[cfe-dev] [libc++] link errors on Windows

[Shoaib Meenai via cfe-dev]

Even though basic_string::data is marked __forceinline, clang (and I believe
it emulates cl's behavior here) won't inline dllimport functions which call
non-dllimport functions [1]. Over here, the call to _VSTD::__to_raw_pointer
will prevent inlining. (This is a pretty unnecessary pessimization in this
case, since __to_raw_pointer is a trivial inline function, and we can try to
make it better, but that's a different topic.)

The function you're getting a link error against demangles to the *non-const*
data overload, which is only enabled when _LIBCPP_STD_VER > 14. I believe
what's happening is that you compiled libc++ with _LIBCPP_STD_VER <= 14, so
the non-const overload won't be present in the library, but then you're using
the headers with _LIBCPP_STD_VER > 14, so the compiler will attempt to use the
non-const overload, which won't be inlined as discussed above, and also won't
be present in the library, hence the link error. There's no non-const overload
of c_str, and c_str will call the const overload of data, which is why it
fixes the issue you're seeing.

You should ensure your C++ standard version is consistent across the build of
libc++ and its uses. I'm adding Marshall and Eric in case they have thoughts
on libc++ headers being used with a different C++ standard version than the
library was compiled with, since the problem over here is a Windows-specific
issue.

[1] https://reviews.llvm.org/diffusion/L/browse/cfe/trunk/lib/CodeGen/CodeGenModule.cpp;311991$1871


On 8/28/17, 11:17 PM, "cfe-dev on behalf of Hamza Sood via cfe-dev" <cfe-dev-bounces at lists.llvm.org on behalf of cfe-dev at lists.llvm.org> wrote:

    I’ve built libc++ on Windows as shown in the documentation, but I’m having trouble linking a simple program.
    
    #include <string>
    int main() {
      std::string s = “Hello world”;
      const char *data = s.data();
    }
    
    LLD fails to link with the following error: undefined symbol: __imp_?data@?$basic_string at DU?$char_traits at D@__1 at std@@V?$allocator at D@23@@__1 at std@@QEAAPEADXZ.
    MSVC link.exe errors in the same way.
    
    Interestingly it works fine if s.data() is changed to s.c_str().
    
    Am I doing something wrong or is this a bug?
    It looks like the string header marks basic_string::data as having inline visibility, so I’m not sure why this doesn’t work.
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