On 05/02/2015 05:39 AM, Peter Zijlstra wrote:
> On Fri, May 01, 2015 at 01:49:52PM -0700, Linus Torvalds wrote:
>> On Fri, May 1, 2015 at 12:02 PM, Vladimir Makarov <vmaka...@redhat.com>
>> wrote:
>>>
>>> GCC RA is a major reason to prohibit output operands for asm goto.
>>
>> Hmm.. Thinking some more about it, I think that what would actually
>> work really well at least for the kernel is:
>>
>> (a) allow *memory* operands (ie "=m") as outputs and having them be
>> meaningful even at any output labels (obviously with the caveat that
>> the asm instructions that write to memory would have to happen before
>> the branch ;)
>>
>> This covers the somewhat common case of having magic instructions that
>> result in conditions that can't be tested at a C level. Things like
>> "bit clear and test" on x86 (with or without the lock) .
>
> Would not something like:
>
> static inline bool __test_and_clear_bit(long nr, volatile unsigned long *addr)
> {
> bool oldbit;
>
> asm volatile ("btr %2, %1"
> : "CF" (oldbit), "+m" (*addr)
> : "Ir" (nr));
>
> return oldbit;
> }
>
> Be the far better solution for this? Bug 59615 comment 7 states that
> they actually modeled the flags in the .md file, so the above should be
> possible to implement.
>
> Now GCC can decide to use "sbb %0, %0" to convert CF into a register
> value or use "jnc" / "jc" for branches, depending on what
> __test_and_clear_bit() was used for.
>
> We don't have to (ab)use asm goto for these things anymore; furthermore
> I think the above will naturally work with our __builtin_expect() hints,
> whereas the asm goto stuff has a hard time with that (afaik).
>
> That's not to say output operants for asm goto would not still be useful
> for other things (like your EXTABLE example).
>
I agree that being able to model flags outputs, and thus minimize the amount of
code actually within the asm, is superior to the complexity of asm goto.
r~
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