ast added inline comments.
================
Comment at: llvm/test/CodeGen/BPF/atomics_2.ll:14
+; int test_xchg_64(long *p, long v) {
+; return __sync_lock_test_and_set(p, v);
+; }
----------------
yonghong-song wrote:
> ast wrote:
> > test_and_set is not the same as xchg.
> > xchg doesn't do comparison.
> I am looking at here:
> https://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Atomic-Builtins.html
> which mentions:
> type __sync_lock_test_and_set (type *ptr, type value, ...)
> This builtin, as described by Intel, is not a traditional test-and-set
> operation, but rather an atomic exchange operation. It writes value into
> *ptr, and returns the previous contents of *ptr.
>
> Many targets have only minimal support for such locks, and do not support
> a full exchange operation. In this case, a target may support reduced
> functionality here by which the only valid value to store is the immediate
> constant 1. The exact value actually stored in *ptr is implementation defined.
>
> This builtin is not a full barrier, but rather an acquire barrier. This
> means that references after the builtin cannot move to (or be speculated to)
> before the builtin, but previous memory stores may not be globally visible
> yet, and previous memory loads may not yet be satisfied.
>
> So it does not do compare.
>
> Or alternatively for llvm atomic builtin,
> https://llvm.org/docs/Atomics.html
>
> We have:
> iN __atomic_load_N(iN *ptr, iN val, int ordering)
> void __atomic_store_N(iN *ptr, iN val, int ordering)
> iN __atomic_exchange_N(iN *ptr, iN val, int ordering)
> bool __atomic_compare_exchange_N(iN *ptr, iN *expected, iN desired, int
> success_order, int failure_order)
>
> But as I mentioned in bpf office hour meeting, a "ordering" is required and I
> do not know how to deal with it.
Thanks for the info. gcc builtin has a misleading name.
Please mention this quirk in the tests comment and in the commit log.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D72184/new/
https://reviews.llvm.org/D72184
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