lock/lock_free decisions in compiled time
Summary:
Enables atomic.c in compiler_rt and forces clang to not emit a call for runtime
decision about lock/lock_free. At compiling time, if clang can't decide if
atomic operation can be lock free, it emits calls to external functions like
`__atomic_is_lock_free`, `__c11_atomic_is_lock_free` and
`__atomic_always_lock_free`, postponing decision to a runtime check. According
to LLVM code documentation, the mechanism exists due to differences between
x86_64 processors that can't be decided at runtime.
On PowerPC and PowerPCSPE (32 bits), we already know in advance it can't be lock
free, so we force the decision at compile time and avoid having to implement it
in an external library.
This patch was made after 32 bit users testing the PowePC32 bit ISO reported
llvm could not be compiled with in-base llvm due to `__atomic_load8` not
implemented.
Submitted by: alfredo.junior_eldorado.org.br
Reviewed by: jhibbits, dim
Differential Revision: https://reviews.freebsd.org/D22549
This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.
Updating the vendor area to match this layout is next.
