From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Lobakin Subject: Re: [PATCH v2 4/6] bitops: define const_*() versions of the non-atomics Date: Wed, 15 Jun 2022 15:55:06 +0200 Message-ID: <20220615135506.1264880-1-alexandr.lobakin@intel.com> References: <20220610113427.908751-1-alexandr.lobakin@intel.com> <20220610113427.908751-5-alexandr.lobakin@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1655301419; x=1686837419; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=l3Dz9pCGYsKL9M71ANGJFS/yhgtgmIg63N13O2SpgOE=; b=ZIWSrq97lWkxdwWPN3eRZs8ibstsomDnaF3tgvvIEHOvnq1xblOCBrCp yk5n80X3PULcZ7WzqL9rQzrdP+t/gG5jhMkIXhb0DZYLlqw0KtqoHxg1g M9+RGxjdrYrjV+cVsvtVrcEqv1QXat75qk2orzn/iCY3T1iEr0nunDk0P a98fp6N4xWTpZGaqosb7MECLSJG/66B71mhSQ7frw9gvmpIZjyyEqYV72 YGNPTGgLlZXqAQvELv5WIGBhbPgh+WSERTM2vkRx/N/dFcQEtrsAdoubK +NledGY7xj20xztT7FGKKbcsybIsXEL10tpCYoX681H2ZWtK8GB+Y4Tlu g==; In-Reply-To: List-ID: To: Yury Norov Cc: Alexander Lobakin , Arnd Bergmann , Andy Shevchenko , Mark Rutland , Matt Turner , Brian Cain , Geert Uytterhoeven , Yoshinori Sato , Rich Felker , "David S. Miller" , Kees Cook , "Peter Zijlstra (Intel)" , Marco Elver , Borislav Petkov , Tony Luck , Greg Kroah-Hartman , linux-alpha@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-sh@vger.ke From: Yury Norov 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 > > Signed-off-by: Alexander Lobakin > > --- > > .../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