Comment by gpderetta

Casting a char pointer to a vector pointer and doing vector loads doesn't violate provenance, although it might violate TBAA.

Regarding provenance, consider this:

  void bar();
  int foo() {
    int * ptr = malloc(sizeof(int));
    *ptr = 10;
    bar();
    int result = *ptr;
    free(ptr);
    return result;
  }

If the compiler can track the lifetime of the dynamically allocated int, it can remove the allocation and covert this function to simply

  int foo() { 
      bar();
      return 10;
  }

It can't if arbitrary code (for example inside bar()) can forge pointers to that memory location. The code can seem silly, but you could end up with something similar after inlining.

Can't reply to the sibling comment, for some reason.

If you don't know the extents of the object pointed to by the char*, using an aligned vector load can reach outside the bounds of the object. Keeping provenance makes that undefined behavior.

Using integer arithmetic, and pointer-to-integer/integer-to-pointer conversions would make this implementation defined, and well defined in all of the hardware platforms where an aligned vector load can never possibly fail.

So you can't do some optimizations to functions where this happens? Great. Do it. What else?

As for why you'd want to do this. C makes strings null-terminated, and you can't know their extents without strlen first. So how do you implement strlen? Similarly your example. Seems great until you're the one implementing malloc.

But I'm sure "let's create undefined behavior for a libc implemented in C" is a fine goal.