[cfe-dev] [libcxx] atomic header and low-level intrinsics

[Al Grant]

Hi,

I've been looking at libcxx's <atomic> and experimenting with an implementation
of the low-level intrinsics it uses, as described at

  http://libcxx.llvm.org/atomic_design_a.html

where the intrinsics include

  type __atomic_load(const type* atomic_obj, int mem_ord);

These are similar to but incompatible with the new GCC atomic intrinsics:

  http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

which include

  type __atomic_load_n (type *ptr, int memmodel);
  void __atomic_load (type *ptr, type *ret, int memmodel);

Is there any reason not to move libcxx <atomic> to use these GCC intrinsics,
and have clang implement them?

Also, I noticed a couple of minor issues with libcxx <atomic> and its tests:

The pointer operations e.g.

    _Tp* fetch_add(ptrdiff_t __op, memory_order __m = memory_order_seq_cst)
        {return __atomic_fetch_add(&this->__a_, __op, __m);}

don't seem to match the defined intrinsics:

  void* __atomic_fetch_add(void** atomic_obj, ptrdiff_t operand, int mem_ord);

Either the low-level intrinsics need to be polymorphic, or <atomic> needs
some casts and also needs to scale the ptrdiff_t value.

Finally, various libcxx atomic tests attempt to instantiate atomic<> on a
struct A which is not trivially-copyable and hence not valid (29.5#1 in N3290).
Is libcxx intending its <atomic> to support such types on all platforms?

Al

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