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x86: Slightly tweak the access_ok() C variant for better code

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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@linux-foundation.org>
Link: http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2BOKJFac9XW2awegogTkVTA@mail.gmail.com
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
This commit is contained in:
H. Peter Anvin 2013-12-27 16:52:47 -08:00
parent c5fe5d8068
commit a740576a4a
1 changed files with 5 additions and 3 deletions
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8
arch/x86/include/asm/uaccess.h
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@ -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