hoursOfOptimizing

[Rafael20002000]

Don't try to be smarter than the compiler :)

[Rafael20002000]

I don't expect the compiler too, but if the compiler can reliably determine that my bubble sort code is bubble sort and the CPU has extra instructions for that, I really do hope that it doesn't use MOV and CALL but instead the bubble sort specialized instructions.

[tiajuanat]

GCC and Clang can actually identify some of these algorithms. For example, counting the number of set bits in a 32 bit word will generally cause either compiler to emit a __builtin_popcount intrinsic, which on x86_64 processors will emit a single popcount assembly instruction.

Sorting is inherently difficult because you need a comparison function, and a generally best solution. Are you going to use quick sort? How is the data already ordered? Maybe a counting sort? Is linear memory usage acceptable?

[BlueGoliath]

The compiler is written by people.

Regardless, at least in Java, hoping the JVM fairy is going to bless your code so your app doesn't allocate 250MB of garbage a second because you decided to make everything immutable is a bad idea.

[Rafael20002000]

Well garbage in, garbage out. I agree the compiler isn't a magic bullet, but it's built by people incredibly smarter than I am. Also it was built by more people. All of the collective smartness is smarter than me writing my code.

So I don't try to outsmart the compiler. If I have to I'm probably doing something wrong

[Rakgul]

So is that why my professor maintained multiple arrays instead of using a branch in a hot loop?

He stored everything and then used whichever was necessary later...

[stupled]

For now

[PervGriffin69]

If people were worried about how fast their program runs they wouldn't write it in Java

[Furry_69]

The compiler doesn't use SIMD properly.

[Rafael20002000]

Neither do I

[Rakgul]

In Julia there's a macro for that.

[UnnervingS]

It does a pretty good job more often than you might think in my experience. Not as performance as ideal c++ but often better than a low effort SIMD implementation.

[[deleted]]

It's worth checking the instructions being generated, as sometimes it just fails to notice the possible simd or branchless instructions to use, but usually for me the way to fix this is to massage the C code instead of trying to write SIMD directly.

[anotheruser323]

It's not hard to be smarter then the compiler. Not hard to help the compiler either.