x86, mm: Redesign get_user with a __builtin_choose_expr hack

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Instead of using a bitfield, use an odd little trick using typeof,
__builtin_choose_expr, and sizeof.  __builtin_choose_expr is
explicitly defined to not convert its type (its argument is required
to be a constant expression) so this should be well-defined.

The code is still not 100% preturbation-free versus the baseline
before 64-bit get_user(), but the differences seem to be very small,
mostly related to padding and to gcc deciding when to spill registers.

Cc: Jamie Lokier <jamie@shareable.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. J. Lu <hjl.tools@gmail.com>
Link: http://lkml.kernel.org/r/511A8922.6050908@zytor.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

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This commit is contained in:

H. Peter Anvin 2013-02-12 11:47:31 -08:00

parent 16640165c9

commit 3578baaed4

1 changed files with 14 additions and 43 deletions

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arch/x86/include/asm/uaccess.h

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