gdb on e.g. aarch64-unknown-linux-gnu/windows-gnu cannot print unused by-value non-immediate arguments

[saethlin]

This test does not work on aarch64: https://github.com/rust-lang/rust/blob/c9bd03cb724e13cca96ad320733046cbdb16fbbe/tests/debuginfo/by-value-non-immediate-argument.rs

Stripped down (a bit):

fn by_val_enum(x: (usize, usize, usize)) {
    zzz(); // #break
}

fn main() {
    by_val_enum((usize::MAX, usize::MAX, usize::MAX));
}

fn zzz() { }

Set a breakpoint on line 2 and try to print x. gdb says it is optimized out. This can be reproduced even with -Zmir-opt-level=0, and doesn't reproduce on x86_64. If the argument is Option<(usize, usize)>, gdb will try to read from address 0x0.

I'm not sure if this is a rustc bug or a gdb bug because if I add any use of the non-immediate argument inside the function, we gain some instructions that look like a function prelude, and gdb is able to print the argument. https://godbolt.org/z/GeGxE7ExE