kevin diggs <diggskevin38@xxxxxxxxx> writes: > The default the compiler chooses will depend on idiosyncrasies of the > processor (i.e. the signed compare instruction is faster or the > instruction is smaller (less bytes to represent it)). Or the jumps for > one or the other are smaller or faster. Or have a larger range. Or are > guaranteed to work during a full moon . ... > > In reading through the internals doc, I stumbled across something I > had not realized: > > Some machines have different compare instruction for signed and > unsigned values but only one 'set' of branches. I think powerpc is an > example (cmp crfD,L,rA,rB for signed and cmpl crfD,L,rA,rB for > unsigned). Others have one compare for both and branches for both > signed and unsigned results. Like x86s conditional jumps: > > jb,jae,jbe,ja for unsigned, and > > jl, jge, jle, jg for signed. > > As far as I know, neither of these archs have any reason > (architecturally speaking) to choose one or the other. Anyone? Most machines have an instruction which loads a single byte from memory into a register which is more than a single byte. Some machines sign extend the loaded value, some machines zero extend, some machines have both kinds of instruction. On machines which only have sign-extend or zero-extend, that is a good choice to use for the default signedness of char. Ian
