[llvm-dev] Reducing the number of ptrtoint/inttoptrs that are generated by LLVM

[Sanjoy Das via llvm-dev]

On Mon, Jan 14, 2019 at 3:23 AM Juneyoung Lee via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Patch https://reviews.llvm.org/D56598 adds llvm.psub(p1,p2) intrinsic function, which subtracts two pointers and returns the difference. Its semantic is as follows.
>
> If p1 and p2 point to different objects, and neither of them is based on a pointer casted from an integer, `llvm.psub(p1, p2)` returns poison. For example,

Are you proposing landing this in conjunction with some of the other
stuff discussed in the twin allocation paper?  Otherwise isn't this
problematic with propagateEquality of pointers?

a = malloc()
free(a)
b = malloc() // Assume b == a numerically
if (a == b) {
  print(psub(b, b)) // prints 0
}

=>

a = malloc()
free(a)
b = malloc() // Assume b == a numerically
if (a == b) {
  print(psub(a, b)) // prints poison
}

Though I admit propagateEquality of pointers has many other problems like this.


-- Sanjoy