[llvm-dev] Preparing BOLT for LLVM monorepo

[Martin Storsjö via llvm-dev]

On Wed, 3 Nov 2021, Maksim Panchenko via llvm-dev wrote:

> The Windows build was disabled due to the usage of threads, IIRC.
> Although, we don't use them directly, only via the C++11 standard
> library. I don't have access to a windows box to test it myself.

C++11 threads should be usable on Windows too, in general.

If building with GCC/libstdc++, there's two compiler configurations, the 
"win32 thread model" which doesn't provide the full C++11 thread support, 
and the "posix thread model" which is configured to run libgcc and 
libstdc++ on top of libwinpthread, which does support C++11 threads. I 
suspect this is what you've run into.

The mingw cross compilers in debian/ubuntu ship both configurations, but 
default to the "win32 thread model" without libwinpthread. There, you can 
choose the other one by invoking e.g. "x86_64-w64-mingw32-g++-posix", i.e. 
there are compiler frontends with -win32 and -posix suffixes in addition 
to the default.

I think the LLVM codebase also uses some bits of C++11 threads/mutexes 
somewhere, because I remember that cross compiling LLVM with the 
debian/ubuntu provided mingw compilers requires using the -posix suffixed 
tools. So with that in mind, this shouldn't be any additional limitation 
compared with what LLVM requires already.

Also, if building for Windows with a toolchain that uses libc++ (which is 
also usable in mingw configurations) or MSVC STL, then there's no 
ambiguity, C++11 threads are always available.

// Martin