linux-kernel - [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Date:	Mon, 23 Jul 2007 21:35:58 +0530
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

From: Satyam Sharma <ssatyam@....iitk.ac.in>

[6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

The goal is to let gcc generate good, beautiful, optimized code.

But test_and_set_bit, test_and_clear_bit, __test_and_change_bit,
and test_and_change_bit unnecessarily mark all of memory as clobbered,
thereby preventing gcc from doing perfectly valid optimizations.

The case of __test_and_change_bit() is particularly surprising, given
that it's a variant where we don't make any guarantees at all.

Even for the other three cases, the (only) instruction that accesses
shared memory is btsl/btrl/btcl and requires locking and atomicity.
But we handle that properly already by the use of the lock prefix.
Also, "=m" is already specified in the output operands of the asm
for the passed bit-string pointer, so the gcc optimizer knows what
to do and what not to do anyway.

So there's no point clobbering all of memory in these functions.

Signed-off-by: Satyam Sharma <ssatyam@....iitk.ac.in>
Cc: David Howells <dhowells@...hat.com>
Cc: Nick Piggin <nickpiggin@...oo.com.au>

---

 include/asm-i386/bitops.h |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/asm-i386/bitops.h b/include/asm-i386/bitops.h
index 0f5634b..f37b8a2 100644
--- a/include/asm-i386/bitops.h
+++ b/include/asm-i386/bitops.h
@@ -164,7 +164,7 @@ static inline int test_and_set_bit(int nr, unsigned long *addr)
 	__asm__ __volatile__( LOCK_PREFIX
 		"btsl %2,%1\n\tsbbl %0,%0"
 		:"=r" (oldbit),"=m" (*addr)
-		:"r" (nr) : "memory");
+		:"r" (nr));
 	return oldbit;
 }
 
@@ -208,7 +208,7 @@ static inline int test_and_clear_bit(int nr, unsigned long *addr)
 	__asm__ __volatile__( LOCK_PREFIX
 		"btrl %2,%1\n\tsbbl %0,%0"
 		:"=r" (oldbit),"=m" (*addr)
-		:"r" (nr) : "memory");
+		:"r" (nr));
 	return oldbit;
 }
 
@@ -254,7 +254,7 @@ static inline int __test_and_change_bit(int nr, unsigned long *addr)
 	__asm__ __volatile__(
 		"btcl %2,%1\n\tsbbl %0,%0"
 		:"=r" (oldbit),"=m" (*addr)
-		:"r" (nr) : "memory");
+		:"r" (nr));
 	return oldbit;
 }
 
@@ -275,7 +275,7 @@ static inline int test_and_change_bit(int nr, unsigned long *addr)
 	__asm__ __volatile__( LOCK_PREFIX
 		"btcl %2,%1\n\tsbbl %0,%0"
 		:"=r" (oldbit),"=m" (*addr)
-		:"r" (nr) : "memory");
+		:"r" (nr));
 	return oldbit;
 }
 
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