[PATCH] D27575: [libcxxabi] Introduce an externally threaded libc++abi variant (take-2)

[Eric Fiselier via Phabricator via cfe-commits]

EricWF accepted this revision.
EricWF added a comment.
This revision is now accepted and ready to land.

LGTM. I'll re-review the mutexor changes post-commit.



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Comment at: CMakeLists.txt:121
 option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
+option(LIBCXXABI_HAS_EXTERNAL_THREAD_API
+  "Build libc++abi with an externalized threading API.
----------------
rmaprath wrote:
> EricWF wrote:
> > Maybe use a dependent option that sets it to the value of `LIBCXX_ENABLE_THREADS` when it is defined?
> Are you referring to `CMAKE_DEPENDENT_OPTION` macro? I don't see this macro used anywhere else in llvm, but I suppose that's not a concern?
> 
> Although, it would have to be something like:
> 
> ```
> CMAKE_DEPENDENT_OPTION(LIBCXXABI_HAS_EXTERNAL_THREAD_API "Externally Threaded libcxxabi" OFF  "LIBCXX_ENABLE_THREADS" OFF)
> ```
> 
> Because, `LIBCXX_ENABLE_THREADS=ON` should not mean `LIBCXXABI_HAS_EXTERNAL_THREAD_API=ON`.
> 
Ah OK. This looks good as is then.

Although it would be really nice if `LIBCXX_HAS_EXTERNAL_THREAD_API=ON` also enabled this option unless otherwise specified, but I don't see an easy way to do that. 


================
Comment at: src/fallback_malloc.cpp:37
 class mutexor {
 public:
----------------
rmaprath wrote:
> EricWF wrote:
> > Can't we replace this class with `std::mutex` directly?
> Again, I should've included more context to the patch :(
> 
> The `mutexor` here is supposed to degrade to a nop when threading is disabled. I think we cannot use `std::mutex` without threading support.
> 
> Will update the patch with more context.
We cannot use `std::mutex` without threading support but I would still rather use it.

I would `typedef std::lock_guard<std::mutex> mutexor` when threading is enabled and otherwise I would just `#ifdef` all usages away.

Also please add `_LIBCPP_SAFE_STATIC` to the heap_mutex declaration.


https://reviews.llvm.org/D27575