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

Alexander Lobakin
2022-06-10T11:35:59Z

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@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.
+ */
+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