[PATCH 1/2] x86/bitops: implement __test_bit

From: Michael S. Tsirkin
Date: Sun Aug 30 2015 - 04:38:41 EST


One little known side effect of test_bit is that it adds
a kind of a compiler barrier since the pointer parameter
is volatile.

It seems risky to change the semantics of test_bit so let's just
add __test_bit (matching __set_bit and __clear_bit) that does
not add such a barrier.

Will be used by kvm on x86, where it shaves several bytes off
the binary size. Small win, but comes at no cost, so why not.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---

x86 maintainers - please specify whether you are ok with
adding this to arch/x86/include/asm/bitops.h
An alternative is to add this to kvm/x86 only.
It might be worth it to add this to all architectures,
though I haven't explored too much.

arch/x86/include/asm/bitops.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index cfe3b95..9229334 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -323,6 +323,24 @@ static inline int variable_test_bit(long nr, volatile const unsigned long *addr)
return oldbit;
}

+static __always_inline int __constant_test_bit(long nr, const unsigned long *addr)
+{
+ return ((1UL << (nr & (BITS_PER_LONG-1))) &
+ (addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
+}
+
+static inline int __variable_test_bit(long nr, const unsigned long *addr)
+{
+ int oldbit;
+
+ asm volatile("bt %2,%1\n\t"
+ "sbb %0,%0"
+ : "=r" (oldbit)
+ : "m" (*addr), "Ir" (nr));
+
+ return oldbit;
+}
+
#if 0 /* Fool kernel-doc since it doesn't do macros yet */
/**
* test_bit - Determine whether a bit is set
@@ -330,6 +348,13 @@ static inline int variable_test_bit(long nr, volatile const unsigned long *addr)
* @addr: Address to start counting from
*/
static int test_bit(int nr, const volatile unsigned long *addr);
+
+/**
+ * __test_bit - Determine whether a bit is set
+ * @nr: bit number to test
+ * @addr: Address to start counting from
+ */
+static int __test_bit(int nr, const volatile unsigned long *addr);
#endif

#define test_bit(nr, addr) \
@@ -337,6 +362,11 @@ static int test_bit(int nr, const volatile unsigned long *addr);
? constant_test_bit((nr), (addr)) \
: variable_test_bit((nr), (addr)))

+#define __test_bit(nr, addr) \
+ (__builtin_constant_p((nr)) \
+ ? __constant_test_bit((nr), (addr)) \
+ : __variable_test_bit((nr), (addr)))
+
/**
* __ffs - find first set bit in word
* @word: The word to search
--
MST

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