linux-kernel - Re: [PATCH] rtc: isl1208: avoid unnecessary rc variable tests

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Date:   Wed, 5 Jan 2022 23:25:56 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Hugo Villeneuve <hugo@...ovil.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: isl1208: avoid unnecessary rc variable tests

On 05/01/2022 15:34:46-0500, Hugo Villeneuve wrote:
> On Wed, 5 Jan 2022 21:01:10 +0100
> Alexandre Belloni <alexandre.belloni@...tlin.com> wrote:
> 
> > On 05/01/2022 14:34:39-0500, Hugo Villeneuve wrote:
> > > From: Hugo Villeneuve <hvilleneuve@...onoff.com>
> > > 
> > > The rc variable doesn't need to be tested a second time when the <if> block
> > > evaluates to false.
> > > 
> > 
> > rc is not tested a second time, here is the relevant listing:
> > 
> > -	if (client->irq > 0)
> > +	if (client->irq > 0) {
> >  ffffffff81aef647:	41 8b b5 bc 01 00 00 	mov    0x1bc(%r13),%esi
> >  ffffffff81aef64e:	85 f6                	test   %esi,%esi
> >  ffffffff81aef650:	0f 8f 35 01 00 00    	jg     ffffffff81aef78b <isl1208_probe+0x314>
> >  		rc = isl1208_setup_irq(client, client->irq);
> >  	if (rc)
> >  		return rc;
> > +	}
> >  
> > -	if (evdet_irq > 0 && evdet_irq != client->irq)
> > +	if (evdet_irq > 0 && evdet_irq != client->irq) {
> >  ffffffff81aef656:	85 db                	test   %ebx,%ebx
> >  ffffffff81aef658:	7e 0d                	jle    ffffffff81aef667 <isl1208_probe+0x1f0>
> >  ffffffff81aef65a:	41 39 9d bc 01 00 00 	cmp    %ebx,0x1bc(%r13)
> > @@ -1663,6 +1664,7 @@ ffffffff81aef661:	0f 85 0a 01 00 00
> >  		rc = isl1208_setup_irq(client, evdet_irq);
> >  	if (rc)
> >  		return rc;
> > +	}
> > 
> > As you can see, no change in assembly but it is worse to read. gcc on
> > arm behaves the same way.
> 
> Hi Alexandre,
> I am not sure that I fully understand your assembly code analysis. Maybe my patch comment was misleading, because I am not talking about a redundant test inside the if block, but ouside of it (after it).
> 

I understood that and what I'm showing is that it doesn't matter to the
compiler, it will not test the same variable twice if it didn't change.

> Here is the original code with my annotations. Let's assume that the variable client->irq = 0:
> 
> ---
> /* If client->irq = 0, then function isl1208_setup_irq() will not be called, and rc will not be modified: */
> if (client->irq > 0)
> 	rc = isl1208_setup_irq(client, client->irq);
> 
> /* If rc hasn't been modified, there is no need to re-test its value here: */
> if (rc)
> 	return rc;
> ---
> 
> After the patch, this code section becomes:
> 
> ---
> if (client->irq > 0) {
> 	rc = isl1208_setup_irq(client, client->irq);
> 	if (rc)
> 		return rc;
> }
> ---
> 
> For me it is more logical and clearer like this. Moreover, you can see that at line 873 of the original driver, the same kind of mechanism is used:
> 
> ---
> if (isl1208->config->has_timestamp) {
> 	rc = rtc_add_group(isl1208->rtc, &isl1219_rtc_sysfs_files);
> 	if (rc)
> 		return rc;
> }
> ---
> 
> Regards,
> Hugo.
> 
> 
> > > Signed-off-by: Hugo Villeneuve <hvilleneuve@...onoff.com>
> > > ---
> > >  drivers/rtc/rtc-isl1208.c | 14 ++++++++------
> > >  1 file changed, 8 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/rtc/rtc-isl1208.c b/drivers/rtc/rtc-isl1208.c
> > > index 182dfa605515..c7f04df5a0b6 100644
> > > --- a/drivers/rtc/rtc-isl1208.c
> > > +++ b/drivers/rtc/rtc-isl1208.c
> > > @@ -880,15 +880,17 @@ isl1208_probe(struct i2c_client *client, const struct i2c_device_id *id)
> > >  	if (rc)
> > >  		return rc;
> > >  
> > > -	if (client->irq > 0)
> > > +	if (client->irq > 0) {
> > >  		rc = isl1208_setup_irq(client, client->irq);
> > > -	if (rc)
> > > -		return rc;
> > > +		if (rc)
> > > +			return rc;
> > > +	}
> > >  
> > > -	if (evdet_irq > 0 && evdet_irq != client->irq)
> > > +	if (evdet_irq > 0 && evdet_irq != client->irq) {
> > >  		rc = isl1208_setup_irq(client, evdet_irq);
> > > -	if (rc)
> > > -		return rc;
> > > +		if (rc)
> > > +			return rc;
> > > +	}
> > >  
> > >  	rc = devm_rtc_nvmem_register(isl1208->rtc, &isl1208->nvmem_config);
> > >  	if (rc)
> > > -- 
> > > 2.30.2
> > > 
> > 
> > -- 
> > Alexandre Belloni, co-owner and COO, Bootlin
> > Embedded Linux and Kernel engineering
> > https://bootlin.com
> > 
> 
> 
> -- 
> Hugo Villeneuve <hugo@...ovil.com>

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com