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

[Eli Friedman via Phabricator via libcxx-commits]

efriedma added inline comments.


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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;
 
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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.


Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D114109