GCC and Clang need to mark functions that use SSE/AVX etc, either with a function attribute or a pragma around the function. strlen uses a template function that either uses SSE2 or AVX2. Previously the template was surrounded with pragmas to allow both SSE2 and AVX2, but that makes GCC assume that it can use AVX2 instructions even in the SSE2 version. To fix this the template instances are now build in individual compilation units for SSE2 and AVX, separate from the "dispatcher" function.
Now ucrtbase doesn't crash anymore on GCC build.
Another issue was the namespace around strnlen_mode, which has confused clang so much, that it forgot to instantiate the template code.
vcruntime contains the code that is linked into ucrtbase (in VS it is also provided as vcruntime140.dll)
vcstartup contains the code that is statically linked into executables that link to ucrtbase.dll. In Visual Studio this is part of msvcrt.lib (the import library for msvcrt), similar to our current msvcrtex, and it gets linked when you link to ucrtbase as well. The name is based on the folder name in the library.
Both libraries share some code, but each file is only compiled once.
Use #pragma section only for MSVC (and Clang-cl), because Clang doesn't like allocating non-const variables in a read-only section, while GCC doesn't understand these pragmas and ignores them.
This is because GCC will generate errors for both const_cast and static_cast depending on the template parameter:
- With const_cast: error: invalid 'const_cast' from type 'std::nullptr_t' to type 'const wchar_t**'
- With static_cast: error: invalid 'static_cast' from type 'wchar_t** const' to type 'const wchar_t**'
On GCC the code using extended ISA cannot be inlined and must either be marked with a function attribute or compiled with the proper target options ('-mavx2' on the command line or '#pragma GCC target("avx2")' inside the code)
