Re: [PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH_BITS

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi George,

On Fri, Mar 15, 2019 at 5:33 AM George Spelvin <lkml@sdf.org> wrote:
> On Thu, 14 Mar 2019 at 11:10:41 +0200, Andy Shevchenko wrote:
> > On Tue, Mar 05, 2019 at 03:06:44AM +0000, George Spelvin wrote:
> >> +            for (bit = 1; count & bit; bit <<= 1) {
> >> +                    cur = merge(priv, (cmp_func)cmp, pending, cur);
> >> +                    pending = pending->prev;  /* Untouched by merge() */
> >>              }
> >
> > Wouldn't be it the same to
> >
> >       bit = ffz(count);
> >       while (bit--) {
> >               ...
> >       }
> > ?
> >
> > Though I dunno which one is generating better code.
>
> One question I should ask everyone: should "count" be 32 or 64 bits
> on 64-bit machines?  That would let x86 save a few REX bytes.  (815
> vs. 813 byte code, if anyone cares.)
>
> Allegedy ARM can save a few pJ by gating the high 32
> bits of the ALU.
>
> Most other 64-bit processors would prefer 64-bit operations as
> it saves masking operations.
>
> If we never sort a list with more than 2^32 entries, it
> makes no difference.
>
> If we use a 32-bit count and we *do* sort a list with more than
> 2^32 entries, then it still sorts, but the performance degrades to
> O((n/2^32)^2).
>
> Just how often do we expect the kernel to face lists that long?
> (Note that the old code was O((n/2^20)^2).)

Using size_t sounds most logical to me (argument of least surprise).

> In the code, I could do something like
>
> #ifdef CONFIG_X86_64
> /* Comment explaining why */
> typedef uint32_t count_t;
> #else
> typedef size_t count_t;
> #endif
>
> ...
>         count_t count = 0;

Using different types makes it more complex, e.g. to print the value
in debug code.
And adding more typedefs is frowned upon.

Just my 0.02€.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds