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

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

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).

Also available on my open GH[2].

[0] https://lore.kernel.org/all/Yp4GQFQYD32Rs9Cw@FVFF77S0Q05N
[1] https://lore.kernel.org/all/20220606114908.962562-1-alexandr.lobakin@intel.com
[2] 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: unify non-atomic bitops prototypes across architectures
  bitops: define const_*() versions of the non-atomics
  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 +-
 arch/x86/include/asm/bitops.h                 |  22 +--
 .../asm-generic/bitops/generic-non-atomic.h   | 155 ++++++++++++++++++
 .../bitops/instrumented-non-atomic.h          |  35 ++--
 include/asm-generic/bitops/non-atomic.h       | 123 ++------------
 include/linux/bitops.h                        |  50 ++++++
 tools/include/asm-generic/bitops/non-atomic.h |  34 ++--
 tools/include/linux/bitops.h                  |  16 ++
 15 files changed, 407 insertions(+), 230 deletions(-)
 create mode 100644 include/asm-generic/bitops/generic-non-atomic.h

base-commit: 874c8ca1e60b2c564a48f7e7acc40d328d5c8733
-- 
2.36.1