[libcxx-commits] [PATCH] D114109: [libc++] Enable <atomic> when threads are disabled

[Eli Friedman via Phabricator via libcxx-commits]

efriedma added inline comments.


================
Comment at: libcxx/include/atomic:2697
 typedef atomic<__libcpp_signed_lock_free> atomic_signed_lock_free;
 typedef atomic<__libcpp_unsigned_lock_free> atomic_unsigned_lock_free;
 
----------------
androm3da wrote:
> efriedma wrote:
> > androm3da wrote:
> > > After this commit landed downstream we encountered a failure building for an arm baremetal target that defines the __CLANG_ATOMIC_*_LOCK_FREE vals to '1' ("sometimes lock free").
> > > 
> > > This results in the `atomic_{,un}signed_lock_free` typedefs here referencing an undefined `_libcpp_{,un}signed_lock_free`.
> > > 
> > > I suspect that before this commit `_LIBCPP_HAS_NO_ATOMIC_HEADER` was false.
> > > 
> > > I think maybe we could consider an `#error:` in the else case above that might make it clearer?  
> > > 
> > > I could use some advice as to how to handle this -- does it make sense to have the atomic header in the case where these types are only designated 'sometimes lock free'?  If not I think the fix would be to change the conditional for _LIBCPP_HAS_NO_ATOMIC_HEADER to consider these?  Otherwise, if it does make sense to have an atomic header, can I just suppress these typedefs?  That does seem to address the build failure so far.
> > The standard suggests we should just suppress the typedefs, not the entire header.
> > 
> > From the current C++ standard, [compliance]p3:
> > 
> > > The supplied version of the header <atomic> shall meet the same requirements as for a hosted implementation except that support for always lock-free integral atomic types ([atomics.lockfree]) is implementation-defined, and whether or not the type aliases atomic_­signed_­lock_­free and atomic_­unsigned_­lock_­free are defined ([atomics.alias]) is implementation-defined. The other headers listed in this table shall meet the same requirements as for a hosted implementation.
> > The standard suggests we should just suppress the typedefs, not the entire header.
> > 
> > From the current C++ standard, [compliance]p3:
> > 
> > > The supplied version of the header <atomic> shall meet the same requirements as for a hosted implementation except that support for always lock-free integral atomic types ([atomics.lockfree]) is implementation-defined, and whether or not the type aliases atomic_­signed_­lock_­free and atomic_­unsigned_­lock_­free are defined ([atomics.alias]) is implementation-defined. The other headers listed in this table shall meet the same requirements as for a hosted implementation.
> 
> Ok, thanks, that clears it up.
> 
> @efriedma @ldionne should I post a change like this?
>  
> 
> ```
> --- a/libcxx/include/atomic
> +++ b/libcxx/include/atomic
> @@ -2691,10 +2691,13 @@ typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, char>::typ
>  typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, unsigned char>::type      __libcpp_unsigned_lock_free;
>  #else
>      // No signed/unsigned lock-free types
> +#define _LIBCPP_NO_LOCK_FREE_TYPES
>  #endif
>  
> +#if !defined(_LIBCPP_NO_LOCK_FREE_TYPES)
>  typedef atomic<__libcpp_signed_lock_free> atomic_signed_lock_free;
>  typedef atomic<__libcpp_unsigned_lock_free> atomic_unsigned_lock_free;
> +#endif
>  
>  #define ATOMIC_FLAG_INIT {false}
>  #define ATOMIC_VAR_INIT(__v) {__v}
> 
> ```
Looks reasonable to me.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D114109/new/

https://reviews.llvm.org/D114109