[fpc-devel] Do bitwise operation (1 or 2) acre about the sign ? (Giving sign related hints on compilation)

[Hans-Peter Diettrich]

Martin schrieb:
>> During above calculation ("or") a sign extension is required because 
>> the result *must* have a definite sign. Else a following comparison of 
>> e.g. (x or j)>0 could not determine a result.
> 
> This is casting a "set of bits" (neither signed, nor unsigned - a set is 
> not a number at all) into a number. This only needs to have a 
> definition, if it should cast to signed or unsigned type.

A set of bits should be incompatible with ordinal values, as is, then 
everything is fine. The compiler would flag any mix as an error, instead 
of doing something unwanted.

Unsigned integer types can be represented as subranges of the next 
bigger signed type, then all problems with unwanted sign extension go 
away. When e.g. a file size exceeds 31 bits, it's not a good idea to 
store it as an unsigned 32 bit value, which will be too small again a 
few weeks later.


> The operation could be done with 32 bit
> set_of_bits := s32 or u32;
> 
> and if you do
>    set_of_bits > int
>    set_of_bits + int
> or anything then set_of_bits must be cast to an int type. It needs to be 
> defined if that should cast in to signed or unsigned.

Right. The intended cast should be inserted by the coder.

DoDi