Re: [boost] [type_traits] Variadic function pointer with __fastcall calling convention

Subject: Re: [boost] [type_traits] Variadic function pointer with __fastcall calling convention
From: Stephan T. Lavavej (stl_at_[hidden])
Date: 2013-10-07 14:05:39

> If this is left out of a function specification, is not the 
> default calling convention user-specified, with __cdecl being the 
> default ?

Yes for ordinary functions (non-varargs, non-member). Member functions (non-varargs) always default to __thiscall. 

> So if the end-user changes the default to something other than 
> __cdecl is that considered the end-user's fault with VC++ ?

Our goal for VC's STL is to be immune to all of the calling-convention switches (/Gd, /Gr, /Gz, /Gv). I believe that 2013's <type_traits> has achieved this (except for /Gv, which we've fixed post-2013-RTM), although I am aware of bugs elsewhere (mostly in <functional>).

Because varargs functions (member and non-member) are always __cdecl, regardless of the calling-convention switches, it doesn't matter if you mark them as such, or leave them unmarked.

> I believe that clang wants to be compatible enough to VC++ on Windows to 
> be able to compile Windows header files, but still wants to follow the 
> C++ standard as closely as possible.

Well, calling conventions are non-Standard.

STL

-----Original Message-----
From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Edward Diener
Sent: Sunday, October 06, 2013 4:02 PM
To: boost_at_[hidden]
Subject: Re: [boost] [type_traits] Variadic function pointer with __fastcall calling convention

On 10/5/2013 4:02 PM, Stephan T. Lavavej wrote:
> [Edward Diener]
>> The problem from clang's perspective is that they must emulate this
>> "feature" <g> to compile Boost type_traits. Their developers feel that
>> Boost type_traits should not depend on this feature, ie. should not
>> specify any VC++ calling conventions for varargs function or function
>> template declarations.
>
> I agree with clang's devs (because that's what VC's <type_traits> does).

Thanks for the encouragement. I will quote you when I propose to John 
Maddock, the main Boost type_traits implementor, that Boost should so 
the same.

>
>> What I mean is that in VC++ the declarations are redundant, being the
>> same template redeclared.
>
> Right. So if you want to absorb all old-style varargs, you need only one overload/specialization.

It looks as if the overload specialization for C++ should specify 
__cdecl. If this is left out of a function specification, is not the 
default calling convention user-specified, with __cdecl being the 
default ? So if the end-user changes the default to something other than 
__cdecl is that considered the end-user's fault with VC++ ?

>
>> In this case clang either has to change to be like VC++
>
> Oh, I thought clang was attempting to be VC-compatible here (like our copy of EDG for Intellisense does).

I believe that clang wants to be compatible enough to VC++ on Windows to 
be able to compile Windows header files, but still wants to follow the 
C++ standard as closely as possible. Of course it is a bit of a 
tightrope act because 3rd party libraries for Windows which test for 
VC++ using _MSC_VER and other VC++ macros which clang on Windows 
supports, may want to be able to use clang to compile the code also.

>
> If clang is supposed to be VC-compatible, then they should treat varargs with calling conventions like VC does, and Boost should do what VC <type_traits> does (i.e. one overload/specialization with no calling convention - or __cdecl would work, I think).

It seems as if __cdecl is better than no calling convention, because the 
end-user has the ability to change the default calling convention on VC++.

>
> If clang wants to be different from VC, then Boost should do something different for clang and VC.

I do not work developing clang but I am just trying to compile Boost 
code with the clang toolset.

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