[patch] [libcxx] _LIBCPP_WEAK

Reid Kleckner rnk at google.com
Wed Sep 25 12:18:57 PDT 2013

I'm OK with this because it helps libc++ build, but I can't yet see a path
forward for actually implementing replaceable operator new on Windows.

I agree that you can drop the visibility("default") bits from new.cpp,
because it's already in the header.  I would probably duplicate
_LIBCPP_NEW_DELETE_VIS on the definitions in new.cpp for consistency and
documentation, since the visibility of new and delete is one of the most
interesting things about it.

The changes to __config to detect RTTI and implement _LIBCPP_WARNING are
unrelated and shouldn't be in this patch.

On Wed, Sep 25, 2013 at 6:42 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:
> 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 /
> 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
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