Using clang-cl to ship Windows builds

David Major dmajor at
Tue Aug 21 22:02:37 UTC 2018

A quick update about performance: ThinLTO and PGO were enabled for our
clang-cl builds over the last few weeks. These optimizations have
cancelled the previous regressions and brought clang well ahead of
To call out a few: Speedometer scores increased by 6% on Win64 and 12%
on Win32, and the most dramatic improvement was displaylist_mutate at

As far as I'm aware, no blockers have come up and clang-cl is still
looking good for the 63 train.

On Tue, Jul 10, 2018 at 4:29 PM David Major <dmajor at> 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.

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