Re: [PATCH 2/2] of: fix side effect in macro expansion

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 4/29/20 10:00 AM, Sascha Hauer wrote:
> On Wed, Apr 29, 2020 at 08:40:40AM +0200, Ahmad Fatoum wrote:
>> On little endian platforms that don't define __arch_swab32p, be32_to_cpu
>> ultimately expands to an expression that evaluates its argument multiple
>> times. Because of the increment, this is wrong (and UB because of
>> unsequenced modification).
>> Hoist the side effect out of the macro argument.
> 
> I'm not sure if the reasoning is correct. That would mean that
> of_read_number() doesn't work on ARM, but I can assure it actually does.

Typo, sorry. arch needs to define __arch_swab32 (no p at the end).

of_read_number -> be32_to_cpu -> __be32_to_cpu -> __swab32 -> __fswab32

then

static inline __attribute_const__ __u32 __fswab32(__u32 val)
{
#ifdef __arch_swab32
        return __arch_swab32(val);
#else
        return ___constant_swab32(val);
#endif
}

where ___constant_swab32 is the macro that evaluates the argument
multiple times.

grep shows '#define __arch_swab32' matches for mips, arm, ppc and x86.


> That said, the implementation looks way better with this patch.
Could you amend the commit message when applying?

Thanks
Ahmad

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