linux-kernel - Re: [PATCH v12 01/11] bitops: Introduce the for_each_set

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Message-ID: <20190326114252.GX9224@smile.fi.intel.com>
Date:   Tue, 26 Mar 2019 13:42:52 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     linus.walleij@...aro.org, bgolaszewski@...libre.com,
        akpm@...ux-foundation.org, linux-gpio@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux@...musvillemoes.dk, yamada.masahiro@...ionext.com,
        linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
        geert@...ux-m68k.org, preid@...ctromag.com.au,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v12 01/11] bitops: Introduce the for_each_set_clump8 macro

On Tue, Mar 26, 2019 at 11:54:59AM +0900, William Breathitt Gray wrote:
> On Mon, Mar 25, 2019 at 03:12:36PM +0200, Andy Shevchenko wrote:
> > On Mon, Mar 25, 2019 at 03:22:23PM +0900, William Breathitt Gray wrote:
> > > This macro iterates for each 8-bit group of bits (clump) with set bits,
> > > within a bitmap memory region. For each iteration, "start" is set to the
> > > bit offset of the found clump, while the respective clump value is
> > > stored to the location pointed by "clump". Additionally, the
> > > bitmap_get_value8 and bitmap_set_value8 functions are introduced to
> > > respectively get and set an 8-bit value in a bitmap memory region.
> > 
> > 
> > This seems to miss Randy's (IIRC) comment about too many const specifiers.
> 
> I disagree with removing the const qualifiers; I believe they are useful
> and do not significantly impact the clarity of the code (in fact, I'd
> argue the opposite). 

Had you checked the assembly? I'm talking about const for values on the stack.

I think if you put less const there compiler can keep something in the
registers instead of using direct constants or accessing stack.

I might be mistaken, so, I can't argue without evidence of either.

> The const qualifiers make it clear these values are
> constant, allowing readers at a glace to know these values never change
> within this function. Although I believe GCC is smart enough in this
> case to deduce implicitly that these are constant values, generally
> speaking const qualifiers do make it easier for compilers to optimize
> sections of code (OoO execution, algorithm simplification, etc.), so I
> believe it's useful in a technical sense as well.

Again, what the difference do you see in assembly if any?

> I added the const qualifier to these variables because they really are
> constant, and I believe there is merit in making it explicit in the
> code. If the primary reason for removing the const qualifiers is for
> aesthetics, then I must dissent with that decision.

The point is, if there is no difference, I would prefer one which will be
better to read, otherwise check the assembly.

> However, it is difficult to read the definitions that wrap around to a
> second line. These definitions are long enough that even removing the
> const qualifiers would not help prevent the wrapping, so perhaps it
> would make to let these stay on a single line. Do you think it would
> help to ignore the 80-character maximum line width coding style rule for
> these cases here?

80-characters rule can be slightly bended depending on the context. Here, I
think, we might continue discussing the matter after having an evidence how
const qualifiers affect the code.

-- 
With Best Regards,
Andy Shevchenko