Re: [PATCH] x86: Optimize variable_test_bit()

[Richard Henderson <rth@redhat.com>]

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 <vmakarov@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~