[patch] [libcxx] _LIBCPP_WEAK

[Howard Hinnant]

On Sep 25, 2013, at 9:42 AM, G M <gmisocpp at gmail.com> wrote:

> Hi Everyone
> 
> The attached patch is for libcxx's new.cpp and __config files. The patch's intent is to make new.cpp compile using MS's cl.exe compiler without changing the meaning of anything for any other compiler.
> 
> The issue this patch seeks to address is that MS's compiler (cl.exe) doesn't support the __attribute__((__weak__)) or __atribute__((__visibility__("default")) syntax; so a solution must be found where cl.exe doesn't see this syntax.
> 
> This patch seeks to solve this problem by changing code patterned like this:
> __attribute__((__weak__, __visibility__("default")))
> void* operator new(size_t size, const std::nothrow_t&) _NOEXCEPT { /*snip*/; return p; }
> 
> to code like this:
> _LIBCPP_WEAK
> void* operator new(size_t size, const std::nothrow_t&) _NOEXCEPT { return p; }
> 
> with the expectation that this change will NOT introduce any functionality change for clang++/g++ etc. That expectation is based on two aspects of the change:
> 
> * The first is the belief that cl.exe doesn't support "weak" in any documented way and that libcxx on Windows doesn't need it anyway. So _LIBCPP_WEAK is defined as nothing when cl.exe is the detected compiler.
> 
> For all other compilers, _LIBCPP_WEAK is defined to be just __attribute__((__weak__)) and nothing more).
> This should mean that cl.exe doesn't see the weak attribute syntax and so won't choke on it; and g++/clang++ will see the same weak attribute that it saw before this patch.
> 
> * The second part is what to do about __attribute__((_visibility__("default"))) as in the proposed change it is dropped from the function definition.
>  
> The expecatation here is that this is ok because it isn't neccessary because the prototype for the modified functions already have it; so the right thing should still happen.
> If all of this is correct, then this patch should fix new.cpp for cl.exe without changing anything else.
>  
> It also provides a pattern that will work with all the compilers libcxx already supports; and without having to introduce alternate #if/#else guards or other uglyness. This should make it better match the patterns libcxx already uses.
> If removing the "default" attribute turns out to be a problem, I believe the default attribute could be added back now that it is decoupled from the "weak" attribute (which I think is a good thing in of itself) by using one of libcxx's existing macro's such as _LIBCPP_FUNC_VIS / _LIBCPP_NEW_DELETE_VIS etc.
> 
> I'm not sure of the neccessity of LIBCPP_NEW_DELETE_VIS or it's realtionship to _LIBCPP_FUNC_VIS at this point, FWIW, but that doesn't matter to the logic of this patch.
> 
> I compiled this patch with cl.exe, g++ and clang++.exe.
> 
> Please let me know what you think. If this patch doesn't get traction, I'd appreciate some advice with real alternative code that could be used to advance things here as I found it hard to produce something actionable from the comments I received to my previous patch for this problem though I did and do appreciate the responses.
> 
> Thanks
> <libcxx_weak.diff>

Committed revision 192007.  See commit comments for minor modifications to the patch.

Thanks,
Howard