asm!: constraints to select specific registers are undocumented

[nodakai]

None of the "a" -- "d" constraints (to use the eax -- edx registers) works as expected.

#![feature(asm)]

fn main() {
    let x: u32;
    unsafe {
        asm!( "xor $0, $0" : "=r"(x) );
//      asm!( "xor $0, $0" : "=a"(x) );
    };
    println!("{}", x);
}

If I enable the commented-out line, I get:

error: couldn't allocate output register for constraint 'a' at line 20

But Clang/LLVM 3.0 (Ubuntu 12.04 package) understands them:

int main(void) {
    int x;
    __asm__ __volatile__ ( "xor %0, %0" : "=a"(x) );
    __asm__ __volatile__ ( "xor %0, %0" : "=b"(x) );
    __asm__ __volatile__ ( "xor %0, %0" : "=c"(x) );
    __asm__ __volatile__ ( "xor %0, %0" : "=d"(x) );
    return 0;
}

00000000004004c0 <main>:
  4004c0:       53                      push   %rbx
  4004c1:       31 c0                   xor    %eax,%eax
  4004c3:       31 db                   xor    %ebx,%ebx
  4004c5:       31 c9                   xor    %ecx,%ecx
  4004c7:       31 d2                   xor    %edx,%edx
  4004c9:       31 c0                   xor    %eax,%eax
  4004cb:       5b                      pop    %rbx
  4004cc:       c3                      retq   
  4004cd:       90                      nop
  4004ce:       90                      nop
  4004cf:       90                      nop

[huonw]

You can get specific registers via {...}, e.g. "={eax}"(x).

[nodakai]

@huonw Great! I thought it was a bug similar to #16337

[nagisa]

This needs A-inline-assembly.

[alexcrichton]

This is now documented!. We're just using LLVM syntax, so their documentation suffices for ours now at least.