On 08/14/2017 01:12 PM, tha...@chromium.org wrote:
Hi,

we (Chromium) are also happy to answer questions if there's interest. We've 
looked at most of these issues in some detail.
Thanks Nico, much appreciated!

(For the record, we have already gotten a lot of help from the Google compiler folks with the LLVM side fixes for the bugs that we discovered while porting Firefox to build with clang-cl. Getting to where we are now would certainly not be possible without that help! I wish I had done a better job at compiling a full list of all of those contributions over the years...)
In bullet points:
* Correctness: You might have some UB here and there but I wouldn't expect this 
to be a big problem.
Yes, we have indeed found and fixed bugs in our Windows specific code where MSVC has been too lenient in accepting bad C++ code, e.g. https://bugzilla.mozilla.org/show_bug.cgi?id=1251226.

* Performance: We switched from msvc+pgo to clang without pgo and got 
comparable perf. We did have to use an order file (/order: flag to link.exe) to 
get comparable startup perf.
That is very interesting! This is one of the aspects that we have been worried about a lot. We should probably also think about using /order as well.

Does Chromium plan to switch to use clang with PGO on Windows by any chance?
*Debuggability: Basically works, see blockers of https://crbug.com/636111 for 
in-progress work. link.exe can produce pdbs with clang's codeview debug info.
Wow, it looks like things have improved quite a bit on this front since the last time I looked at this closely. Really impressive work!
-Z7 and -Zi are aliased to each other in clang-cl, we don't do mspdbsrv)
I think this should be sufficient for Firefox's needs as well.

Cheers,
Ehsan
You can find us on #chromium on freenode, or over email.

Nico
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