3 ms·

Don't compilers, cpu branch prediction, etc all fix the performance issues behind the scenes for the most part?

malux85 3y ago
No, compilers (correctly) prefer correctness over speed, so they can optimise “obvious” things, but they cannot account for domain knowledge or inefficiencies further apart, or that “might” alter some global state, so they can only make optimisations where they can be very sure there’s no side effects, because they have to err on the side of caution. They will only give you micro optimisations which could cumulatively speed up sometimes but the burden of wholistic program efficiency is still very much on the programmer. If you’re emptying the swimming pool using only a glass, the compiler will optimise the glass size, and your arm movements, but it won’t optimise “if you’re emptying the correct pool” or “if you should be using a pump instead” - a correct answer to the latter two could be 100,000 times more efficient than the earlier two, which a compiler could answer.
jchw 3y ago
The short answer is absolutely not, even when you are sure that it should. Even something as simple as a naive byteswap function might wind up generating surprisingly suboptimal code depending on the compiler. If you really want to be sure, you're just going to have to check. (And if you want to check, a good tool is, of course, Compiler Explorer.)
Thaxll 3y ago
Well compilers are good and dumb at the same time.
bee_rider 3y ago
Some of the moves seemed to change what an individual function might do. For example they suggested pulling an if from a function to the calling function. Could the compiler figure it out? My gut says maybe; maybe if it started by inlining the callee? But inlining happens based on some heuristics usually, this seems like an unreliable strategy if it would even work at all.
roywashere 3y ago
Function call overhead can be a real issue in languages like Python and JavaScript. But you can or should measure when in doubt!
- hansvm 3y ago
  It's a real issue in most compiled languages too if you're not careful (also a sort of opposite issue; too few functions causing unnecessary bloat and also killing performance).
ezekiel68 3y ago
This is just a myth promoted by Big Compiler designed to sell you more compiler.
Izkata 3y ago
There's a pretty famous StackOverflow question/answer about branch prediction failure: https://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-processing-an-unsorted-array https://stackoverflow.com/questions/11227809/why-is-processi...

No, compilers (correctly) prefer correctness over speed, so they can optimise “obvious” things, but they cannot account for domain knowledge or inefficiencies further apart, or that “might” alter some global state, so they can only make optimisations where they can be very sure there’s no side effects, because they have to err on the side of caution. They will only give you micro optimisations which could cumulatively speed up sometimes but the burden of wholistic program efficiency is still very much on the programmer. If you’re emptying the swimming pool using only a glass, the compiler will optimise the glass size, and your arm movements, but it won’t optimise “if you’re emptying the correct pool” or “if you should be using a pump instead” - a correct answer to the latter two could be 100,000 times more efficient than the earlier two, which a compiler could answer.

jchw 3y ago

The short answer is absolutely not, even when you are sure that it should. Even something as simple as a naive byteswap function might wind up generating surprisingly suboptimal code depending on the compiler. If you really want to be sure, you're just going to have to check. (And if you want to check, a good tool is, of course, Compiler Explorer.)

Thaxll 3y ago

Well compilers are good and dumb at the same time.

bee_rider 3y ago

Some of the moves seemed to change what an individual function might do. For example they suggested pulling an if from a function to the calling function. Could the compiler figure it out? My gut says maybe; maybe if it started by inlining the callee? But inlining happens based on some heuristics usually, this seems like an unreliable strategy if it would even work at all.

roywashere 3y ago

Function call overhead can be a real issue in languages like Python and JavaScript. But you can or should measure when in doubt!

hansvm 3y ago
It's a real issue in most compiled languages too if you're not careful (also a sort of opposite issue; too few functions causing unnecessary bloat and also killing performance).

ezekiel68 3y ago

This is just a myth promoted by Big Compiler designed to sell you more compiler.

Izkata 3y ago

There's a pretty famous StackOverflow question/answer about branch prediction failure: https://stackoverflow.com/questions/11227809/why-is-processing-a-sorted-array-faster-than-processing-an-unsorted-array https://stackoverflow.com/questions/11227809/why-is-processi...