regparm

The tracking issue for this feature is: https://github.com/rust-lang/rust/issues/131749.

Option -Zregparm=N causes the compiler to pass N arguments in registers EAX, EDX, and ECX instead of on the stack for "C", "cdecl", and "stdcall" fn. It is UNSOUND to link together crates that use different values for this flag. It is only supported on x86.

It is equivalent to Clang's and GCC's -mregparm.

Supported values for this option are 0-3.

Implementation details: For eligible arguments, llvm inreg attribute is set.