[PATCH] D18738: Add new !unconditionally_dereferenceable load instruction metadata

[Sanjoy Das via llvm-commits]

sanjoy added a comment.

In https://reviews.llvm.org/D18738#592084, @hfinkel wrote:

> >   void bar(int* ptr) {
> >     int* k1 = foo1(ptr, true);
> >     // inlined int* k2 = foo_common(alloca(...), false);
> >     t = alloca(...)
> >     int* val;
> >     if (false) {
> >       val = *t, unconditionally_dereferenceable
> >     } else {
> >       val = *ptr
> >     }
> >   }
>
> The assumption is placed on the return value, and val would not unconditionally have the assumption, and so never will really, so there's not a problem here (AFAIKT).


Sure, but isn't that the same as `!dereferenceable` then?

I thought the idea here was to be able to hoist the load of `val` out of the never-taken branch and still preserve the `!unconditionally_dereferenceable` attribute.  That would mean if the example was a bit more complicated:

  void bar(int* ptr) {
    int* k1 = foo1(ptr, true);
    // inlined int* k2 = foo_common(alloca(...), false);
    t = alloca(...)
    int* val;
    if (false) {
      val = *t, unconditionally_dereferenceable
      int k = *val;
      print(k)
    } else {
      val = *ptr
    }
  }

(hoisting)

  void bar(int* ptr) {
    int* k1 = foo1(ptr, true);
    // inlined int* k2 = foo_common(alloca(...), false);
    t = alloca(...)
    int* val;
    val = *t, unconditionally_dereferenceable
    if (false) {
      int k = *val;
      print(k)
    } else {
      val = *ptr
    }
  }

(hoisting, since `k` is a load from a known dereferenceable pointer)

  void bar(int* ptr) {
    int* k1 = foo1(ptr, true);
    // inlined int* k2 = foo_common(alloca(...), false);
    t = alloca(...)
    int* val;
    val = *t, unconditionally_dereferenceable
    int k = *val; // FAULT / UB
    if (false) {
      print(k)
    } else {
      val = *ptr
    }
  }


Repository:
  rL LLVM

https://reviews.llvm.org/D18738