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.

▲ rurban 3 days ago | parent | next [-]

Then show me the compiler which tells the user that it removed this dead code. There is even an assignment removed, which violates all expectations

▲ torstenvl 4 days ago | parent | prev [-]

> It can't if arbitrary code (for example inside bar()) can forge pointers to that memory location.

Yes. It absolutely can. What are you even talking about?

C is not the Windows Start Menu. This habit of thinking it needs to do what it thinks I might expect instead of what I told it is deeply psychotic.

▲

gpderetta 4 days ago | parent [-]

I litterally have no idea what are you trying to say. Do you mean that bar should be allowed to access *ptr with impunity or not?

▲

torstenvl 4 days ago | parent [-]

I'm not trying to say anything. I said and meant exactly what I said. No more, no less. Your logic is obviously flawed. There is nothing preventing that optimization in the presence of a forged pointer in bar().

▲

gpderetta 4 days ago | parent [-]

Either there is no provenance, forging is allowed and the optimization is disallowed; or there is provenance and forging the pointer and attempting to inspect (or modify) the value of *ptr in bar() is UB.

	▲	ncruces 4 days ago \| parent \| next [-]
		You never converted ptr to an integer. If you did, if the pointer escapes, yes, I claim that then the allocation can't be optimized away. Why is that so bad?
	▲	torstenvl 4 days ago \| parent \| prev [-]
		Attempting to inspect or modify the value of *ptr in bar() through a forged pointer was always UB. You are saying absolutely nothing meaningful.