

[commit-bot: I haz the power, 2016-03-15 09:54:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1806443002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1806443002/1

[Yang, 2016-03-15 10:01:15]

On 2016/03/15 09:54:59, commit-bot: I haz the power wrote:
> CQ is trying da patch. Follow status at
>  https://chromium-cq-status.appspot.com/patch-status/1806443002/1
> View timeline at
>  https://chromium-cq-status.appspot.com/patch-timeline/1806443002/1

Any opinion on the comment I added?

[commit-bot: I haz the power, 2016-03-15 10:15:33]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-03-15 10:15:36]

Try jobs failed on following builders:
  v8_linux_rel_ng on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_linux_rel_ng/builds/2945)
  v8_linux_rel_ng_triggered on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_linux_rel_ng_triggered/b...)

[Toon Verwaest, 2016-03-15 10:19:27]

Sure, this code can be templatized and optimized more. This change by itself
gets rid of much of the unnecessary overhead. In particular, we shouldn't need
to use WriteToFlat for separators ever; we could just once check what it is and
remember it. I'm not sure it's worth the complexity right now though.

[Toon Verwaest, 2016-03-15 10:28:14]

The CQ bit was checked by verwaest@chromium.org

[Toon Verwaest, 2016-03-15 10:28:15]

The patchset sent to the CQ was uploaded after l-g-t-m from yangguo@chromium.org
Link to the patchset: https://codereview.chromium.org/1806443002/#ps20001
(title: "Don't index into buffer unless we're going to write")

[commit-bot: I haz the power, 2016-03-15 10:28:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1806443002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1806443002/20001

[commit-bot: I haz the power, 2016-03-15 10:47:51]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-03-15 10:48:57]

Description was changed from

==========
Speed up repeating the separator between elements in sparse array .join

BUG=
==========

to

==========
Speed up repeating the separator between elements in sparse array .join

BUG=

Committed: https://crrev.com/a20f793ff4394470aa781fc91b1e9a5770123e5a
Cr-Commit-Position: refs/heads/master@{#34778}
==========

[commit-bot: I haz the power, 2016-03-15 10:48:58]

Patchset 2 (id:??) landed as
https://crrev.com/a20f793ff4394470aa781fc91b1e9a5770123e5a
Cr-Commit-Position: refs/heads/master@{#34778}