Can't designate registers in asm! macro

[edwardw]

The x86 instruction in requires specific registers as its operands, i.e. input in dx and output to ax.

#[crate_type="staticlib"];
#[feature(asm)];

#[no_mangle]
pub fn inb(port: u16) -> u8 {
  let data: u8;
  unsafe { asm!("inb $1,$0" : "=a" (data) : "d" (port) :: "volatile"); }
  data
}

But it seems that rustc can't allocate these registers for it:

$ rustc --version
rustc 0.9-pre
host: x86_64-apple-darwin
$ rustc -c asm.rs
error: couldn't allocate output register for constraint 'a'

Trying to relax the register allocation constraints simply leads to invalid assembly:

$ cat asm.rs
...
unsafe { asm!("inb $1,$0" : "=q" (data) : "q" (port) :: "volatile"); }
...
$ rustc -c asm.rs
<inline asm>:1:6: error: invalid operand for instruction
    inb %ax,%cl
LLVM ERROR: Error parsing inline asm

Why is it so? Is it a bug in rustc or in llvm? Any suggestion of where in the rust source code to look into would also be appreciated.

[alexcrichton]

Closing, because sadly this is more relevant to LLVM than rust (we just pass things through to LLVM)

I've gotten inb to work before with a function like this (unsure of how correct it actually is)

pub unsafe fn inb(port: u16) -> u8 {                                              
    let ret;                                                                      
    asm!("inb $1, $0" : "={ax}"(ret) : "{dx}"(port) :: "(eax),(edx)" : "volatile")
    return ret;                                                                   
}                                                                                 

[alexcrichton]

The best resource I've found so far for learning inline assembly is looking at the relevant tests inside of LLVM sadly, otherwise I know of no other good documentation for all the syntax.

[edwardw]

Thanks, Alex. This is good enough for me.

[edwardw]

Oh, it definitely works correctly. There's a boot-loader written in rust with inb / outb: https://github.com/edwardw/rumos/tree/master/boot

[carlpaten]

Alex: do you suggest looking at any tests in particular? There are quite a few.