[clang] [libc] [llvm] [NVPTX] Implement variadic functions using IR lowering (PR #96015)

[Tom Honermann via cfe-commits]

tahonermann wrote:

The change to `NVPTXTargetInfo::getBuiltinVaListKind()` in `clang/lib/Basic/Targets/NVPTX.h` caused a regression in Intel's downstream Clang-based compiler when compiling SYCL code with device compilation targeting NVPTX. CUDA might be similarly impacted, but I haven't verified. There is no need to revert the change; we've backed out the relevant part of the change in our downstream fork for now. But we need to figure out a proper solution for the problem as described below.

SYCL compilation uses the host standard library headers for both host and device compilation. Microsoft's standard library headers define `va_list` as `char*` as shown at line 72 below:
```
...\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\vadefs.h:
 67 #ifndef _VA_LIST_DEFINED
 68    #define _VA_LIST_DEFINED
 69    #ifdef _M_CEE_PURE
 70        typedef System::ArgIterator va_list;
 71    #else
 72        typedef char* va_list;
 73    #endif
 74 #endif
 ..
 97     void  __cdecl __va_start(va_list*, ...);
 98     void* __cdecl __va_arg(va_list*, ...);
 99     void  __cdecl __va_end(va_list*);
100 
101     #define __crt_va_start_a(ap, v) ((void)(__va_start(&ap, _ADDRESSOF(v), _SLOTSIZEOF(v), __alignof(v), _ADDRESSOF(v))))
102     #define __crt_va_arg(ap, t)     (*(t *)__va_arg(&ap, _SLOTSIZEOF(t), _APALIGN(t,ap), (t*)0))
103     #define __crt_va_end(ap)        ((void)(__va_end(&ap)))
```

The Clang driver interposes on Microsoft's `vadefs.h` header file by placing its own `vadefs.h` header file earlier in the header search path. This header overrides some of the macros defined by Microsoft's `vadefs.h` header file with ones that are intended to work with Clang's builtin variadic function support.
```
<llvm-project>/clang/lib/Headers/vadefs.h:
 18 #include_next <vadefs.h>
 19
 20 /* Override macros from vadefs.h with definitions that work with Clang. */
 ..
 34 /* VS 2015 switched to double underscore names, which is an improvement, but now
 35  * we have to intercept those names too.
 36  */
 37 #ifdef __crt_va_start
 38 #undef __crt_va_start
 39 #define __crt_va_start(ap, param) __builtin_va_start(ap, param)
 40 #endif
 41 #ifdef __crt_va_end
 42 #undef __crt_va_end
 43 #define __crt_va_end(ap)          __builtin_va_end(ap)
 44 #endif
 45 #ifdef __crt_va_arg
 46 #undef __crt_va_arg
 47 #define __crt_va_arg(ap, type)    __builtin_va_arg(ap, type)
 48 #endif
```

The result is that invocations of the `__crt_va_start`, `__crt_va_end`, and `__crt_va_arg` macros in Microsoft standard library headers end up passing objects of the Microsoft defined `va_list` type (aka, `char*`) to builtin functions like `__builtin_va_start()` that expect objects of type `__builtin_va_list` (aka, `void*`) thus leading to compilation failures when compiling in C++ modes.

There are at least a few options available to try to address this.

1. Revert the change to `NVPTXTargetInfo::getBuiltinVaListKind()` so that `TargetInfo::CharPtrBuiltinVaList` is returned. I don't know what motivated that particular change, so I don't know how feasible this option is.
2. Modify `NVPTXTargetInfo::getBuiltinVaListKind()` to conditionally return a value based on which standard library is being used. I'm not sure how feasible this is either. There are currently two targets that conditionally return different values from their `getBuiltinVaListKind()` implementations; Hexagon (see [here](https://github.com/llvm/llvm-project/blob/8fd9624cf729dd722a170a9dfd8f725966515231/clang/lib/Basic/Targets/Hexagon.h#L106-L110)] and ARM (see [here](https://github.com/llvm/llvm-project/blob/8fd9624cf729dd722a170a9dfd8f725966515231/clang/lib/Basic/Targets/ARM.cpp#L1096-L1101)).
3. Modify Clang's `vadefs.h` header file to also interpose on Microsoft's definition of `va_list` with a change like the following. This could still lead to problems if `_VA_LIST_DEFINED` is already defined, if `va_list` is already declared (as `char*`), or in code that assumes that `va_list` is defined as `char*` in Microsoft environments.
```
 +  #ifndef _VA_LIST_DEFINED
 +      #define _VA_LIST_DEFINED
 +      typedef __builtin_va_list va_list;
 +  #endif
 18 #include_next <vadefs.h>
```

Any thoughts on the above are much appreciated!

https://github.com/llvm/llvm-project/pull/96015