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

Geert Uytterhoeven
2022-06-13T07:35:34Z

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 |= 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:

CC kernel/bounds.s
In file included from include/linux/bits.h:22,
from include/linux/ratelimit_types.h:5,
from include/linux/printk.h:9,
from include/asm-generic/bug.h:22,
from arch/m68k/include/asm/bug.h:32,
from include/linux/bug.h:5,
from include/linux/page-flags.h:10,
from kernel/bounds.c:10:
include/linux/bitops.h:72:21: error: ‘arch___set_bit’ undeclared here
(not in a function); did you mean ‘const___set_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:75:1: note: in expansion of macro ‘__check_bitop_pr’
75 | __check_bitop_pr(__set_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:75:1: note: in expansion of macro ‘__check_bitop_pr’
75 | __check_bitop_pr(__set_bit);
| ^~~~~~~~~~~~~~~~
include/linux/bitops.h:72:21: error: ‘arch___clear_bit’ undeclared
here (not in a function); did you mean ‘const___clear_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:76:1: note: in expansion of macro ‘__check_bitop_pr’
76 | __check_bitop_pr(__clear_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:76:1: note: in expansion of macro ‘__check_bitop_pr’
76 | __check_bitop_pr(__clear_bit);
| ^~~~~~~~~~~~~~~~
include/linux/bitops.h:72:21: error: ‘arch___change_bit’ undeclared
here (not in a function); did you mean ‘const___change_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:77:1: note: in expansion of macro ‘__check_bitop_pr’
77 | __check_bitop_pr(__change_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:77:1: note: in expansion of macro ‘__check_bitop_pr’
77 | __check_bitop_pr(__change_bit);
| ^~~~~~~~~~~~~~~~
include/linux/bitops.h:72:21: error: ‘arch___test_and_set_bit’
undeclared here (not in a function); did you mean
‘const___test_and_set_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:78:1: note: in expansion of macro ‘__check_bitop_pr’
78 | __check_bitop_pr(__test_and_set_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:78:1: note: in expansion of macro ‘__check_bitop_pr’
78 | __check_bitop_pr(__test_and_set_bit);
| ^~~~~~~~~~~~~~~~
include/linux/bitops.h:72:21: error: ‘arch___test_and_clear_bit’
undeclared here (not in a function); did you mean
‘const___test_and_clear_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:79:1: note: in expansion of macro ‘__check_bitop_pr’
79 | __check_bitop_pr(__test_and_clear_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:79:1: note: in expansion of macro ‘__check_bitop_pr’
79 | __check_bitop_pr(__test_and_clear_bit);
| ^~~~~~~~~~~~~~~~
include/linux/bitops.h:72:21: error: ‘arch___test_and_change_bit’
undeclared here (not in a function); did you mean
‘const___test_and_change_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:80:1: note: in expansion of macro ‘__check_bitop_pr’
80 | __check_bitop_pr(__test_and_change_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:80:1: note: in expansion of macro ‘__check_bitop_pr’
80 | __check_bitop_pr(__test_and_change_bit);
| ^~~~~~~~~~~~~~~~
include/linux/bitops.h:72:21: error: ‘arch_test_bit’ undeclared here
(not in a function); did you mean ‘_test_bit’?
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:72:9: note: in expansion of macro ‘__same_type’
72 | __same_type(arch_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:81:1: note: in expansion of macro ‘__check_bitop_pr’
81 | __check_bitop_pr(test_bit);
| ^~~~~~~~~~~~~~~~
include/linux/compiler_types.h:293:27: error: expression in static
assertion is not an integer
293 | #define __same_type(a, b)
__builtin_types_compatible_p(typeof(a), typeof(b))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:78:56: note: in definition of macro ‘__static_assert’
78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
| ^~~~
include/linux/bitops.h:71:2: note: in expansion of macro ‘static_assert’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~~~
include/linux/bitops.h:71:16: note: in expansion of macro ‘__same_type’
71 | static_assert(__same_type(const_##name, generic_##name) && \
| ^~~~~~~~~~~
include/linux/bitops.h:81:1: note: in expansion of macro ‘__check_bitop_pr’
81 | __check_bitop_pr(test_bit);
| ^~~~~~~~~~~~~~~~

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