[cfe-dev] BUG: complete misunterstanding of the MS-ABI

[Stefan Kanthak via cfe-dev]

"Jean-Daniel" <mailing at xenonium.com> wrote:

>> Le 3 sept. 2020 à 23:25, Stefan Kanthak via cfe-dev <cfe-dev at lists.llvm.org> a écrit :
>>
>> "Anton Korobeynikov" <anton at korobeynikov.info> wrote:
>>
>>>> Since MSVC does NOT support an (unsigned) __int128 data type, __uint128_t
>>   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>> is a user-defined type there, for which the cited specs clearly state how
>>   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>> to return it.
>>>>
>>> WRONG USER INTERPRETATION:  __uint128_t is not a user-defined type.
>>
>> OUCH!
>> Which part of my underlined words are not understood?
>> In the MS-ABI, it is an user-defined type.
>> clang claims to be ABI-compatible, but it's implementations fails
>> to support that claim!
>>
>
> If MSVC does not support __int128, how could compile code that use it in the first place ?

Please read my initial post, CAREFULLY: it shows the source I used.

>> You probably define a custom type (user defined).

Yes, with MSVC: see above!

> The fact that you choose to use an other (incompatible) type when compiling with
> clang (a compiler define __int128) can hardly be considered a compiler bug.

Don't try to make up any "facts": READ the source I posted!

LLVM/clang CLAIMS(!) to be ABI-compatible with MSVC, for example there
<https://blog.llvm.org/2018/03/clang-is-now-used-to-build-chrome-for.html>

| Clang is the first-ever open-source C++ compiler that's ABI-compatible with
| Microsoft Visual C++ (MSVC) - meaning you can build some parts of your program
| (for example, system libraries) with the MSVC compiler ("cl.exe"), other parts
| with Clang, and when linked together (either by MSVC's linker, "link.exe", or
| LLD, the LLVM project's linker - see below) the parts will form a working program.

Now read the specification of the Microsoft ABI
<https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention>

| User-defined types can be returned by value from global functions and
| static member functions. To return a user-defined type by value in RAX,
| it must have a length of 1, 2, 4, 8, 16, 32, or 64 bits. [...]
| Otherwise, the caller must allocate memory for the return value and
| pass a pointer to it as the first argument. The remaining arguments
| are then shifted one argument to the right. The same pointer must be
| returned by the callee in RAX.

> Just use the same user defined type with clang, and you will get the right behaviour.

Again: start to read what I already posted!
I use clang's own __uint128_t data type and show the assembly it generates,
which is but INCOMPATIBEL with the MS-ABI.

> That the kind of issue you get when defining custom type using compiler
> reserved name (double underscore).

OUCH: read again what I wrote, CAREFUL, especially the source I posted.
I don't use a compiler reserved name!

Stefan