Add safe cstring rules

[lucic71]

No description provided.

[lucic71]

@nunoplopes do you think it's better to implement most of the cstring functions as part of Ptr<u8>? The current rules are not very idiomatic

[nunoplopes]

why can't we use memcpy here?

[lucic71]

f4 is memmove. memmove allows overlap between src and dst, memcpy does not (it's UB). Maybe it would be a good idea to add an assert inside Ptr::memcpy to panic on overlapping regions.

[nunoplopes]

But our implementation of memcpy is ok with overlaps. There's no point in reinventing the wheel.

[lucic71]

It's not ok if dst > src. For the following code:

#include <string.h>

int main(void) {
    char buf[6] = {1, 2, 3, 4, 5, 6};

    memmove(buf + 2, buf, 4);

    for (int i = 0; i < 6; i++) {
      printf("%d ", buf[i]);
    }
    printf("\n");

    return 0;
}

The output is:

C:                                 1 2 1 2 3 4 (correct, it's a shift right)
memmove translated as Ptr::memcpy: 1 2 1 2 1 2 (wrong)

[lucic71]

I think an assert on overlapping Ptr::memcpy would be useful. Memcpy with overlapping memory is UB in C

[nunoplopes]

I know it's UB in C. But our safe Rust implementation can handle overlapping memory. So let's use it.

[lucic71]

In this case I will change the implementation of Ptr::memcpy to fully behave as memmove

[nunoplopes]

unsafe

[lucic71]

Done

[nunoplopes]

you can decrement a2; there's no need for __i

[lucic71]

It makes no difference, a2 is textually replaced inside the rule. We cannot decrement a2 until it reaches 0, we need to save it in a variable eitherway

[nunoplopes]

same here; can be decremented

[lucic71]

It makes no difference, a2 is textually replaced inside the rule. We cannot decrement a2 until it reaches 0, we need to save it in a variable eitherway

[nunoplopes]

I would rather read a1 into a local vector. seems easier to optimize.

[lucic71]

No longer relevant. I replaced the __hit = loop idiom with CStringIterator

[nunoplopes]

this code should be using CStringIterator instead. becomes a lot smaller.

[lucic71]

Done. Also used CStringIterator in other rules