Clang in VS 17.7p1 doesn't grok _addcarry_u64/_subborrow_u64

[CaseyCarter]

The compiler is emitting calls as if these were separately-compiled functions instead of intrinsics that it should expand inline. This is not the case when installing LLVM 16.0.1 directly, so I suspect some kind of corruption of the VS in-box Clang. We need to figure out what's going on here, and communicate with the MS-internal folks who handle the in-box Clang to get it straightened out before 17.7 ships.

EDIT:
This bug has the same root cause as DevCom-10369658/VSO-1822920 "Clang 16.0 offsetof behavior difference". The issue is in how VS packages LLVM.

[CaseyCarter]

Compiling this file (from DevCom-10369658):

#include <cstddef>

struct X { int a; int b; };

static_assert(offsetof(X, b) == 4);

with clang-cl /c /std:c++20 derp.cpp /showIncludes produces:

• for the Clang 15.0.1 included in VS 17.6:

  Note: including file: c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\cstddef
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\yvals_core.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\vcruntime.h
  Note: including file:    c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\sal.h
  Note: including file:     c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\concurrencysal.h
  Note: including file:    c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\Llvm\x64\lib\clang\15.0.1\include\vadefs.h
  Note: including file:     c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\vadefs.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\xkeycheck.h
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\Llvm\x64\lib\clang\15.0.1\include\stddef.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\Llvm\x64\lib\clang\15.0.1\include/__stddef_max_align_t.h
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.36.32532\include\xtr1common
  

• for the Clang 16.0.1 included in LLVM-16.0.1:

  Note: including file: c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\cstddef
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\yvals_core.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\vcruntime.h
  Note: including file:    c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\sal.h
  Note: including file:     c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\concurrencysal.h
  Note: including file:    C:\Program Files\LLVM\lib\clang\16\include\vadefs.h
  Note: including file:     c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\vadefs.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\xkeycheck.h
  Note: including file:  C:\Program Files\LLVM\lib\clang\16\include\stddef.h
  Note: including file:   C:\Program Files\LLVM\lib\clang\16\include/__stddef_max_align_t.h
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\xtr1common  ```
  

• for the Clang 16.0.1 in-box with VS17.7p1:

  Note: including file: c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\cstddef
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\yvals_core.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\vcruntime.h
  Note: including file:    c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\sal.h
  Note: including file:     c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\concurrencysal.h
  Note: including file:    c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\vadefs.h
  Note: including file:   c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\xkeycheck.h
  Note: including file:  C:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt\stddef.h
  Note: including file:   C:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt\corecrt.h
  Note: including file:  c:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.37.32705\include\xtr1common
  derp.cpp(5,15): error: static assertion expression is not an integral constant expression
  static_assert(offsetof(X, b) == 4);
                ^~~~~~~~~~~~~~~~~~~
  C:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt\stddef.h(47,31): note: expanded from macro 'offsetof'
          #define offsetof(s,m) ((::size_t)&reinterpret_cast<char const volatile&>((((s*)0)->m)))
                                ^
  derp.cpp(5,15): note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
  C:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt\stddef.h(47,32): note: expanded from macro 'offsetof'
          #define offsetof(s,m) ((::size_t)&reinterpret_cast<char const volatile&>((((s*)0)->m)))
                                ^
  1 error generated.
  

The vs17.7p1 Clang isn't including its override headers??!?

[CaseyCarter]

Aha! The bug is in the above: Clang 16 now expects to find its includes in $\lib\clang\16\include instead of $lib\clang\16.0.1\include where VS puts them.

[CaseyCarter]

The layout problem has been fixed for VS 2022 v17.7 Preview 3. I'm going to leave this issue open as a reminder for us to remove the workarounds when 17.7p3 ships.