On Fri, Jun 24, 2022 at 02:13:10PM +0200, Alexander Lobakin wrote:
Currently, 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. 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.
In order to let the compiler optimize out such cases, expand the
bitop() macro to use the "constant" 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, so that it produces more efficient and simple
code in 100% cases, comparing to the architecture-specific
counterparts.
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):
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)
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
Building i386:allyesconfig ... failed
--------------
Error log:
arch/x86/platform/olpc/olpc-xo1-sci.c: In function 'send_ebook_state':
arch/x86/platform/olpc/olpc-xo1-sci.c:83:63: error: logical not is only applied to the left hand side of comparison
Bisect log attached.
Guenter
---
# bad: [4662b7adea50bb62e993a67f611f3be625d3df0d] Add linux-next specific files for 20220713
# good: [32346491ddf24599decca06190ebca03ff9de7f8] Linux 5.19-rc6
git bisect start 'HEAD' 'v5.19-rc6'
# good: [8b7e002d8bc6e17c94092d25e7261db4e6e5f2cc] Merge branch 'drm-next' of git://git.freedesktop.org/git/drm/drm.git
git bisect good 8b7e002d8bc6e17c94092d25e7261db4e6e5f2cc
# good: [07f6d21d6e33c1e28e24ae84e9d26e4e7d4853f5] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git
git bisect good 07f6d21d6e33c1e28e24ae84e9d26e4e7d4853f5
# good: [5ff085e5d4f6700e03635d5e700f52163a6dc2a7] Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
git bisect good 5ff085e5d4f6700e03635d5e700f52163a6dc2a7
# good: [eb9e3fdbdd8b61ef0f4bee23259fe6ab69e463ab] Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
git bisect good eb9e3fdbdd8b61ef0f4bee23259fe6ab69e463ab
# good: [9f2183cd961e5ddb7954eafb6bb01a495c6a9c7b] hexagon/mm: enable ARCH_HAS_VM_GET_PAGE_PROT
git bisect good 9f2183cd961e5ddb7954eafb6bb01a495c6a9c7b
# bad: [e878aa5faf9ac8c0b5d0c3f293389c194c250fff] Merge branch 'mm-nonmm-stable' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
git bisect bad e878aa5faf9ac8c0b5d0c3f293389c194c250fff
# good: [cf95d50205f62c4f5f538676def847292cf39fa9] fs: don't call ->writepage from __mpage_writepage
git bisect good cf95d50205f62c4f5f538676def847292cf39fa9
# good: [5103cbfd92d3587713476f94f9485b96e02f0146] Merge branch 'for-next/execve' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git
git bisect good 5103cbfd92d3587713476f94f9485b96e02f0146
# good: [ee56c3e8eec166f4e4a2ca842b7804d14f3a0208] Merge branch 'master' into mm-nonmm-stable
git bisect good ee56c3e8eec166f4e4a2ca842b7804d14f3a0208
# bad: [dc34d5036692c614eef23c1130ee42a201c316bf] lib: test_bitmap: add compile-time optimization/evaluations assertions
git bisect bad dc34d5036692c614eef23c1130ee42a201c316bf
# good: [bb7379bfa680bd48b468e856475778db2ad866c1] bitops: define const_*() versions of the non-atomics
git bisect good bb7379bfa680bd48b468e856475778db2ad866c1
# bad: [b03fc1173c0c2bb8fad61902a862985cecdc4b1b] bitops: let optimize out non-atomic bitops on compile-time constants
git bisect bad b03fc1173c0c2bb8fad61902a862985cecdc4b1b
# good: [e69eb9c460f128b71c6b995d75a05244e4b6cc3e] bitops: wrap non-atomic bitops with a transparent macro
git bisect good e69eb9c460f128b71c6b995d75a05244e4b6cc3e
# first bad commit: [b03fc1173c0c2bb8fad61902a862985cecdc4b1b] bitops: let optimize out non-atomic bitops on compile-time constants
