Justification around `nomem` specifier

[conradludgate]

This crate specifies nomem in the asm!(), and casts pointers to usize.

usdt/usdt-impl/src/no-linker.rs

Line 112 in 4329591

options(nomem, nostack, preserves_flags)

usdt/usdt-impl/src/common.rs

Line 182 in 4329591

quote! { .as_ptr() as usize },

---

The inline assembly docs specify

The assembly code does not read from or write to any memory accessible outside of the assembly code. This allows the compiler to cache the values of modified global variables in registers across execution of the assembly code since it knows that they are not read from or written to by it.

It's quite clear that reading from a pointer value when nomem is set is UB. The probe function would necessarily need to read the pointer when tracing.

Wouldn't it be more appropriate to use readonly?

Thanks

[ahl]

The definition of nomem is slightly ambiguous for this use case i.e. the assembly code certainly does not load or store to global variables at least for some definition of "the assembly code" and "global variables", but the example was more illustrative:

let mut x = 0i32;
let z: i32;
// Accessing outside memory from assembly when `nomem` is
// specified is disallowed
unsafe {
    core::arch::asm!("mov {val:e}, dword ptr [{ptr}]",
        ptr = in(reg) &mut x,
        val = lateout(reg) z,
        options(nomem)
    )
}

Yeah, I think readonly would be more appropriate. Please see #65 which covers this as well as other theoretical (i.e. un-observed) issues, if you intend to offer a PR.