Re: [PATCH] x86: Optimize variable_test_bit()

[Peter Zijlstra]

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@xxxxxxxxxx> 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).
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