Re: [PATCH v2 10/30] Add x86-specific parity functions

[Zeng Zhaoxiu]

å 2016å04æ06æ 18:13, Borislav Petkov åé:
> On Wed, Apr 06, 2016 at 05:14:45PM +0800, zengzhaoxiu@xxxxxxx wrote:
> > From: Zhaoxiu Zeng <zhaoxiu.zeng@xxxxxxxxx>
> >
> > Use alternatives, lifted from arch_hweight
> >
> > Signed-off-by: Zhaoxiu Zeng <zhaoxiu.zeng@xxxxxxxxx>
> > ---
> >   arch/x86/include/asm/arch_hweight.h |   5 ++
> >   arch/x86/include/asm/arch_parity.h  | 102 ++++++++++++++++++++++++++++++++++++
> >   arch/x86/include/asm/bitops.h       |   4 +-
> >   arch/x86/lib/Makefile               |   8 +++
> >   arch/x86/lib/parity.c               |  32 ++++++++++++
> >   5 files changed, 150 insertions(+), 1 deletion(-)
> >   create mode 100644 arch/x86/include/asm/arch_parity.h
> >   create mode 100644 arch/x86/lib/parity.c
> ...
>
> > +static __always_inline unsigned int __arch_parity32(unsigned int w)
> > +{
> > +	unsigned int res;
> > +
> > +	asm(ALTERNATIVE("call __sw_parity32", POPCNT32 "; and $1, %0", X86_FEATURE_POPCNT)
> > +		: "="REG_OUT (res)
> > +		: REG_IN (w)
> > +		: "cc");
> So why all that churn instead of simply doing:
>
> static __always_inline unsigned int __arch_parity32(unsigned int w)
> {
> 	return hweight32(w) & 1;
> }
>
> Ditto for the 64-bit version.

__sw_parity32 is faster than __sw_hweight32.
I don't know how many CPUs do not support the popc, if they are outdated,
use __arch_hweight32 is the easiest way.