[LLVMdev] lifetime.start/end clarification

[Reid Kleckner]

On Wed, Nov 5, 2014 at 11:17 AM, Philip Reames <listmail at philipreames.com>
wrote:

>
> On 11/05/2014 10:54 AM, Reid Kleckner wrote:
>
>   This seems fine to me.  The optimizer can (soundly) conclude that %p is
>> dead after the "lifetime.end" (for the two instructions), and dead before
>> the "lifetime.start" (for the *single* instruction in that basic block,
>> *not* for the previous BB).  This seems like the proper result for this
>> example, am I missing something?
>>
>
>  What if I put that in a loop, unroll it once, and prove that the
> lifetime.start is unreachable? We would end up with IR like:
>
>  loop:
>   ... use %p
>   call void @lifetime.end( %p )
>    ... use %p
>   call void @lifetime.end( %p )
>    br i1 %c, label %loop, label %exit
>
>  Are the second uses of %p uses of dead memory?
>
> It's hard to discuss this without being specific about the starting IR and
> transforms.  My general response is that either a) such a transform
> wouldn't be valid or b) the behaviour of the original program was
> undefined.
>

The starting IR would be something that jumps into the middle of a lifetime
region, like the example that Arnaud gave. This was assuming the current
state of the world where we haven't added a second lifetime start call
prior to the goto branch.

Start with something like:

void f(int x) {
  while (x) {
    goto skip_start;
    {
      int y; // lifetime.start
skip_start:
      y = g();
      x -= y;
      // lifetime.end
    }
  }
}

The block containing the lifetime start of y is unreachable and can be
deleted.
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