linux-kernel - [PATCH 1/2] x86/bitops: implement __test

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Date:	Sun, 30 Aug 2015 11:38:09 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: [PATCH 1/2] x86/bitops: implement __test_bit

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@...hat.com>
---

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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