Hi Olek,
On Fri, Jun 17, 2022 at 6:51 PM Alexander Lobakin
<alexandr.lobakin@xxxxxxxxx> 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@xxxxxxxxxxxxxx
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
