offsetof() is not constant in clang-cl

[danakj]

I use offsetof in a template parameter, this works on all compilers (Clang, MSVC, GCC) except clang-cl.

error: non-type template argument is not a constant expression

C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\stddef.h(47,31): note: 
      expanded from macro 'offsetof'
  ...offsetof(s,m) ((::size_t)&reinterpret_cast<char const volatile&>((((s*)0)->m)))    
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    

Obviously reinterpret_cast is not constant evaluable. So I guess MSVC provides a builtin, but clang-cl does not?

[danakj]

Simple repro:

struct S { int i; };
constexpr auto x = offsetof(S, i);

error: 
      constexpr variable 'x' must be initialized by a constant expression
constexpr auto x = offsetof(S, i);
               ^   ~~~~~~~~~~~~~~
note: cast that
      performs the conversions of a reinterpret_cast is not allowed in a constant       
      expression
constexpr auto x = offsetof(S, i);
                   ^

[danakj]

MSVC working: https://godbolt.org/z/4jnvY9o9r
Clang working: https://godbolt.org/z/448G6W3x7
GCC working: https://godbolt.org/z/sn3v619q9

[efriedma-quic]

How are you invoking clang-cl? In the past, we made this work by making clang-cl use its own stddef.h, which has the proper definition of offsetof.

Apparently recent SDKs guard the definition of offsetof with a define _CRT_USE_BUILTIN_OFFSETOF; if you define that, it uses __builtin_offsetof just like clang's stddef.h. Maybe clang-cl should be defining that.

[danakj]

I am invoking it by constructing a ClangTool and invoking tool.buildASTs(), on Windows with LLVM/Clang and my tool compiled by MSVC.

I am running my tool against a compile database, and here's an example CommandLine from the compile database:

C:\PROGRA~1\MICROS~2\2022\COMMUN~1\VC\Tools\MSVC\1434~1.319\bin\Hostx86\x64\cl.exe \
    --driver-mode=cl /nologo /TP /DWIN32 /D_WINDOWS /GR /EHsc /Zi /Ob0 /Od /RTC1 -MDd \
    /Zc:preprocessor /wd5105 -std:c++20 /Fomy_input_file.obj /FdTARGET_COMPILE_PDB /FS \
    -c my_input_file.cc

[danakj]

Passing it to MSVC with /D_CRT_USE_BUILTIN_OFFSETOF makes the compiler unhappy.

[build] command line(1): error C2220: the following warning is treated as an error
[build] command line(1): warning C4117: macro name '_CRT_USE_BUILTIN_OFFSETOF' is reserved, '#define' ignored

Since I am using MSVC that means it will not be able to appear in my compile database. I will need to

1. insert in my library's CMakelists.txt only if building with clang-cl (so that clang-cl works with the lib)
2. also insert it in my tool unconditionally since the compdb may not be created by clang-cl.

It'd be nice if clang-cl did define that itself, if that's the answer.

[danakj]

I do confirm that adding /D_CRT_USE_BUILTIN_OFFSETOF to the command line inside my ClangTool does resolve this for my tool, would be great to see clang-cl address this itself.