Messages in this thread |  | | Date | Thu, 8 Feb 2001 17:37:45 +0100 | From | David Weinehall <> | Subject | Re: [PATCH] micro-opt DEBUG_ADD_PAGE |
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On Thu, Feb 08, 2001 at 04:24:23PM +0000, Hugh Dickins wrote: > On Wed, 7 Feb 2001, Linus Torvalds wrote: > > On Wed, 7 Feb 2001, Hugh Dickins wrote: > > > > > > None of those optimizes this: I believe the semantics of "||" (don't > > > try next test if first succeeds) forbid the optimization "|" gives? > > > > No. The optimization is entirely legal - but the fact that > > "constant_test_bit()" uses a "volatile unsigned int *" is the reason why > > gcc thinks it can't optimize it. > > Ah, yes, I hadn't noticed that, the "volatile" is indeed why it ends up > with three "mov"s. But take the "volatile"s out of constant_test_bit(), > and DEBUG_ADD_PAGE still expands to three tests and three (four if 2.97) > jumps - which is what originally offended me. > > But Mark (in test program in private mail) shows gcc combining bits > into one test and one jump, just as we'd hope (and I wrongly thought > forbidden). Perhaps the inline function nature of constant_test_bit() > (which Mark didn't use) gets in the way of combining those tests. > > > You could try to remove the volatile from test_bit, and see if that fixes > > it - but then we'd have to find and add the proper "rmb()" calls to people > > who do the endless loop kind of thing like above. > > That is not an inviting path to me, at least not any time soon! > > I think this all argues for the little patch I suggested - just avoid > test_bit() here. But it was only intended as a quick little suggestion: > looks like our tastes differ, and you prefer taking the _tiny_ hit of > using the regular macros, to seeing "1<<PG_bitshift"s in DEBUG_ADD_PAGE.
Well, after all, it's debugging code, and the code now is easy to read. Your code, while more efficient, isn't. I think that clarity takes priority over efficiency in non-critical code such as debugging code. Of course, this is my personal opinion...
/David _ _ // David Weinehall <tao@acc.umu.se> /> Northern lights wander \\ // Project MCA Linux hacker // Dance across the winter sky // \> http://www.acc.umu.se/~tao/ </ Full colour fire </ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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