Re: [PATCH v5 6/9] bitops: let optimize out non-atomic bitops on compile-time constants

Guenter Roeck
2022-07-15T00:04:02Z

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@arm.com>
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> Reviewed-by: Marco Elver <elver@google.com>

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

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# bad: [4662b7adea50bb62e993a67f611f3be625d3df0d] Add linux-next specific files for 20220713
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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
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git bisect good eb9e3fdbdd8b61ef0f4bee23259fe6ab69e463ab
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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
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# 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