On Fri, Jun 10, 2022 at 01:34:25PM +0200, Alexander Lobakin wrote: > Define const_*() variants of the non-atomic bitops to be used when > the input arguments are compile-time constants, so that the compiler > will be always to resolve those to compile-time constants as well. will be always able? > Those are mostly direct aliases for generic_*() with one exception > for const_test_bit(): the original one is declared atomic-safe and > thus doesn't discard the `volatile` qualifier, so in order to let > optimize the code, define it separately disregarding the qualifier. > Add them to the compile-time type checks as well just in case. > > Suggested-by: Marco Elver <elver@xxxxxxxxxx> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx> > --- > .../asm-generic/bitops/generic-non-atomic.h | 31 +++++++++++++++++++ > include/linux/bitops.h | 3 +- > 2 files changed, 33 insertions(+), 1 deletion(-) > > diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h > index 3ce0fa0ab35f..9a77babfff35 100644 > --- a/include/asm-generic/bitops/generic-non-atomic.h > +++ b/include/asm-generic/bitops/generic-non-atomic.h > @@ -121,4 +121,35 @@ generic_test_bit(unsigned long nr, const volatile unsigned long *addr) > return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1))); > } > > +/* > + * const_*() definitions provide good compile-time optimizations when > + * the passed arguments can be resolved at compile time. > + */ > +#define const___set_bit generic___set_bit > +#define const___clear_bit generic___clear_bit > +#define const___change_bit generic___change_bit > +#define const___test_and_set_bit generic___test_and_set_bit > +#define const___test_and_clear_bit generic___test_and_clear_bit > +#define const___test_and_change_bit generic___test_and_change_bit > + > +/** > + * const_test_bit - Determine whether a bit is set > + * @nr: bit number to test > + * @addr: Address to start counting from > + * > + * A version of generic_test_bit() which discards the `volatile` qualifier to > + * allow the compiler to optimize code harder. Non-atomic and to be used only > + * for testing compile-time constants, e.g. from the corresponding macro, or > + * when you really know what you are doing. Not sure I understand the last sentence... Can you please rephrase? > + */ > +static __always_inline bool > +const_test_bit(unsigned long nr, const volatile unsigned long *addr) > +{ > + const unsigned long *p = (const unsigned long *)addr + BIT_WORD(nr); > + unsigned long mask = BIT_MASK(nr); > + unsigned long val = *p; > + > + return !!(val & mask); > +} > + > #endif /* __ASM_GENERIC_BITOPS_GENERIC_NON_ATOMIC_H */ > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 87087454a288..51c22b8667b4 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -36,7 +36,8 @@ extern unsigned long __sw_hweight64(__u64 w); > > /* Check that the bitops prototypes are sane */ > #define __check_bitop_pr(name) \ > - static_assert(__same_type(arch_##name, generic_##name) && \ > + static_assert(__same_type(const_##name, generic_##name) && \ > + __same_type(arch_##name, generic_##name) && \ > __same_type(name, generic_##name)) > > __check_bitop_pr(__set_bit); > -- > 2.36.1
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