* Mark Dickinson: > On Tue, Dec 1, 2009 at 12:39 AM, Lawrence Crowl <crowl@xxxxxxxxxx> wrote: >> Gcc does optimizations based on knowing that signed integer overflow >> is undefined behavior. It may not catch conversion right now, >> but given time, it will. > > This surprises me. My understanding was that the result of > a conversion from an unsigned integer type to a signed > integer type, when the unsigned value doesn't fit into the > range of the signed type, is merely implementation defined > rather than undefined behaviour. There are some who think that, when properly documented, implementation-defined behavior can result in arbitrary effects, just as undefined behavior. > Section 4.5 of gcc's manual seems to say that gcc chooses to wrap > modulo 2**(width of the signed type) in this case. Is this likely > to change in future gcc versions? I wouldn't rule it out. Just use -fwrapv (perhaps after benchmarking to make sure that it doesn't make a difference). Other compilers will have similar switches.
