linux-kernel - Re: [v2 3/5] arm64: mte: implement CONFIG_ARM64_MTE

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Message-ID: <ZLEA3oxst2FBDWhC@smile.fi.intel.com>
Date:   Fri, 14 Jul 2023 11:01:34 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Alexander Potapenko <glider@...gle.com>, catalin.marinas@....com,
        will@...nel.org, pcc@...gle.com, andreyknvl@...il.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com
Subject: Re: [v2 3/5] arm64: mte: implement CONFIG_ARM64_MTE_COMP

On Thu, Jul 13, 2023 at 12:27:15PM -0700, Yury Norov wrote:

...

> > > +	void *storage = ea0_storage(handle);
> > > +	int size = ea0_storage_size(handle);

> > > +	if (!handle || !storage)
> > > +		return;
> > 
> > You use handle before this check. Haven't you run static analysers?
> 
> This approach is called 'defensive programming' as I learned from
> previous iteration.

No, this approach called "let compiler optimize the check away". :-)

When compiler sees something like

	TYPE bar = MACRO(foo); // FUNC(foo);

	if (!foo)
		blalblabla;

the conditional can be eliminated as the optimizer thinks the way "okay,
developer already _used_ the foo in some code, it means it can't not be NULL,
let's drop a dead code".

(Yes, I know what defensive programming means and we actually quite rarely
 use it in the kernel for the internal APIs)

-- 
With Best Regards,
Andy Shevchenko