linux-kernel - [PATCH v2 4/6] bitops: define const

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Date:   Fri, 10 Jun 2022 13:34:25 +0200
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Matt Turner <mattst88@...il.com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-alpha@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 4/6] bitops: define const_*() versions of the non-atomics

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.
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@...gle.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.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.
+ */
+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