On Tue, Mar 13, 2018 at 3:10 AM, Henri Sivonen <hsivo...@hsivonen.fi> wrote:
> On Tue, Mar 13, 2018 at 2:56 AM, Nathan Froyd <nfr...@mozilla.com> wrote:
>> (Our release builds use -O2 for Rust code.)
> What does cargo bench use by default?
> (https://internals.rust-lang.org/t/default-opt-level-for-release-builds/4581
> suggests -O3.)

As mentioned by Alexis, -O3 is indeed what gets used by default.

> That is, is cargo bench for a crate that's vendored into m-c
> reflective of that crate's performance when included in a Firefox
> release?

I do not know.  I know that folks working on WebRender were finding
that -O3 produces *much* better code for certain things; they filed
bugs for rustc that I can't find right now.  I don't know if those
bugs have been addressed.

When Stylo was getting off the ground a year ago, mbrubeck did some
-O2 vs. -O3 size analysis, which led to some -O2 vs. -O3 performance
analysis in https://bugzilla.mozilla.org/show_bug.cgi?id=1328954.  The
conclusion there is that -O3 wasn't worth it and came with a
reasonably large codesize increase.

I guess the answer is "probably, but you should measure to make sure"?

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