3 ms·

So the main point here is we save some time with a direct jump at least for a few values of x? It's cool I guess but the extra confusion might not be worth it.

Sanddancer 10y ago
The author is coming from the embedded world, where saving time in an important loop can determine whether or not your code can operate in the required time windows. Keep in mind that there are a lot of micros out there that are being used for new projects that have memory amounts in the hundreds or even tens of bytes. When you start getting into those areas, sometimes you have to cheat to get things working.
- noobermin 10y ago
  Sure, in that case, then I agree with the saurik. The solution is you just put in more labels in your switch statement. And, if you aren't so keen on that sort of discipline, use a macro ALL_CMDS so you don't have to change nine places in the code. Go "O(1)" on all the CMDs or don't, for the sake of ease of comprehension. In the hypothetical, or may be practical world[0] where size+computation constraints require you must goto some of them and if-else the rest of them, I suppose the "1/2goto,1/2if-else" optimization would be one of the latter considerations for optimization over a host of other things you can consider. [0] I admit I don't hack in that world, but I do computational physics, which is more about vectorizing things, not optimization of branching typically.
julian_1 10y ago
A compiler like gcc will (or at least used to) translate small switch statements into compare/jmp assembler, and larger switch statements into a jump table using indexes/modulos on the test expression. The tipping point for when the compiler decides to switch strategies, is obviously architecture dependent but could also be influenced by what flags are set and whether to favor for small code side, or highest speed. I suspect, the code could have been written using two switch statements - and probably achieved the same effect in terms of the generated assembler, as well as be more idiomatic.

Sanddancer 10y ago

The author is coming from the embedded world, where saving time in an important loop can determine whether or not your code can operate in the required time windows. Keep in mind that there are a lot of micros out there that are being used for new projects that have memory amounts in the hundreds or even tens of bytes. When you start getting into those areas, sometimes you have to cheat to get things working.

noobermin 10y ago
Sure, in that case, then I agree with the saurik. The solution is you just put in more labels in your switch statement. And, if you aren't so keen on that sort of discipline, use a macro ALL_CMDS so you don't have to change nine places in the code. Go "O(1)" on all the CMDs or don't, for the sake of ease of comprehension. In the hypothetical, or may be practical world[0] where size+computation constraints require you must goto some of them and if-else the rest of them, I suppose the "1/2goto,1/2if-else" optimization would be one of the latter considerations for optimization over a host of other things you can consider. [0] I admit I don't hack in that world, but I do computational physics, which is more about vectorizing things, not optimization of branching typically.

julian_1 10y ago

A compiler like gcc will (or at least used to) translate small switch statements into compare/jmp assembler, and larger switch statements into a jump table using indexes/modulos on the test expression. The tipping point for when the compiler decides to switch strategies, is obviously architecture dependent but could also be influenced by what flags are set and whether to favor for small code side, or highest speed. I suspect, the code could have been written using two switch statements - and probably achieved the same effect in terms of the generated assembler, as well as be more idiomatic.