In cases where you need a floored result, or need it rounded in a certain direction, or know that the divisor is always positive, the compiler will often give you sub-optimal assembly. And sometimes the compiler just randomly fails to inline or constant-fold and outputs a big fat IDIV.
Also, if you have to debug with optimizations disabled, the compiler will give you deliberately garbage code, which can make the program you're debugging unusably slow. So you often end up hand-optimizing for that case.
Of course, this depends on where the hot path is, but I've had to do a lot of optimization for code that gets run billions of times per second. I used to think compilers were really smart, but after staring at enough assembly output and learning all their tricks, they don't seem that smart anymore. Especially Microsoft's compiler; I've seen it output redundant division instructions!
The author mentions that this is for a variable length (i.e. bigint) format. Sure, I bet mature bigint libraries do stuff like this automatically when sensible, but I still think it's interesting to read about someone figuring out such things on their own.
In cases where you need a floored result, or need it rounded in a certain direction, or know that the divisor is always positive, the compiler will often give you sub-optimal assembly. And sometimes the compiler just randomly fails to inline or constant-fold and outputs a big fat IDIV.
Also, if you have to debug with optimizations disabled, the compiler will give you deliberately garbage code, which can make the program you're debugging unusably slow. So you often end up hand-optimizing for that case.
Of course, this depends on where the hot path is, but I've had to do a lot of optimization for code that gets run billions of times per second. I used to think compilers were really smart, but after staring at enough assembly output and learning all their tricks, they don't seem that smart anymore. Especially Microsoft's compiler; I've seen it output redundant division instructions!