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

[Sanjoy Das via llvm-commits]

sanjoy added a comment.

I haven't done a full review, but one aspect of this change worries me
at a theoretical level -- after this change it is possible to cause
miscompiles by introducing dynamically dead code.

E.g. if we have

  void @foo() {
    %t = alloca i32*
  }

and say we change it to

  void @foo() {
    %t = alloca i32*
    if (false) {
      %ptr = load i32*, i32** %t, !unconditionally_dereferenceable
      %val = load i32, i32* %ptr
    }
  }

In theory the second program should be equivalent to the first, since
only dynamically dead code was added (that would never execute at
runtime).  But, given the semantics of the
`!unconditionally_dereferenceable` attribute, I can further transform
the program to

  void @foo() {
    %t = alloca i32*
    %ptr = load i32*, i32** %t, !unconditionally_dereferenceable
        ;; allocas are always dereferenceable
    %val = load i32, i32* %ptr
        ;; load from unconditionally dereferenceable value
    if (false) {
    }
  }

which would has undefined behavior.

Is there a way you can change the semantics of this attribute so that
one of the above transforms isn't possible?


Repository:
  rL LLVM

http://reviews.llvm.org/D18738