[LLVMdev] [announce] Chrome now uses clang for production builds on Linux

[Nico Weber]

Hi,

As of Chrome m38, which shipped to stable a bit over a month ago, we are
using clang as our production compiler for Linux – meaning that the bits we
ship to users on Linux are now created by clang. It's now also our default
host compiler for Chrome/Android builds (the target compiler is still gcc).
We used to use gcc 4.6 before. Maybe this is interesting to others, hence
this (somewhat belated) announcement.

We're using the same model we use on OS X: We use a pinned clang revision
without any changes (these days, we sometimes cherrypick a few newer
versions every now and then) that we update to ~trunk every few weeks.

The switch went off without larger problems, except one: The linux binaries
that clang produces by default don't run on older 32-bit debians, an issue
that we didn't find until our stable release. This is due to clang making
assumptions about alignment that aren't true on 32-bit in practice. We
worked around this and got an updated stable out relatively quickly, but
producing broken 32-bit binaries by default is a great opportunity for
improvement :-) I filed http://llvm.org/PR21414 for this.

With the switch to clang, binary size dropped about 8% while performance
mostly stayed the same. Startup time improved a bit, some microbenchmarks
got faster, others got slower (I debugged the biggest slowdown, that turned
out to be a benchmark bug). If you're interested
https://chromeperf.appspot.com/group_report?rev=282246 has details, ignore
all rows that aren't for "linux-release" or "chromium-rel-linux".
page_cycler.tough_layout_cases for example got 2.5% faster, which is nice.

Thanks everyone for all your work building such a great compiler!

Nico
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