[cfe-dev] Inlining dllimport functions

[Hamza Sood via cfe-dev]

Would it break ABI compatibility if we let Clang inline things like this? What about putting it behind a Clang-specific optimisation flag?

> On 13 Sep 2018, at 21:29, Reid Kleckner wrote:
> 
> That would be a shame. I wouldn't want to throw away such an important code size optimization just because we can't export the things the inline functions reference.
> 
> I understand it's tough, though. libc++'s visibility and export macros have many consumers, many vendors, many linkage models, and basically too many cooks in the kitchen. I don't have any brilliant suggestions for solving that.
> 
>> On Thu, Sep 13, 2018 at 1:05 PM Hamza Sood <hamza_sood at me.com> wrote:
>> I’ve looked into it a bit more and it seems like the main optimisation barrier with libc++ on Windows is the extern template usage; it’s preventing a lot of trivial inlining. What about making _LIBCPP_DISABLE_EXTERN_TEMPLATE the default for Windows DLL builds?
>> 
>>> On 13 Sep 2018, at 17:23, Reid Kleckner wrote:
>>> 
>>> It would cause problems if 'f' in turn referenced functions that were neither inline nor exported:
>>> 
>>> bool not_exported();
>>> inline bool f() { return not_exported(); }
>>> __declspec(dllimport) inline bool g() { return f(); }
>>> 
>>> I'm pretty sure MSVC will not inline g in this or your example, so I would suggest adjusting dllimport usage patterns to something that satisfies both compilers rather than changing clang.
>>> 
>>>> On Thu, Sep 13, 2018 at 8:43 AM Hamza Sood via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>>> Currently, Clang won’t inline a dllimport function that references a non-dllimport function. This has a big performance impact on using libc++ on Windows because even something as simple as vector::data() won’t be inlined at the max optimisation level.
>>>> 
>>>> Would anything break if Clang were to allow inlining of a dllimport function that references a non-dllimport function that’s also declared inline?
>>>> 
>>>> E.g:
>>>> 
>>>> inline bool f() { return false; }
>>>> 
>>>> __declspec(dllimport) inline bool g() { return f(); }
>>>> 
>>>> bool h() {
>>>>   // can we inline this without issue?
>>>>   return g();
>>>> }
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