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

[Guenter Roeck <linux@roeck-us.net>]

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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# 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
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# 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
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# first bad commit: [b03fc1173c0c2bb8fad61902a862985cecdc4b1b] bitops: let optimize out non-atomic bitops on compile-time constants