On Thu, Sep 23, 2021 at 3:34 AM John Emmas via cfe-users < cfe-users@lists.llvm.org> wrote:
> On 22/09/2021 18:21, Reid Kleckner via cfe-users wrote: > > Looking back in the thread, I found the example code, and I see that > > MSVC refuses to inline this helper, but clang will inline it. I > > believe clang is permitted to inline it, MSVC will export the static > > data member (_the_keyboard), so everything should work as long as you > > provide a definition of _the_keyboard in the DLL and set -DBUILDING_DLL. > > > > Thanks for coming in on this, Reid. As Jeff suggested, I transferred the > question to llvm-dev where there's a small discussion going on now. I > must admit though, my personal view is that declaring something as > __declspec(dllimport) should automatically exclude it from being inlined > (I'm pretty sure that's what Microsoft itself does...) > > Ultimately, the main advantage of a DLL is that it offers dynamic > linkage. In other words, DLL features can be updated simply by updating > the DLL (i.e. without needing to update all the apps which use it). In > the dim old days this could lead to something called "DLL Hell" but > WinSxS has largely consigned that to history now. So whilst inlining > code from a DLL might seem like a good idea, it throws away the main > advantage of DLL's without offering anything better. > Presumably it offers the benefit of inlining optimizations - so there's probably some execution speed improvement to tradeoff. (& a way the user can address the issue by moving functions out of line) Over on llvm-dev I'm trying to persuade them that declaring something as > __declspec(dllimport) should automatically exclude it from being > inlined. And to be honest, I'd be quite surprised if that's not what > Microsoft intended. > I think if it's clearly demonstrated that that's Microsoft's implementation - that no matter how hard you ask it to optimize and how simple the function is, that it won't inline a dllexported function that's inline in a header (both implicitly inline in a class definition, and probably check the case of a standalone dllexported inline non-member function in a header) that I'd say (though I have little sway/weight in this design decision) clang-cl should implement the same behavior, because it is observable/can be relied upon as you have (though also - dllexported variables should be defined somewhere, generally) & an opt-in flag to what is the current behavior (dllexport-inlining). - Dave
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