efriedma added a comment. `atomicrmw add [...] sync_seq_cst` is supposed to be equivalent to `fence seq_cst; atomicrmw add seq_cst; fence seq_cst`, I think.
The extra fences aren't necessary for most uses of `__sync_*`, but the difference is theoretically visible for exotic synchronization primitives. At least the gcc developers thought the difference might be visible. They therefore decided to add the extra barriers to `__sync_*` on AArch64 (and maybe other targets?). As far as I know, the issue is entirely theoretical; nobody has demonstrated any issue on actual hardware, even with a synthetic testcase. If we do decide we need to do something here, I'd prefer to model this using explicit fences in the IR, as opposed to a new atomic ordering. Adding extra atomic orderings just makes everything harder to reason about. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D129802/new/ https://reviews.llvm.org/D129802 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
