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

Jeremy Fitzhardinge
2013-07-14T19:41:10Z

On 07/14/2013 12:30 PM, Tim Northover wrote:
>> And that is why I think you should just consider "bt $x,y" to be
>> trivially the same thing and not at all ambiguous. Because there is
>> ABSOLUTELY ZERO ambiguity when people write
>>
>> bt $63, mem
>>
>> Zero. Nada. None. The semantics are *exactly* the same for btl and btq
>> in this case, so why would you want the user to specify one or the
>> other?
> I don't think you've actually tested that, have you? (x86-64)
>
> int main() {
> long val = 0xffffffff;
> char res;
>
> asm("btl $63, %1\n\tsetc %0" : "=r"(res) : "m"(val));
> printf("%d\n", res);
>
> asm("btq $63, %1\n\tsetc %0" : "=r"(res) : "m"(val));
> printf("%d\n", res);
> }

Blerk. It doesn't undermine the original point - that gas can
unambiguously choose the right operation size for a constant bit offset
- but yes, the operation size is meaningful in the case of a immediate
bit offset. Its pretty nasty of Intel to hide that detail in Table 3-2,
far from the instructions which use it...

J

>
> Tim.
>