Messages in this thread | | | From | "Perez-Gonzalez, Inaky" <> | Subject | RE: [PATCH] include/asm-ARCH/page.h:get_order() Reorganize and op timize | Date | Wed, 13 Nov 2002 16:43:37 -0800 |
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> > > > s = --s >> PAGE_SHIFT; > > > > > > This code has undefined behaviour. > > > > Do you mean that this: > > > > s = (s-1) >> PAGE_SHIFT > > > > is more deterministic? If so, I agree -- if you mean something else, > > I am kind of lost. > > I mean that this code violates the rule that you may modify a value > only once between two sequence points. Newer gccs have a warning for > this (-Wsequence-point), the info page tells more.
Agreed, and it was a poor choice from me. (s-1) should be correct, and gcc-3.2 with -Wsequence-point is happy about it.
> Also, did I understand it right that you want to use fls even on > architectures that don't have it as a builtin? I would guess that will > actually be noticeably slower, since generic_fls is so complicated.
Well, it is not that bad, and it is still faster [did a quick test now]
0.144s, 0.241s and 0.358s
[get_order on all numbers from 0 to 90000000 with the optimized version, the one that uses generic_fls and the old one].
Inaky Perez-Gonzalez -- Not speaking for Intel - opinions are my own [or my fault]
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