Re: Using clang-cl to ship Windows builds

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Re: Using clang-cl to ship Windows builds

Chris Peterson-12
How does the performance of clang-cl builds compare to MSVC builds on
benchmarks like Speedometer?


On 2018-07-10 1:29 PM, David Major wrote:

> Bug 1443590 is switching our official Windows builds to use clang-cl
> as the compiler.
>
> Please keep an eye out for regressions and file a blocking bug for
> anything that might be fallout from this change. I'm especially
> interested in hearing about the quality of the debugging experience.
>
> It's possible that the patch may bounce and we'll go back and forth to
> MSVC for a while. You can check your build's compiler at
> `about:buildconfig`. Treeherder is running an additional set of MSVC
> jobs on mozilla-central to make sure we can fall back to a green MSVC
> if needed.
>
> Watch for more toolchain changes to come. The next steps after this
> will be to switch to lld-link and enable ThinLTO. That will open the
> door to a cross-language LTO that could inline calls between Rust and
> C++. In the longer term we can look into cross-compiling from Linux.
>
> But for now, shipping our most-used platform with an open-source
> compiler is a huge milestone in and of itself. Big thanks to everyone
> who has contributed to this effort on the Mozilla side, and also big
> thanks to the developers of LLVM and Chromium who helped make clang on
> Windows a realistic possibility.
> _______________________________________________
> firefox-dev mailing list
> [hidden email]
> https://mail.mozilla.org/listinfo/firefox-dev

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Re: Using clang-cl to ship Windows builds

David Major
At the moment, performance is a mixed bag. Some tests are up and some
are down. In particular I believe Speedometer is down a few percent.

Note however that clang-cl is punching above its weight. These builds
currently have neither LTO nor PGO, while our MSVC builds use both of
those. Any regressions that we're seeing ought to be short-lived. Once
we enable LTO and PGO, I expect clang to be a clear performance win.

If the short-term regressions end up being unacceptable, we can revert
to MSVC later in the release, but at this early point in the cycle
that shouldn't prevent us from collecting data on Nightly.

On Tue, Jul 10, 2018 at 4:31 PM Chris Peterson <[hidden email]> wrote:

>
> How does the performance of clang-cl builds compare to MSVC builds on
> benchmarks like Speedometer?
>
>
> On 2018-07-10 1:29 PM, David Major wrote:
> > Bug 1443590 is switching our official Windows builds to use clang-cl
> > as the compiler.
> >
> > Please keep an eye out for regressions and file a blocking bug for
> > anything that might be fallout from this change. I'm especially
> > interested in hearing about the quality of the debugging experience.
> >
> > It's possible that the patch may bounce and we'll go back and forth to
> > MSVC for a while. You can check your build's compiler at
> > `about:buildconfig`. Treeherder is running an additional set of MSVC
> > jobs on mozilla-central to make sure we can fall back to a green MSVC
> > if needed.
> >
> > Watch for more toolchain changes to come. The next steps after this
> > will be to switch to lld-link and enable ThinLTO. That will open the
> > door to a cross-language LTO that could inline calls between Rust and
> > C++. In the longer term we can look into cross-compiling from Linux.
> >
> > But for now, shipping our most-used platform with an open-source
> > compiler is a huge milestone in and of itself. Big thanks to everyone
> > who has contributed to this effort on the Mozilla side, and also big
> > thanks to the developers of LLVM and Chromium who helped make clang on
> > Windows a realistic possibility.
> > _______________________________________________
> > firefox-dev mailing list
> > [hidden email]
> > https://mail.mozilla.org/listinfo/firefox-dev
>
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