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asm!: constraints to select specific registers are undocumented #17885
Description
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
Activity
huonw commentedon Oct 9, 2014
You can get specific registers via
{...}, e.g."={eax}"(x).nodakai commentedon Oct 9, 2014
@huonw Great! I thought it was a bug similar to #16337
[-]x86_64 asm!: some types of constraints don't work[/-][+]asm!: constraints to select specific registers are undocumented[/+]nagisa commentedon Dec 21, 2014
This needs
A-inline-assembly.alexcrichton commentedon Sep 28, 2015
This is now documented!. We're just using LLVM syntax, so their documentation suffices for ours now at least.
Auto merge of rust-lang#17885 - Wilfred:op_queue_docs, r=lnicola