I don't think the argument "Compilers are tools. They can reliably vectorize code if it is written in an amenable-to-vectorization form." is proved correct here. I think I'll stay at level 2.
In my experience relying on auto-vectorization is brittle:
- Even if the compiler should be able to auto-vectorize code written in an amenable-to-vectorization form, that doesn't mean it will. Maybe the particular compiler and version you're using does auto-vectorize, but that doesn't mean other compilers will. This might be a less of a problem in Rust, where you effectively have only compiler (although there are still compiler updates).
- Even if you managed to once write the code in an amenable-to-vectorization form, that doesn't mean that the code will stay that way forever. Code tends to change.
- Sometimes it's far less obvious what the amenable-to-vectorization form should look like, or what the reasons are for the compiler failing to auto-vectorize. The classic example is the compiler failing to auto-vectorize a floating point sum, since the order in which floating point numbers are added affects the result.
In my experience relying on auto-vectorization is brittle:
- Even if the compiler should be able to auto-vectorize code written in an amenable-to-vectorization form, that doesn't mean it will. Maybe the particular compiler and version you're using does auto-vectorize, but that doesn't mean other compilers will. This might be a less of a problem in Rust, where you effectively have only compiler (although there are still compiler updates).
- Even if you managed to once write the code in an amenable-to-vectorization form, that doesn't mean that the code will stay that way forever. Code tends to change.
- Sometimes it's far less obvious what the amenable-to-vectorization form should look like, or what the reasons are for the compiler failing to auto-vectorize. The classic example is the compiler failing to auto-vectorize a floating point sum, since the order in which floating point numbers are added affects the result.