linux-kernel - Re: [PATCH v4 1/5] lib/bitmap: add bitmap_{set,get}

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Date:   Fri, 4 Aug 2023 12:55:01 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, andriy.shevchenko@...ux.intel.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4 1/5] lib/bitmap: add bitmap_{set,get}_value()

On Fri, Aug 04, 2023 at 06:07:00PM +0200, Alexander Potapenko wrote:
> >         space >= nbits <=>
> >         BITS_PER_LONG - offset >= nbits <=>
> >         offset + nbits <= BITS_PER_LONG
> >
> > >         map[index] &= (fit ? (~(GENMASK(nbits - 1, 0) << offset)) :
> >
> > So here GENMASK(nbits + offset - 1, offset) is at max:
> > GENMASK(BITS_PER_LONG - 1, offset). And it never overflows, which is my
> > point. Does it make sense?
> 
> It indeed does. Perhaps pulling offset inside GENMASK is not a bug
> after all (a simple test does not show any difference between their
> behavior.
> But `GENMASK(nbits - 1 + offset, offset)` blows up the code (see below).
> My guess is that this happens because the compiler fails to reuse the
> value of `GENMASK(nbits - 1, 0)` used to clamp the value to write, and
> calculates `GENMASK(nbits - 1 + offset, offset)` from scratch.

OK. Can you put a comment explaining this? Or maybe would be even
better to use BITMAP_LAST_WORD_MASK() here:

         mask = BITMAP_LAST_WORD_MASK(nbits);
         value &= mask;
         ...
         map[index] &= (fit ? (~mask << offset)) :

> > > ~BITMAP_FIRST_WORD_MASK(start));
> >
> > As I said, ~BITMAP_FIRST_WORD_MASK() is the same as BITMAP_LAST_WORD_MASK()
> > and vise-versa.
> 
> Surprisingly, ~BITMAP_FIRST_WORD_MASK() generates better code than
> BITMAP_LAST_WORD_MASK().
 
Wow... If that's consistent across different compilers/arches, we'd
just drop the latter. Thanks for pointing that. I'll check.

> > >         map[index] |= value << offset;
> > >         if (fit)
> > >                 return;
> > >
> > >         map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> 
> OTOH I managed to shave three more bytes off by replacing
> ~BITMAP_LAST_WORD_MASK with a BITMAP_FIRST_WORD_MASK here.
> 
> > >         map[index + 1] |= (value >> space);
> > > }
> 
> I'll post the implementations together with the disassembly below.
> I used some Clang 17.0.0 version that is a couple months behind
> upstream, but that still produces sustainably shorter code (~48 bytes
> less) than the trunk GCC on Godbolt.
> 
> 1. Original implementation of bitmap_write() from this patch - 164
> bytes (interestingly, it's 157 bytes with Clang 14.0.6)

I spotted that too in some other case. Newer compilers tend to
generate bigger code, but the result usually works faster. One
particular reason for my case was a loop unrolling.

[...]

> 3. My improved version built on top of yours and mentioned above under
> the name bitmap_write_new() - 116 bytes:

30% better in size - that's impressive!
 
> ==================================================================
> void bitmap_write_new(unsigned long *map, unsigned long value,
>                       unsigned long start, unsigned long nbits)
> {
>         unsigned long offset;
>         unsigned long space;
>         size_t index;
>         bool fit;
> 
>         if (unlikely(!nbits))
>                 return;
> 
>         value &= GENMASK(nbits - 1, 0);
>         offset = start % BITS_PER_LONG;
>         space = BITS_PER_LONG - offset;
>         index = BIT_WORD(start);
>         fit = space >= nbits;
> 
>         map[index] &= (fit ? (~(GENMASK(nbits - 1, 0) << offset)) :
> ~BITMAP_FIRST_WORD_MASK(start));
>         map[index] |= value << offset;
>         if (fit)
>                 return;
> 
>         map[index + 1] &= BITMAP_FIRST_WORD_MASK(start + nbits);
>         map[index + 1] |= (value >> space);
> }

Thanks,
Yury