"Clean" Code, Horrible Performance in Rust (chrs.dev)

submitted 5 months ago by chrs@programming.dev to c/rust@programming.dev

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[-] zweieuro@lemmy.world 16 points 5 months ago

Correct me if I am wrong but isn't "loop unrolling/unwinding" something that the c++ and rust compilers do? Why does the loop here not get unwound?

[-] Giooschi@lemmy.world 14 points 5 months ago

Loop unrolling is not really the speedup, autovectorization is. Loop unrolling does often help with autovectorization, but is not enough, especially with floating point numbers. In fact the accumulation operation you're doing needs to be associative, and floating point numbers addition is not associative (i.e. (x + y) + z is not always equal to (x + (y + z)). Hence autovectorizing the code would change the semantics and the compiler is not allowed to do that.