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Comment by Sohcahtoa82 | Hacker News Reader

> Because there are no pointers in Fortran, only arrays, and because arguments to functions aren't allowed to alias [...], aggressive optimization and vectorization is easier.

Can you explain this like I'm 12?

What's an example of an aggressive optimization you can make based on arguments not being aliased and there being no pointers?

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  int *x;
  int *y;
  // ...
  *x = 5;

Because of aliasing, x and y might point to the same memory location, so the compiler must assume that when you modified x to 5, y also potentially got modifed, so at next access it will read y again from memory and discard any cached value in a register it might have.

The compiler will try proving that no such aliasing is present, by tracking your pointer usage, but it's not always possible, in those cases it will assume the worse and re-read values from memory.

Ah that makes sense. Thank you!

I finally understand—-thanks for this example.

Not a direct compiler optimization, but consider memcpy() vs memmove() as an example. If you know two regions of memory do not overlap you can call memcpy() for a direct optimized copy, but if they overlap you must call memmove() and introduce an intermediate copy.

It makes sense, but when would you ever memcpy with overlap? I would think any situation that lets that happen is from a bug, like you have an incorrect buffer length or an incorrect destination address.

Inserting an element in an array is something along the lines of memmove(arr + idx + 1, arr + idx, (length - idx) * sizeof(*arr)); arr[idx] = foo;

memmove does not (in any implementation I’ve ever heard of) introduce an intermediate copy, it just performs the copy loop in the reverse direction to handle the overlap (and can’t always vectorize in the same way memcpy can).

Here's a relevant thread: https://users.rust-lang.org/t/possible-rust-specific-optimiz...

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