Commit-ID: 34f873a773c59f738aff8f412fa59d528f1b3e3f
Gitweb: http://git.kernel.org/tip/34f873a773c59f738aff8f412fa59d528f1b3e3f
Author: H. Peter Anvin <hpa@xxxxxxxxx>
AuthorDate: Fri, 27 Dec 2013 16:52:47 -0800
Committer: H. Peter Anvin <hpa@xxxxxxxxx>
CommitDate: Fri, 27 Dec 2013 16:58:17 -0800
x86: Slightly tweak the access_ok() C variant for better code
gcc can under very specific circumstances realize that the code
sequence:
foo += bar;
if (foo < bar) ...
... is equivalent to a carry out from the addition. Tweak the
implementation of access_ok() (specifically __chk_range_not_ok()) to
make it more likely that gcc will make that connection. It isn't
fool-proof (sometimes gcc seems to think it can make better code with
lea, and ends up with a second comparison), still, but it seems to be
able to connect the two more frequently this way.
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2BOKJFac9XW2awegogTkVTA@xxxxxxxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxx>
---
arch/x86/include/asm/uaccess.h | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 84ecf1d..4f02113 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -41,7 +41,7 @@
* Test whether a block of memory is a valid user space address.
* Returns 0 if the range is valid, nonzero otherwise.
*/
-static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, unsigned long limit)
+static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, unsigned long limit)
{
/*
* If we have used "sizeof()" for the size,
@@ -55,7 +55,9 @@ static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, uns
/* Arbitrary sizes? Be careful about overflow */
addr += size;
- return (addr < size) || (addr > limit);
+ if (addr < size)
+ return true;
+ return addr > limit;
}
#define __range_not_ok(addr, size, limit) \
@@ -84,7 +86,7 @@ static inline int __chk_range_not_ok(unsigned long addr, unsigned long size, uns
* this function, memory access functions may still return -EFAULT.
*/
#define access_ok(type, addr, size) \
- (likely(__range_not_ok(addr, size, user_addr_max()) == 0))
+ unlikely(!__range_not_ok(addr, size, user_addr_max()))
/*
* The exception table consists of pairs of addresses relative to the
--
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