On Sat, Aug 12, 2017 at 11:22:30PM -0400, Jeff Muizelaar wrote:
> On Sat, Aug 12, 2017 at 9:40 PM, Ehsan Akhgari <ehsan.akhg...@gmail.com> 
> wrote:
> > Last but not least, you may ask yourself why would we want to spend this
> > much effort to switch to clang-cl on Windows?  I believe this is an
> > important long term shift that is beneficial for us.  First and foremost,
> > clang is a vibrant open source compiler, and being able to use open source
> > toolchains on our most important platforms is really important for us in
> > terms of being able to contribute to the compiler where needed
> It's worth emphasizing the value of using an open source compiler.
> Being able to find and fix bugs in the compiler instead of having to
> work around them without knowing the true cause is enormously
> valuable. A recent example of this happened to me yesterday with
> https://bugzilla.mozilla.org/show_bug.cgi?id=1382857. Once I had
> reported the issue (https://bugs.llvm.org/show_bug.cgi?id=34163) a fix
> was committed to clang trunk in less than 6 hours. That's something
> not ever possible with MSVC.

That bugs can be fixed in a few hours is nice, but that's not the main
advantage. (also, not all bugs are fixed in a few hours. The one that
makes Firefox fail to build with current clang trunk has been open for 2
weeks and hasn't been fixed yet).

The main advantage is that you can take that fix, and apply it to your
compiler *right now*. As opposed to "whenever the vendor makes a new
release". We're regularly applying patches to clang and GCC.  We can't
do the same to MSVC, even when we file bugs to Microsoft and they fix

Another advantage is that if you're so inclined, you can fix the
it yourself.


PS: And once clang-cl+lld actually works, it will also be possible to
build for Windows on Linux.
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