Re: [PATCH v3 0/7] bitops: let optimize out non-atomic bitops on compile-time constants

Geert Uytterhoeven
2022-06-20T12:07:51Z

Hi Olek,

On Fri, Jun 17, 2022 at 6:51 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 |= 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 (which is being checked now at build time),
> so that we can now 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 v2[1]:
> * collect several Reviewed-bys (Andy, Yury);
> * add a comment to generic_test_bit() that it is atomic-safe and
> must always stay like that (the first version of this series
> errorneously tried to change this) (Andy, Marco);
> * unify the way how architectures define platform-specific bitops,
> both supporting instrumentation and not: now they define only
> 'arch_' versions and asm-generic includes take care of the rest;
> * micro-optimize the diffstat of 0004/0007 (__check_bitop_pr())
> (Andy);
> * add compile-time tests to lib/test_bitmap to make sure everything
> works as expected on any setup (Yury).

Thanks for the update!

Still seeing
add/remove: 49/13 grow/shrink: 280/137 up/down: 6464/-3328 (3136)

on m68k atari_defconfig (i.e.CONFIG_CC_OPTIMIZE_FOR_SIZE=y)
with gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04).

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