Messages in this thread |  | | Date | Thu, 8 Feb 2001 16:24:23 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: [PATCH] micro-opt DEBUG_ADD_PAGE |
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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.
Hugh
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