linux-kernel - Re: [PATCH v2 2/3] lib/find_bit: create find_first_zero_bit

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Date:   Wed, 24 Aug 2022 15:09:35 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>,
        Dennis Zhou <dennis@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexey Klimov <aklimov@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Whitcroft <apw@...onical.com>
Subject: Re: [PATCH v2 2/3] lib/find_bit: create find_first_zero_bit_le()

> Just make the macro take that "last word op" as another argument, and
> then you don't need these tricks, and you can do the _le() version in
> lib/find.c file *exactly* like the regular version, using just
> 
>   #ifdef __BIG_ENDIAN
>   unsigned long find_first_zero_bit_le(const unsigned long *addr,
> unsigned long size)
>   { return FIND_FIRST_BIT(~addr[idx], swab(val), size); }
>   #else
>   unsigned long find_first_zero_bit_le(const unsigned long *addr,
> unsigned long size) __alias(find_first_zero_bit);
>   #endif
> 
> or something like that.
> 
> And yes, it means that the "regular" versions need to pass in just
> "val" as a last-word expression, but who cares? It's still cleaner,
> and easier to explain: "The first expression finds the word that
> matches our requirement, the second expression munges the word we
> found into the bit order we use".

OK. I'll do like this:

   #define FIND_FIRST_BIT(FETCH, MUNG, size) ...

   unsigned long find_first_zero_bit(const unsigned long *addr, unsigned long size)
   {
         return FIND_FIRST_BIT(~addr[idx], /* NOP */, size);
   }

   unsigned long find_first_zero_bit_le(const unsigned long *addr, unsigned long size)
   {
         return FIND_FIRST_BIT(~addr[idx], swab, size);
   }

It doesn't actually look that worse.

Thanks,
Yury