On Wed, Mar 11, 2015 at 4:41 PM, <paul_kon...@dell.com> wrote:
>
>> On Mar 11, 2015, at 7:19 PM, Ian Lance Taylor <i...@google.com> wrote:
>>
>> On Wed, Mar 11, 2015 at 3:58 PM, David Wohlferd <d...@limegreensocks.com>
>> wrote:
>>>
>>> Why does gcc allow you to specify clobbers using numbers:
>>>
>>> asm ("" : : "r" (var) : "0"); // i386: clobbers eax
>>>
>>> How is this better than using register names?
>>>
>>> This makes even less sense when you realize that (apparently) the indices of
>>> registers aren't fixed. Which means there is no way to know which register
>>> you have clobbered in order to use it in the template.
>>>
>>> Having just seen someone trying (unsuccessfully) to use this, it seems like
>>> there is no practical way you can.
>>>
>>> Which makes me wonder why it's there. And whether it still should be.
>>
>> I don't know why it works. It should be consistent, though. It's
>> simply GCC's internal hard register number, which doesn't normally
>> change.
>>
>> I would agree that one should avoid it. I'd be wary of removing it
>> from GCC at this point since it might break working code.
>
> It certainly would. It’s not all that common, but I have seen this done in
> production code. Come to think of it, this certainly makes sense in machines
> where some instructions act on fixed registers.
>
> Register names would be nice as an additional capability.
Register names are supported.
Ian
