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.

[–] [email protected] 7 points 7 months ago (1 children)

so if (somehow) the accumulator was an integer, this loop would autovectorize and the performance differences would be smaller ?

[–] [email protected] 4 points 7 months ago

Very likely yes