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

Alexander Lobakin
2022-06-06T11:50:24Z

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 on x86_64 with LLVM are insane (.text):

$ scripts/bloat-o-meter -c vmlinux.{base,test}
add/remove: 72/75 grow/shrink: 182/518 up/down: 53925/-137810 (-83885)

$ scripts/bloat-o-meter -c vmlinux.{base,mod}
add/remove: 7/1 grow/shrink: 1/19 up/down: 1135/-4082 (-2947)

$ scripts/bloat-o-meter -c vmlinux.{base,all}
add/remove: 79/76 grow/shrink: 184/537 up/down: 55076/-141892 (-86816)

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.

The series has been in intel-next for a while with no reported issues.
Also available on my open GH[0].

[0] https://github.com/alobakin/linux/commits/bitops

Alexander Lobakin (6):
ia64, processor: fix -Wincompatible-pointer-types in ia64_get_irr()
bitops: always define asm-generic non-atomic bitops
bitops: define gen_test_bit() the same way as the rest of functions
bitops: unify non-atomic bitops prototypes across architectures
bitops: wrap non-atomic bitops with a transparent macro
bitops: let optimize out non-atomic bitops on compile-time constants

arch/alpha/include/asm/bitops.h | 28 ++--
arch/hexagon/include/asm/bitops.h | 23 ++--
arch/ia64/include/asm/bitops.h | 40 +++---
arch/ia64/include/asm/processor.h | 2 +-
arch/m68k/include/asm/bitops.h | 47 +++++--
arch/sh/include/asm/bitops-op32.h | 32 +++--
arch/sparc/include/asm/bitops_32.h | 18 +--
arch/sparc/lib/atomic32.c | 12 +-
.../asm-generic/bitops/generic-non-atomic.h | 128 ++++++++++++++++++
.../bitops/instrumented-non-atomic.h | 35 +++--
include/asm-generic/bitops/non-atomic.h | 123 ++---------------
include/linux/bitops.h | 45 ++++++
tools/include/asm-generic/bitops/non-atomic.h | 34 +++--
tools/include/linux/bitops.h | 16 +++
14 files changed, 363 insertions(+), 220 deletions(-)
create mode 100644 include/asm-generic/bitops/generic-non-atomic.h

base-commit: f2906aa863381afb0015a9eb7fefad885d4e5a56
--
2.36.1