This will be a fairly short blog post about a neat trick that I found today that makes sign extending numbers in C correctly trivial. It is based on the version found in this great list of bit hacks, but generalized.
What is sign extending, anyway?
Sign extension means growing a number—say, for instance, from 32 to 64 bits— while preserving the sign bit. The sign bit is always the highest bit in the numerical formats used today—this is purely by convention, but to my knowledge always true. This means that we have to shift the most significant bit up to the new highest position, while keeping the rest of the number intact.
If you’re coming from C, you’re probably saying “But C does that automatically!” right around now. And that’s true for builtin types. But say you’re building your own numerical format, like I’ve done in the past; then you’ll either need to do that manually or rely on this beautifully elegant trick I’m about to show you.
Remember bitfields?
Bitfields are a great way of controlling just how many bits a particular value should take up. Of course they’re useless if you want to save space that’s less than the size of an int—or are they? Enter sign extension:
#define extend(O,I,N) {\
struct {signed int x:N;} tmp_;\
O = tmp_.x = I;\
}
/* Later... */
int x; // A 9 bit number
int y; // A regular int I want to assign to
extend(y, x, 9);Before I get into what I just did there, let me address some of the concerns of the people who already know and are about to ragequit. I do introduce a new binding tmp_ here and hope for the best. Admittedly, that’s not great. A somewhat less crude, but still not entirely secure way would be emulating gensym, although in my opinion that, too, is a duct tape solution at best. I also assign to a thing that was given to me as input, which is a little shady, I’ll admit. It will, however, make the API more pleasant to work with, and so I’ll gladly trade one vice for another.
Now, let’s get back to business: What are we doing here? We are basically exploiting the fact that through bitfields we can defer the problem of sign extending to a higher authority—the compiler, namely, which in turn will probably defer the problem to the processor by using a dedicated instruction. So, by having an immediate assignment only—and the preceding definition of a bitfield—we can stop worrying about sign extension. Pretty cool, huh?
C++ folks are of course free to use templates and make this a little less fragile.