asm! macro does not mark variable as used

[Soveu]

I tried this code:

unsafe fn rep_movsb(mut dest: *mut u8, mut src: *const u8, mut n: usize) -> *mut u8 {
    while n != 0 {
        asm!(
            "rep movsb",
            inout("rcx") n,
            inout("rsi") src,
            inout("rdi") dest,
        );
    }

    return dest;
}

I expected to see this happen:
Compiler doesn't generate any warnings

Instead, this happened:
Rustc says that src is unused, even though it is declared as inout in asm.
Adding arrows (inout("rsi") src => src) fixes it

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (8dae8cdcc 2020-10-12)
binary: rustc
commit-hash: 8dae8cdcc8fa879cea6a4bbbfa5b32e97be4c306
commit-date: 2020-10-12
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly
LLVM version: 11.0

[oliviacrain]

Interesting bug! I used cargo bisect-rustc to determine that this bug has been around since the commit introducing the asm! syntax.

Looking at the MCVE case from above and doing some debugging, I found that for all live nodes in rep_movsb , src is never marked as being used in the RWU table. It only gets marked as read/written. Later on, this is part of the information used to determine that src is unused. That doesn't really make much sense though- the definition of a use does seem to fit the case where a variable is used as both an input and an output to some arbitrary assembly instruction.

I'll go ahead and claim this. I have a solution in mind, just going to ask around zulip first before putting out a PR.

@rustbot claim