commit a5d33b00e277364ba0a49c81d5e4c16c83010f17
author yiyix <yiyix@chromium.org> Tue Sep 25 20:15:39 2018
committer Commit Bot <commit-bot@chromium.org> Tue Sep 25 20:15:39 2018
tree d063adfe0fe82b3312d3c8361121bcb49bd30535
parent 525b54bb14a47817a32b18a1e0d442e5b77ef0c6

FrameMetrics: Improve SquareRoot calculation Efficiency on Android

Enabling FrameMetrics caused 5-20% regression on rendering.
(ref: https://bugs.chromium.org/p/chromium/issues/detail?id=876709).
One of many reasons is that square root calculation took too long.
In this cl, I implemented Newton's method to approximate the square
root value.

I ran perftest on a Nexus 4 device and got the following result:
average of running square_root using std::sqrt: 1601-01-01 00:00:23.160 UTC
average of running square_root using fastsqrt: 1601-01-01 00:00:10.629 UTC
Conclusion: It's 2.3x faster with newton's approximation instead of
standard library.

I also ran perftests on linux desktop and got 3%-5% improvements.

(perftests ref: https://chromium-review.googlesource.com/c/chromium/src/+/1222676)
Bug: 881863

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I5feebc32053650f338a92a2e1db6f4f67b20f995
Reviewed-on: https://chromium-review.googlesource.com/1225105
Commit-Queue: Yi Xu <yiyix@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594077}