Re: [PATCH v2 0/6] bitops: let optimize out non-atomic bitops on compile-time constants

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Mon, 13 Jun 2022 09:35:16 +0200

> Hi Alexander,
> 
> On Fri, Jun 10, 2022 at 1:35 PM Alexander Lobakin
> <alexandr.lobakin@intel.com> wrote:
> > While I was working on converting some structure fields from a fixed
> > type to a bitmap, I started observing code size increase not only in
> > places where the code works with the converted structure fields, but
> > also where the converted vars were on the stack. That said, the
> > following code:
> >
> >         DECLARE_BITMAP(foo, BITS_PER_LONG) = { }; // -> unsigned long foo[1];
> >         unsigned long bar = BIT(BAR_BIT);
> >         unsigned long baz = 0;
> >
> >         __set_bit(FOO_BIT, foo);
> >         baz |=3D BIT(BAZ_BIT);
> >
> >         BUILD_BUG_ON(!__builtin_constant_p(test_bit(FOO_BIT, foo));
> >         BUILD_BUG_ON(!__builtin_constant_p(bar & BAR_BIT));
> >         BUILD_BUG_ON(!__builtin_constant_p(baz & BAZ_BIT));
> >
> > triggers the first assertion on x86_64, which means that the
> > compiler is unable to evaluate it to a compile-time initializer
> > when the architecture-specific bitop is used even if it's obvious.
> > I found that this is due to that many architecture-specific
> > non-atomic bitop implementations use inline asm or other hacks which
> > are faster or more robust when working with "real" variables (i.e.
> > fields from the structures etc.), but the compilers have no clue how
> > to optimize them out when called on compile-time constants.
> >
> > So, in order to let the compiler optimize out such cases, expand the
> > test_bit() and __*_bit() definitions with a compile-time condition
> > check, so that they will pick the generic C non-atomic bitop
> > implementations when all of the arguments passed are compile-time
> > constants, which means that the result will be a compile-time
> > constant as well and the compiler will produce more efficient and
> > simple code in 100% cases (no changes when there's at least one
> > non-compile-time-constant argument).
> > The condition itself:
> >
> > if (
> > __builtin_constant_p(nr) &&     /* <- bit position is constant */
> > __builtin_constant_p(!!addr) && /* <- compiler knows bitmap addr is
> >                                       always either NULL or not */
> > addr &&                         /* <- bitmap addr is not NULL */
> > __builtin_constant_p(*addr)     /* <- compiler knows the value of
> >                                       the target bitmap */
> > )
> >         /* then pick the generic C variant
> > else
> >         /* old code path, arch-specific
> >
> > I also tried __is_constexpr() as suggested by Andy, but it was
> > always returning 0 ('not a constant') for the 2,3 and 4th
> > conditions.
> >
> > The savings are architecture, compiler and compiler flags dependent,
> > for example, on x86_64 -O2:
> >
> > GCC 12: add/remove: 78/29 grow/shrink: 332/525 up/down: 31325/-61560 (-30235)
> > LLVM 13: add/remove: 79/76 grow/shrink: 184/537 up/down: 55076/-141892 (-86816)
> > LLVM 14: add/remove: 10/3 grow/shrink: 93/138 up/down: 3705/-6992 (-3287)
> >
> > and ARM64 (courtesy of Mark[0]):
> >
> > GCC 11: add/remove: 92/29 grow/shrink: 933/2766 up/down: 39340/-82580 (-43240)
> > LLVM 14: add/remove: 21/11 grow/shrink: 620/651 up/down: 12060/-15824 (-3764)
> >
> > And the following:
> >
> >         DECLARE_BITMAP(flags, __IP_TUNNEL_FLAG_NUM) = { };
> >         __be16 flags;
> >
> >         __set_bit(IP_TUNNEL_CSUM_BIT, flags);
> >
> >         tun_flags = cpu_to_be16(*flags & U16_MAX);
> >
> >         if (test_bit(IP_TUNNEL_VTI_BIT, flags))
> >                 tun_flags |= VTI_ISVTI;
> >
> >         BUILD_BUG_ON(!__builtin_constant_p(tun_flags));
> >
> > doesn't blow up anymore, so that we now can e.g. use fixed bitmaps
> > in compile-time assertions etc.
> >
> > The series has been in intel-next for a while with no reported issues.
> >
> > From v1[1]:
> > * change 'gen_' prefixes to '_generic' to disambiguate from
> >   'generated' etc. (Mark);
> > * define a separate 'const_' set to use in the optimization to keep
> >   the generic test_bit() atomic-safe (Marco);
> > * unify arch_{test,__*}_bit() as well and include them in the type
> >   check;
> > * add more relevant and up-to-date bloat-o-meter results, including
> >   ARM64 (me, Mark);
> > * pick a couple '*-by' tags (Mark, Yury).
> 
> Thanks for the update!
> 
> On m68k, using gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04), this
> blows up immediately with:

Yeah I saw the kernel bot report already, sorry for that >_< Fixed
in v3 already, will send in 1-2 days.

> 
>   CC      kernel/bounds.s
> In file included from include/linux/bits.h:22,

[...]

>       | ^~~~~~~~~~~~~~~~
> 
> 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

Thanks,
Olek