Question

When I read this question I remembered someone once telling me (many years ago) that from an assembler-point-of-view, these two operations are very different:

n = 0;

n = n - n;

Is this true, and if it is, why is it so?

EDIT: As pointed out by some replies, I guess this would be fairly easy for a compiler to optimize into the same thing. But what I find interesting is why they would differ if the compiler had a completely general approach.

Answer 1

In the early days, memory and CPU cycles were scarce. That lead to a lot of so called “peep-hole optimizations”. Let’s look at the code:

    move.l #0,d0

    moveq.l #0,d0

    sub.l a0,a0

The first instruction would need two bytes for the op-code and then four bytes for the value (0). That meant four bytes wasted plus you’d need to access the memory twice (once for the opcode and once for the data). Sloooow.

moveq.l was better since it would merge the data into the op-code but it only allowed to write values between 0 and 7 into a register. And you were limited to data registers only, there was no quick way to clear an address register. You’d have to clear a data register and then load the data register into an address register (two op-codes. Bad.).

Which lead to the last operation which works on any register, need only two bytes, a single memory read. Translated into C, you’d get

n = n - n;

which would work for most often used types of n (integer or pointer).