jyknight added a comment. In D97224#2621328 <https://reviews.llvm.org/D97224#2621328>, @rjmccall wrote:
> In D97224#2604537 <https://reviews.llvm.org/D97224#2604537>, @jyknight wrote: > >>> I'm less certain about what to do with the `__atomic_*` builtins >> >> The `__atomic` builtins have already been supporting under-aligned pointers >> all along -- and that behavior is unchanged by this patch. > > How so? Clang hasn't been passing down an alignment, which means that it's > been building `atomicrmw` instructions with the natural alignment for the IR > type, which means we've been assuming that all pointers have a least that > alignment. The LLVM documentation even says that `atomicrmw` doesn't allow > under-alignment. We construct a libcall to `__atomic_*` routines in the frontend upon seeing an underaligned argument, instead of letting the backend handle it -- there's a bunch of code at https://github.com/llvm/llvm-project/blob/bc4a5bdce4af82a5522869d8a154e9e15cf809df/clang/lib/CodeGen/CGAtomic.cpp#L790 to handle that. I'd like to rip most of that out in the future, and just let the backend handle it in more cases. E.g. typedef int __attribute__((aligned(1))) unaligned_int; bool cmpxchg_u(unaligned_int *x) { int expected = 2; return __atomic_compare_exchange_n(x, &expected, 2, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); } generates a libcall to `__atomic_compare_exchange_4` (in IR, generated in the Clang code), instead of creating a cmpxchg IR instruction, due to the under-alignment. (Although, sigh, I've only just noticed we actually have a problem here -- the `__atomic_*_SIZE` libcalls are supposed to require an aligned argument -- so we should be using `__atomic_compare_exchange` (without size suffix) instead. Gah.) Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D97224/new/ https://reviews.llvm.org/D97224 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
