Re: [PATCH] MdePkg: correct and clarify documentation of VA_LIST in Base.h

[Zenith432 <zenith432@users.sourceforge.net>]

Yes, that's right, in mixed ABI on GCC 7.2 (which is what GCC5 toolchain in tools_def.sample creates...), VA_COPY when 
used inside native (non-EFIAPI) function causes incorrect code generation that leads to crash (it mistreats the 
__builtin_ms_va_list as a __builtin_sysv_va_list).

The same source code works in current versions of LLVM and Apple clang - so it's a compiler-specific bug.

The text I wrote
 >> +//  -- VA_END, VA_ARG, VA_COPY, va_end, va_arg, and va_copy may be used
 >> +//     in functions of either calling conventions.

is the hypothetical desired condition, but it depends on compilers generating proper code in mixed ABI.  It obviously 
does not hold for current GCC.

However, the part of the documentation to add EFIAPI to ExampleVarArg should definitely be fixed.  Using 
__builtin_ms_va_list (=VA_LIST) in a sysv_abi (native) function probably yields a compile-time diagnostic, and if not - 
generates code that crashes for sure.

On 11/12/2017 04:35 PM, Gao, Liming wrote:
> Hi,
>    I see you and Laszlo are still in discussion on GCC behavior. Because there is one bug in GCC compiler, the following functions in edk2 must be EFIAPI. Right?
> 
> - functions that have a variable argument list and call VA_START / VA_END
> - functions that call VA_COPY
> 
> Thanks
> Liming
>> -----Original Message-----
>> From: Zenith432 [mailto:zenith432@users.sourceforge.net]
>> Sent: Sunday, December 10, 2017 11:43 PM
>> To: Gao, Liming <liming.gao@intel.com>; edk2-devel@lists.01.org
>> Cc: Kinney, Michael D <michael.d.kinney@intel.com>
>> Subject: Re: [PATCH] MdePkg: correct and clarify documentation of VA_LIST in Base.h
>>
>> On 10/12/2017 03:53 PM, Gao, Liming wrote:
>>> Could you add bug 457 link in the commit message?
>> ---
>> Subject: [PATCH] MdePkg: correct and clarify documentation of VA_LIST in Base.h
>>
>> This is to resolve bug 457.
>> https://bugzilla.tianocore.org/show_bug.cgi?id=457
>>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Zenith432 <zenith432 at users.sourceforge.net>
>> ---
>>     MdePkg/Include/Base.h | 24 ++++++++++++++++++++----
>>     1 file changed, 20 insertions(+), 4 deletions(-)
>>
>> diff --git a/MdePkg/Include/Base.h b/MdePkg/Include/Base.h
>> index 02140a5a..19f36872 100644
>> --- a/MdePkg/Include/Base.h
>> +++ b/MdePkg/Include/Base.h
>> @@ -560,13 +560,14 @@ struct _LIST_ENTRY {
>>     //  VA_LIST  - typedef for argument list.
>>     //  VA_START (VA_LIST Marker, argument before the ...) - Init Marker for use.
>>     //  VA_END (VA_LIST Marker) - Clear Marker
>> -//  VA_ARG (VA_LIST Marker, var arg size) - Use Marker to get an argument from
>> -//    the ... list. You must know the size and pass it in this macro.
>> +//  VA_ARG (VA_LIST Marker, var arg type) - Use Marker to get an argument from
>> +//    the ... list. You must know the type and pass it in this macro.
>>     //  VA_COPY (VA_LIST Dest, VA_LIST Start) - Initialize Dest as a copy of Start.
>>     //
>> -//  example:
>> +//  Example:
>>     //
>>     //  UINTN
>> +//  EFIAPI
>>     //  ExampleVarArg (
>>     //    IN UINTN  NumberOfArgs,
>>     //    ...
>> @@ -582,7 +583,7 @@ struct _LIST_ENTRY {
>>     //    VA_START (Marker, NumberOfArgs);
>>     //    for (Index = 0, Result = 0; Index < NumberOfArgs; Index++) {
>>     //      //
>> -//      // The ... list is a series of UINTN values, so average them up.
>> +//      // The ... list is a series of UINTN values, so sum them up.
>>     //      //
>>     //      Result += VA_ARG (Marker, UINTN);
>>     //    }
>> @@ -591,6 +592,21 @@ struct _LIST_ENTRY {
>>     //    return Result
>>     //  }
>>     //
>> +//  Notes:
>> +//
>> +//  This set of macros is intended to support variadic functions that
>> +//  use the EFIAPI calling convention.  Variadic functions that use a
>> +//  native calling convention should use stdarg.h.
>> +//  In particular:
>> +//  -- VA_START may only be used in a variadic EFIAPI function.
>> +//  -- va_start may only be used in a variadic native function.
>> +//  -- VA_START, VA_END, VA_ARG and VA_COPY may only be used on a VA_LIST.
>> +//  -- va_start, va_end, va_arg and va_copy may only be used on a va_list.
>> +//  -- Both VA_LIST or va_list may be passed as arguments to functions
>> +//     of either EFIAPI or native calling conventions.
>> +//  -- VA_END, VA_ARG, VA_COPY, va_end, va_arg, and va_copy may be used
>> +//     in functions of either calling conventions.
>> +//
>>
>>     /**
>>       Return the size of argument that has been aligned to sizeof (UINTN).
>> --
>> 2.14.3
>