Re: [PATCH v2 4/6] bitops: define const_*() versions of the non-atomics

Alexander Lobakin
2022-06-15T13:57:06Z

From: Yury Norov <yury.norov@gmail.com>
Date: Tue, 14 Jun 2022 19:57:43 -0700

> 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?

Right, ooops.

>
> > 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@google.com>
> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> > ---
> > .../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?

I basically want to tell that there potentinally might be cases for
using those outside of the actual macros from 6/6. But it might be
redundant at all to mention this.

>
> > + */
> > +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

Thanks,
Olek