LLVM does dead code elimination using pointer validity assumptions

[haved]

In the following program, clang is able to fully remove the second if statement. Instead it creates a select using a[0] to select between constant 1 and constant 2. The final load of a[1] is completely removed.

int func(int* a)
{
  if (a[0])
    return 1;

  if (a)
    return 2;

  return a[1];
}

When we load from a[0], we can assume that a is not a nullptr, and LLVM is able to use this assumption to determine that the "else"-branch of the second if is unreachable. We could also do this. For reference, the relevant RVSDG regions of f are shown here:

In the same vein, clang is able to convert the below code into a single ret 2:

int func(int* a)
{
  if (a)
    return 2;

  return a[0];
}

[phate]

I think a good way to deal with this would be value range propagation. In this case, the value range would just be degenerated as one would only be interested in the fact NULL or not NULL.

FYI @caleridas