linux-kernel - coding for optimizations (Re: [PATCH 1/2] i386: mce cleanup part1: functional change)

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Message-ID: <20071009173317.GD22435@flower.upol.cz>
Date:	Tue, 9 Oct 2007 19:33:17 +0200
From:	Oleg Verych <olecom@...wer.upol.cz>
To:	"LKML (Cc removed)" <linux-kernel@...r.kernel.org>
Subject: coding for optimizations (Re: [PATCH 1/2] i386: mce cleanup part1: functional change)

On Tue, Oct 09, 2007 at 06:06:05PM +0200, Joerg Roedel wrote:
> > >  void mcheck_init(struct cpuinfo_x86 *c)
> > >  {
> > > +	uint32_t mca, mce;
> > > +
> > >  	if (mce_disabled==1)
> > >  		return;
> > >  
> > > +	mca = cpu_has(c, X86_FEATURE_MCA);
> > > +	mce = cpu_has(c, X86_FEATURE_MCE);
> > > +
> > > +	if (!mca || !mce) {
> > > +		printk(KERN_INFO "CPU%i: No machine check support available\n",
> > > +			smp_processor_id());
> > > +		return;
> > > +	}
> > > +
> > 
> > cpu_has() returns int,
> > but would it be better to have something like
> > 
> >   	if (!mce_disabled &&
> > 	    !(c->x86_capability & (X86_FEATURE_MCA | X86_FEATURE_MCE)) {
> > 		printk(KERN_INFO "CPU%i: No machine check support available\n",
> > 			smp_processor_id());
> 
> This looks complicated and is harder to read. Its exactly the purpose of the
> cpu_has() macro to avoid such constructs.
> 
> > 		return;
> > 	} else
> > 		return;
> 
> Return unconditionaly here?

Kind of typo.

Due to arch code, i think, it must be coded in non-gcc optimistic way. As
practice and Linus shows, gcc's optimizations sometimes produce very
unexpected results. People do spaghetti-like coding, without thinking
about text size / run time.

Compile time payment was noted by Andrew many years ago:

"As you know, I'd be more concerned about moves to drop support for the
older and much faster gcc versions.  If you're not using egcs-1.1.2,
you're already a very patient person." <http://lkml.org/lkml/2001/12/29/163>

So, i actually wanted to write this:

	if (!mce_disabled) {
	if (!(c->x86_capability & (X86_FEATURE_MCA | X86_FEATURE_MCE)) {
		printk(KERN_INFO "CPU%i: No machine check support available\n",
 			smp_processor_id());
		return;
	}
	/* function code */
	}

(ugly, because `mce_disabled == 1' check is even in gcc is likely by
default). But changed my mind, due to violation of the coding style.
Of course this my be no appropriate at all, but i'd like to bring
this, if anyone would be like to discuss this.
____
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