Fix libcxx build on musl

[raj.khem]

Summary

constexpr is not available on musl

[rengolin]

I'm adding a few library folks to make sure this is the correct behaviour. I honestly don't know.

cheers,
--renato

include/__config

397 ↗

(On Diff #50692)

Unnecessary whitespace change

[bcraig]

If I understand it correctly, GLIBC is defined in features.h, so this won't work.

I suspect that the build isn't broken. You likely just need to define LIBCXX_HAS_MUSL_LIBC on your cmake line. That will cause __config_site.in to #define _LIBCPP_HAS_MUSL_LIBC for you.

[raj.khem]

@bcraig, right, check for GLIBC can be moved after including features.h.
https://chromium.googlesource.com/native_client/pnacl-libcxx/+/master%5E!/
seems to add needed tweaks which can be enabled by passing -Dmusl in CFLAGS
may be it can be backported to 3.8 branch

[raj.khem]

3.8 has it in there. So may be this is just not required. I will add
-DLIBCXX_HAS_MUSL_LIBC=ON to Cmake and see what comes out

[bcraig]

The trouble is that features.h doesn't exist in musl (as far as I know). That's the biggest reason why _LIBCPP_HAS_MUSL_LIBC is needed.

[raj.khem]

I think my problem was that while compiling libcxxabi, it wants to peek into libcxx headers but then libcxxabi cmake infra doesnt have the musl support like libcxx. So Now I solved it by adding -D_LIBCPP_HAS_MUSL_LIBC to CXXFLAGS.

[EricWF]

@raj.khem Can you file a bug against libc++abi please?

[raj.khem]

libcxx still has problem compiling on musl/aarch64 though. I fixed it with this patch

https://github.com/kraj/meta-clang/blob/master/recipes-devtools/clang/libcxx/0001-use-constexpr-when-using-glibc.patch

does this make sense ?

[EricWF]

That patch looks hugely dangerous because you've now changed when your static mutex's get initialized. Please file a bug against libc++.

[bcraig]

In D18174#375813, @raj.khem wrote:

I think my problem was that while compiling libcxxabi, it wants to peek into libcxx headers but then libcxxabi cmake infra doesnt have the musl support like libcxx. So Now I solved it by adding -D_LIBCPP_HAS_MUSL_LIBC to CXXFLAGS.

When cross-compiling, I have found it to be best to build libcxx and libcxxabi as sub-projects of LLVM (i.e. I place libcxx under llvm/projects/libcxx). I can then configure libcxxabi and libcxx at once. When I get around to building, I specifically dodge building LLVM and/or clang by running "make cxxabi cxx".

If you go with that approach, I think all of the _LIBCPP_HAS_MUSL_LIBC stuff gets worked out, because libcxxabi looks at the __ config header that #includes __config_site.

[EricWF]

The method @bcraig is describing is documented here: http://libcxx.llvm.org/docs/BuildingLibcxx.html#id4

[EricWF]

This patch is not OK, since it affects way more that just MUSL libc.

Also can you please provide a reduced reproducer as two why pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER is not a constant expression with MUSL?

include/__mutex_base

51

Limiting constexpr to GLIBC implementations only is incorrect; you want to exclude MUSL.

Also MUSL is wrong for not allowing pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER to be a constant expression, and MUSL should fix that.

[smeenai]

include/__mutex_base

51

I've used __builtin_constant_p to wok around a similarly non-conforming PTHREAD_MUTEX_INITIALIZER (from pthread-win32, which defines it to be (void *)-1): see the last part of https://stackoverflow.com/a/10376574/382079. It's a terrible hack, but it works.

[raj.khem]

@EricWF you are right. I have made the changes accordingly. This patch can be ignored.

[bcraig]

include/__mutex_base

51

musl's pthread_mutex_t has volatile members if I recall correctly. It's hard to validate that statement without building musl, because the header that defines pthread_mutex_t is code generated.

[raj.khem]

this is no longer needed.

include/__mutex_base

 public:
     _LIBCPP_INLINE_VISIBILITY
 #ifndef _LIBCPP_CXX03_LANG
-    constexpr mutex() = default;
+#ifdef __GLIBC__
+    constexpr
+#endif
+    mutex() = default;
 #else
     mutex() _NOEXCEPT {__m_ = (__libcpp_mutex_t)_LIBCPP_MUTEX_INITIALIZER;}
 #endif

 public:
     _LIBCPP_INLINE_VISIBILITY
 #ifndef _LIBCPP_CXX03_LANG
-    constexpr condition_variable() _NOEXCEPT = default;
+#ifdef __GLIBC__
+    constexpr
+#endif
+    condition_variable() _NOEXCEPT = default;
 #else
     condition_variable() _NOEXCEPT {__cv_ = (__libcpp_condvar_t)_LIBCPP_CONDVAR_INITIALIZER;}
 #endif