On 1/4/07, Segher Boessenkool <[email protected]> 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.
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