> Summing floats should by default be taken to have error bounds, and any answer in those bounds is valid. If you know something special about the floats you are inputting, the language should have some means to explicitly encode that. It shouldn’t be the most basic loop, that’s the default case, so it should give the best performance.

This is a lot of "should" for something that basically doesn't happen. The only information you give is the order of arithmetic in the original expression.

It is a total nightmare if arithmetic is not stable between builds. Rebuilding the software and running it on the same input should not produce different results.

(Long ago I encountered the special Intel version of this: because the FPU used 80-bit registers internally but 64-bit in memory, changing when a register fill/spill happened would change when your answer got rounded, and thereby change the results. You can set a global FPU flag at the start of the program to force rounding on every operation.)