linux-kernel - Re: [PATCH 1/2] string: add mem_is_zero() helper to check if memory area is all zeros

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Date: Mon, 27 May 2024 17:06:28 +0300
From: Jani Nikula <jani.nikula@...el.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 intel-gfx@...ts.freedesktop.org, Kees Cook <keescook@...omium.org>, Andy
 Shevchenko <andy@...nel.org>
Subject: Re: [PATCH 1/2] string: add mem_is_zero() helper to check if memory
 area is all zeros

On Mon, 27 May 2024, Andy Shevchenko <andy.shevchenko@...il.com> wrote:
> On Mon, May 27, 2024 at 12:43 PM Jani Nikula <jani.nikula@...el.com> wrote:
>>
>> Almost two thirds of the memchr_inv() usages check if the memory area is
>> all zeros, with no interest in where in the buffer the first non-zero
>> byte is located. Checking for !memchr_inv(s, 0, n) is also not very
>> intuitive or discoverable. Add an explicit mem_is_zero() helper for this
>> use case.
>
> ...
>
>> +static inline bool mem_is_zero(const void *s, size_t n)
>> +{
>> +       return !memchr_inv(s, 0, n);
>> +}
>
> There are potential users for the 0xff check as well. Hence the
> following question:
> Are we going to have a new function per byte in question, or do we
> come up with a common denominator, like mem_is_all_of(mem, byte)?

No. As I wrote in the commit message rationale, "Almost two thirds of
the memchr_inv() usages check if the memory area is all zeros". This is
by far the most common use case of memchr_inv().

BR,
Jani.


-- 
Jani Nikula, Intel