value range propagation for _bitwise_ OR

[bearophile]

Sorry for the slow answer, I have some things to catch up.

Fawzi Mohamed:

>integral overflow are helpful only if you have automatic conversion to a larger type,<

I don't understand what you mean here. There are various ways to detect overflows, from the simple ones like using a long to compute operations on int, and then looking for int overflows, to looking at CPU overflow and carry flags, and so on. Such strategies to don't change the size of the variables on the stack or on the heap, it's mostly a matter of different operations done inside CPU registers.



>but that breaks the compile time knowledge of the size of such an integer,<

Nope, the compiler knows statically the variable sizes, returns the same values regardless integral overflow is used or not.


>so that you have to assume that it might need to be pushed to the heap.<

It requires no heap activity.


>Yes you might use some tricks (like using size_t.sizeof*8-1 bits, or a pointer to spare some place), but I don't think that D wants to go that way for the basic integers...<

Tagging values is almost a science, it's not a trick :-)
Tagged values have some advantages over the C-style fixnums.
(Related: Bigint can use a similar tagging scheme to avoid heap allocation when the integer is small.)
But you don't need tagged values to perform overflow tests, both Dephi and C# use normal untagged values.

Bye,
bearophile