[Android] Avoid unnecessray ObserverList compacting

[Android] Avoid unnecessray ObserverList compacting

Currently, when an ObserverList iterator is rewound or reaches the end
of the list, it will trigger list compacting. However, the majority of
the time, no compacting is required as no elements were removed.

Change the policy to only compact if an element was removed during
list iteration, avoiding unnecessary O(N) list walks.

BUG=440134
Committed: https://crrev.com/4fd0d810432901cfacf58ca054a11f9fd91b6dc5
Cr-Commit-Position: refs/heads/master@{#317725}

[jdduke (slow), 2015-02-23 18:24:54]

jdduke@chromium.org changed reviewers:
+ nyquist@chromium.org

[jdduke (slow), 2015-02-23 18:24:55]

nyquist@: PTAL, thanks. This isn't really a bottleneck, but it does occur
relatively frequently and it just happens that none of our observer lists are
super long.

[nyquist, 2015-02-24 00:26:49]

lgtm

https://codereview.chromium.org/943303005/diff/1/base/android/java/src/org/ch...
File base/android/java/src/org/chromium/base/ObserverList.java (right):

https://codereview.chromium.org/943303005/diff/1/base/android/java/src/org/ch...
base/android/java/src/org/chromium/base/ObserverList.java:118: mNeedsCompact =
true;
Nit: Is always setting this field in the loop cheaper than doing
mNeedsCompact |= size != 0;
before the loop?

[jdduke (slow), 2015-02-24 00:31:52]

https://codereview.chromium.org/943303005/diff/1/base/android/java/src/org/ch...
File base/android/java/src/org/chromium/base/ObserverList.java (right):

https://codereview.chromium.org/943303005/diff/1/base/android/java/src/org/ch...
base/android/java/src/org/chromium/base/ObserverList.java:118: mNeedsCompact =
true;
On 2015/02/24 00:26:48, nyquist wrote:
> Nit: Is always setting this field in the loop cheaper than doing
> mNeedsCompact |= size != 0;
> before the loop?

Hmm, not sure, that suggestion looks good. Done.

[jdduke (slow), 2015-02-24 00:32:01]

The CQ bit was checked by jdduke@chromium.org

[jdduke (slow), 2015-02-24 00:32:02]

The patchset sent to the CQ was uploaded after l-g-t-m from nyquist@chromium.org
Link to the patchset: https://codereview.chromium.org/943303005/#ps20001 (title:
"Code review")

[commit-bot: I haz the power, 2015-02-24 00:33:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/943303005/20001

[commit-bot: I haz the power, 2015-02-24 01:34:22]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-24 01:35:15]

Patchset 2 (id:??) landed as
https://crrev.com/4fd0d810432901cfacf58ca054a11f9fd91b6dc5
Cr-Commit-Position: refs/heads/master@{#317725}