Messages in this thread | | | Date | Mon, 7 May 2007 13:09:42 +0200 (CEST) | From | Esben Nielsen <> | Subject | Re: [patch] CFS scheduler, -v8 |
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On Sat, 5 May 2007, Linus Torvalds wrote:
> > > On Sat, 5 May 2007, Esben Nielsen wrote: >> >> I have been wondering why you use usigned for timers anyway. It is also like >> that in hrtimers. Why not use signed and avoid (almost) all worries about wrap >> around issues. The trick is that when all >> a < b >> is be replaced by >> a - b < 0 >> the code will work on all 2-complement machines even if the (signed!) integers >> a and b wrap around. > > No. BOTH of the above are buggy. > > The C language definition doesn't allow signed integers to wrap (ie it's > undefined behaviour), so "a-b < 0" can be rewritten by the compiler as a > simple signed "a < b". > > And the unsigned (or signed) "a < b" is just broken wrt any kind of > wrap-around (whether wrapping around zero or the sign bit). > > So the _only_ valid way to handle timers is to > - either not allow wrapping at all (in which case "unsigned" is better, > since it is bigger) > - or use wrapping explicitly, and use unsigned arithmetic (which is > well-defined in C) and do something like "(long)(a-b) > 0". > > Notice? The signed variant is basically _never_ correct. >
What is (long)(a-b) ? I have tried to look it up in the C99 standeard but I can't find it. Maybe it is in the referred LIA-1 standeard, which I can't find with google.
I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b" as that should be completely portable.
According to C99 Appendix H2.2 (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf) an implementation can choose to do modulo signed integers as it is mandatory for unsigned integers. If an implementation have choosen to do that it must be a bug to to do the "a-b < 0" -> "a<b" optimization.
I have never experienced a compiler/architecture combination _not_ doing wrapped signed integers.
Esben
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