Messages in this thread | | | Date | Thu, 4 Jan 2007 12:04:18 -0500 | From | "Albert Cahalan" <> | Subject | Re: kernel + gcc 4.1 = several problems |
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On 1/4/07, Segher Boessenkool <segher@kernel.crashing.org> wrote: > > Adjusting gcc flags to eliminate optimizations is another way to go. > > Adding -fwrapv would be an excellent start. Lack of this flag breaks > > most code which checks for integer wrap-around. > > Lack of the flag does not break any valid C code, only code > making unwarranted assumptions (i.e., buggy code).
Right, if "C" means "strictly conforming ISO C" to you. (in which case, nearly all real-world code is broken)
FYI, the kernel also assumes that a "char" is 8 bits. Maybe you should run away screaming.
> > The compiler "knows" > > that signed integers don't ever wrap, and thus eliminates any code > > which checks for values going negative after a wrap-around. > > You cannot assume it eliminates such code; the compiler is free > to do whatever it wants in such a case. > > You should typically write such a computation using unsigned > types, FWIW. > > Anyway, with 4.1 you shouldn't see frequent problems due to
Right, it gets much worse with the current gcc snapshots.
IMHO you should play such games with "g++ -O9", but that's a discussion for a different mailing list.
> "not using -fwrapv while my code is broken WRT signed overflow" > yet; and if/when problems start to happen, to "correct" action > to take is not to add the compiler flag, but to fix the code.
Nope, unless we decide that the performance advantages of a language change are worth the risk and pain. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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