Re: [PATCH] x86/asm: avoid mnemonics without type suffix

Ramkumar Ramachandra
2013-07-14T18:26:51Z

Linus Torvalds wrote:
>> btrl $1, 0
>> btr $1, 0
>> btsl $1, 0
>> bts $1, 0
>
> What the heck is that supposed to show?

I was trying to show a reduced case where gas doesn't complain, but
llvm-mc does. Try compiling this with llvm-mc, and you'll get:

.text
btrl $1, 0
in.s:2:1: error: ambiguous instructions require an explicit suffix
(could be 'btrw', 'btrl', or 'btrq')
btr $1, 0
^
btsl $1, 0
in.s:4:1: error: ambiguous instructions require an explicit suffix
(could be 'btsw', 'btsl', or 'btsq')
bts $1, 0
^

Obviously, I misunderstood something major and screwed up the commit message.

> int main(int argc, char **argv)
> {
> asm("bt %1,%0":"=m" (**argv): "a" (argc));
> asm("bt %1,%0":"=m" (**argv): "a" ((unsigned long)(argc)));
> }

Right, so in:

int main(int argc, char **argv)
{
asm("bts %1,%0":"=m" (**argv): "r" (argc));
asm("btsl %1,%0":"=m" (**argv): "r" (argc));
asm("btr %1,%0":"=m" (**argv): "r" ((unsigned long)(argc)));
asm("btrq %1,%0":"=m" (**argv): "r" ((unsigned long)(argc)));
}

bts disambiguates to btsl, and btr disambiguates to btrq, as
advertised. Is it dependent on whether I have a 32-bit machine or
64-bit machine, or just on the operand lengths?

Either way, this is not a very enlightening example, because clang
also compiles this fine, and doesn't complain about any ambiguity. To
see the ambiguity I'm talking about, try to compile linux.git with
clang; I'll paste one error:

arch/x86/include/asm/bitops.h:129:15: error: ambiguous instructions
require an explicit suffix (could be 'btrw', 'btrl', or 'btrq')
asm volatile("btr %1,%0" : ADDR : "Ir" (nr));
^
<inline asm>:1:2: note: instantiated into assembly here
btr $0,(%rsi)
^

Since nr is an int, and ADDR is *(volatile long *), this should
disambiguate to btrl, right? Any clue why clang is complaining?