[LLVMdev] Proposal for safe-to-execute meta-data for heap accesses

[Chandler Carruth]

On Thu, Nov 7, 2013 at 9:39 PM, Filip Pizlo <fpizlo at apple.com> wrote:

> NEW PROPOSAL
>
> The solution is to introduce meta-data that is explicit about how the
> safe-to-execute condition ought to be evaluated.  Instead of an SSA use, we
> can have meta-data that says:
>
>         %v = load %p !notrap !{ @f, <args> }
>
> where @f is a function in the current module and this function returns i1,
> and <args> is zero or more arguments to pass to @f.  As with any meta-data,
> this doesn’t imply anything different if you wanted to just execute the
> code: executing the load doesn’t imply calling @f; indeed if you dropped
> the meta-data the execution of this would still be the same.
>

So, first a clarifying question:

Is the expectation that to utilize this metadata an optimization pass would
have to inspect the body of @f and reason about its behavior given <args>?

If so, then I think this is pretty bad. If we ever want to parallelize
function passes, then they can't inspect the innards of other functions. So
this would significantly constrain the utility here.

Also, this would create uses of the arguments that were "ephemeral" uses.
It's not clear how that is better than any of the other proposals to
represent constraint systems in the IR via "ephemeral" uses.
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