[SkiFire13]

> int bar(int num) { return num / 2; } > > Doesn't get optimized to a single shift right, because the that won't work if num is negative.

Nit: some might think the reason this doesn't work is because the shift would "move" the sign bit, but actually arithmetic shifting instructions exist for this exact purpose. The reason they are not enough is because shifting provides the wrong kind of division rounding for negative numbers. This can however be fixed up by adding 1 if the number is negative (this can be done with an additional logical shift for moving the sign bit to the rightmost position and an addition).

[abainbridge]

Good point. I guess there are more cases than just this one where I'd like to be able to tell the compiler I don't care about rounding behaviour and would prefer the fastest code. Like -ffast-math but for integer operations. I don't think that exists. I wonder why.

[kragen]

Will shift, shift, and add be slower or faster than a divide instruction on machines with a divide instruction?

[SkiFire13]

Most likely no, division instructins generally take as much as 10-20 other arithmetic/logic instruction.