[tip:x86/asm] x86: Slightly tweak the access_ok() C variant for better code

[tip-bot for H. Peter Anvin]

Commit-ID:  a740576a4abf933de8f50787f24f24456cebd761
Gitweb:     http://git.kernel.org/tip/a740576a4abf933de8f50787f24f24456cebd761
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 17:02:53 -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..6f1bb74 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))
+	likely(!__range_not_ok(addr, size, user_addr_max()))
 
 /*
  * The exception table consists of pairs of addresses relative to the
--
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