forked from Minki/linux
1a750e0cd7
On Wed, 18 Jun 2008, Linus Torvalds wrote: > > And yes, the "lock andl" should be noticeably faster than the xchgl. I dunno. Here's a untested (!!) patch that turns constant-bit set/clear_bit ops into byte mask ops (lock orb/andb). It's not exactly pretty. The reason for using the byte versions is that a locked op is serialized in the memory pipeline anyway, so there are no forwarding issues (that could slow down things when we access things with different sizes), and the byte ops are a lot smaller than 32-bit and particularly 64-bit ops (big constants, and the 64-bit ops need the REX prefix byte too). [ Side note: I wonder if we should turn the "test_bit()" C version into a "char *" version too.. It could actually help with alias analysis, since char pointers can alias anything. So it might be the RightThing(tm) to do for multiple reasons. I dunno. It's a separate issue. ] It does actually shrink the kernel image a bit (a couple of hundred bytes on the text segment for my everything-compiled-in image), and while it's totally untested the (admittedly few) code generation points I looked at seemed sane. And "lock orb" should be noticeably faster than "lock bts". If somebody wants to play with it, go wild. I didn't do "change_bit()", because nobody sane uses that thing anyway. I guarantee nothing. And if it breaks, nobody saw me do anything. You can't prove this email wasn't sent by somebody who is good at forging smtp. This does require a gcc that is recent enough for "__builtin_constant_p()" to work in an inline function, but I suspect our kernel requirements are already higher than that. And if you do have an old gcc that is supported, the worst that would happen is that the optimization doesn't trigger. Signed-off-by: Ingo Molnar <mingo@elte.hu> |
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