MSVC ZEND_FASTCALL not portable?

Description

When building with MSVC (or clang-cl), ZEND_FASTCALL is defined as __vectorcall. While __vectorcall is similar to the default x64 calling convention, and __fastcall on x86, respectively, there are notable differences. It seems to me that our JIT does not cater to these. That may still not be relevant to core functions, but could be an issue with external extensions which declare function as ZEND_FASTCALL.

Furthermore, there is definitely an issue regarding portability, since GCC doesn't support __vectorcall at all. While we do not necessarily support GCC on Windows, compatibility still seems to be a good thing.

Note that even __fastcall appears to be slightly different to the Windows __fastcall convention:

On x86-32 targets, the fastcall attribute causes the compiler to pass the first argument (if of integral type) in the register ECX and the second argument (if of integral type) in the register EDX. Subsequent and other typed arguments are passed on the stack.

The first two DWORD or smaller arguments that are found in the argument list from left to right are passed in ECX and EDX registers; all other arguments are passed on the stack from right to left.

This is possibly just a documentation issue (I haven't checked).

Anyhow, I suggest to change ZEND_FASTCALL with MSVC to use the default x64 calling convention for the x64 ABI; not sure what to suggest regarding x86.

cc @dstogov, @nielsdos, @arnaud-lb

PHP Version

PHP 8.4 (likely older)

Operating System

Windows

The issue names ZEND_FASTCALL, MSVC, clang-cl, GCC, and the JIT but no files or tests. Start by locating the ZEND_FASTCALL definitions and JIT calling-convention handling, then compare the relevant x64 and x86 ABIs. Done means establishing portable behavior for supported compilers and resolving the x86 convention question.