[clang] [clang][CodeGen] Emit atomic IR in place of optimized libcalls. (PR #73176)

[Rainer Orth via cfe-commits]

rorth wrote:

The libcalls per se wouldn't be a problem since on 32-bit Solaris/sparc `-latomic` is always linked due to that.  However, the `__sync_*` libcalls aren't defined in `libatomic.so`, thus the link failure.

https://github.com/llvm/llvm-project/pull/73176