Re: [PATCH] x86: Optimize variable_test_bit()

[Peter Zijlstra]

On Fri, May 01, 2015 at 09:03:32AM -0700, Linus Torvalds wrote:
> On Fri, May 1, 2015 at 8:16 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Since test_bit() doesn't actually have any output variables, we can use
> > asm goto without having to add a memory clobber. This reduces the code
> > to something sensible:
> 
> Yes, looks good, except if we have anything that actually wants to use
> the value rather than branch on it. But a quick grep seems to show
> that the vast majority of them are all about just directly testing the
> result.
> 
> It worries me a bit that gcc now cannot pick the likely branch any
> more. It will always branch out for the bit being set. So code like
> this:
> 
>     net/core/dev.c:         if
> (likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
> 
> wouldn't work, but almost all of those seem to be the constant case
> that doesn't get to this anyway.

Ah yes, that's another thing we've previously discussed with the GCC
people (IIRC). The GCC manual states you can use hot and cold attributes
on the labels (although when we tested that it didn't actually work, it
might now). But that's no good if the hint is one (or more) layer up
from the asm goto.

If would indeed be very good if the likely/unlikely thing would work as
expected.
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