linux-kernel - Re: [PATCH 03/17] x86: Replace open-coded parity calculation with parity8()

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Message-ID: <20250225224623.6edaaaa9@pumpkin>
Date: Tue, 25 Feb 2025 22:46:23 +0000
From: David Laight <david.laight.linux@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Uros Bizjak <ubizjak@...il.com>, Kuan-Wei Chiu <visitorckw@...il.com>,
 tglx@...utronix.de, Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
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 Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH 03/17] x86: Replace open-coded parity calculation with
 parity8()

On Mon, 24 Feb 2025 13:55:28 -0800
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 2/24/25 07:24, Uros Bizjak wrote:
> > 
> > 
> > On 23. 02. 25 17:42, Kuan-Wei Chiu wrote:  
> >> Refactor parity calculations to use the standard parity8() helper. This
> >> change eliminates redundant implementations and improves code
> >> efficiency.  
...
> Of course, on x86, parity8() and parity16() can be implemented very simply:
> 
> (Also, the parity functions really ought to return bool, and be flagged 
> __attribute_const__.)
> 
> static inline __attribute_const__ bool _arch_parity8(u8 val)
> {
> 	bool parity;
> 	asm("and %0,%0" : "=@...p" (parity) : "q" (val));
> 	return parity;
> }
> 
> static inline __attribute_const__ bool _arch_parity16(u16 val)
> {
> 	bool parity;
> 	asm("xor %h0,%b0" : "=@...p" (parity), "+Q" (val));
> 	return parity;
> }

The same (with fixes) can be done for parity64() on 32bit.

> 
> In the generic algorithm, you probably should implement parity16() in 
> terms of parity8(), parity32() in terms of parity16() and so on:
> 
> static inline __attribute_const__ bool parity16(u16 val)
> {
> #ifdef ARCH_HAS_PARITY16
> 	if (!__builtin_const_p(val))
> 		return _arch_parity16(val);
> #endif
> 	return parity8(val ^ (val >> 8));
> }
> 
> This picks up the architectural versions when available.

Not the best way to do that.
Make the name in the #ifdef the same as the function and define
a default one if the architecture doesn't define one.
So:

static inline parity16(u16 val)
{
	return __builtin_const_p(val) ? _parity_const(val) : _parity16(val);
}

#ifndef _parity16
static inline _parity16(u15 val)
{
	return _parity8(val ^ (val >> 8));
}
#endif

You only need one _parity_const().

> 
> Furthermore, if a popcnt instruction is known to exist, then the parity 
> is simply popcnt(x) & 1.

Beware that some popcnt instructions are slow.

	David

> 
> 	-hpa
> 
>