[PATCH] D20116: Add speculatable function attribute

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added a comment.

I'm okay with this as long as this is allowed only on specific intrinsics (i.e. it cannot be used by external clients, but we //know// that certain intrinsics are speculatable).  I don't think we can allow this as generic function attribute, since that allows dead code to affect program behavior.  E.g.:

  if (false)
    puts("hi") speculatable;

which can get transformed to

  puts("hi") speculatable;
  if (false)
    ;

The `speculatable` attribute on `puts` is incorrect, but we need to allow such "bogus" IR down dead paths.  For instance, the original program could have been:

  void do_call(fnptr f, bool is_speculatable) {
    if (is_speculatable)
      f("hi") speculatable;
    else
      f("hi");
  }
  
  // Later
  do_call(puts, false);


https://reviews.llvm.org/D20116