linux-kernel - Re: [PATCH v2] jhash: Deinline jhash, jhash2 and __jhash

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Date:	Fri, 17 Jul 2015 15:53:59 -0700
From:	Joe Perches <joe@...ches.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	David Miller <davem@...emloft.net>, tom <tom@...bertland.com>,
	herbert <herbert@...dor.apana.org.au>, tgraf <tgraf@...g.ch>,
	netdev <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	dvlasenk <dvlasenk@...hat.com>,
	"alexander.h.duyck" <alexander.h.duyck@...hat.com>,
	kadlec <kadlec@...ckhole.kfki.hu>
Subject: Re: [PATCH v2] jhash: Deinline jhash, jhash2 and __jhash_nwords

On Fri, 2015-07-17 at 15:44 +0200, Hagen Paul Pfeifer wrote:
> > On July 16, 2015 at 9:23 PM Joe Perches <joe@...ches.com> wrote:
> > 
> > It might be useful to have these performance impacting
> > changes guarded by something like CONFIG_CC_OPTIMIZE_FOR_SIZE
> > with another static __always_inline __<func> and a function &
> > EXPORT_SYMBOL or just a static inline so that where code size
> > is critical it's uninlined.
> 
> But keep in mind that jhash, jhash2 and __jhash_nwords are *not*
> one-instruction long functions. We duplicate code over and over resulting
> probably in more cache misses. __always_inline__ is probably too strict
> and a vanilla inline is already for 99% of all distribution builds a
>  __always_inline__, see ARCH_SUPPORTS_OPTIMIZED_INLINING and
> CONFIG_CC_OPTIMIZE_FOR_SIZE.

Hello.

Perhaps I wasn't clear/explicit enough.

I tried to suggest using a single __always_inline like this:

Two #ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE (or some other
CONFIG_FOO) functions, one in a .h for the existing behavior
and another in a .c for more code space compact uses.

jhash.h

static __always_inline u32 __jhash(const void *key, u32 length, u32 initval)
{
	[current jhash implementation...]
}

#ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
u32 jhash(const void *key, u32 length, u32 initval);
#else
static inline u32 jhash(const void *key, u32 length, u32 initval)
{
	return __jhash(key, length, initval);	
}
#endif

jhash.c:

#ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
u32 jhash(const void *key, u32 length, u32 initval)
{
	return __jhash(key, length, initval);	
}
EXPORT_SYMBOL(jhash)
#endif

etc...

Perhaps the additional code complexity is not worth it.


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