[LVA] Support access instructions in DCE.

[zoecarver]

A simple check for the following pattern:

x = begin_access
end_access x

[eeckstein]

This will work.
But a better approach would be to add an end_access with a reverse dependency to begin_access.
Usually an instruction is dead of all its uses are dead. With a reverse dependency an instruction is dead if it's operand is dead. And that's what we want for end_access: it should be removed if the its operand (= the begin_access) is dead.

See the handling of cond_fail and fix_lifetime in DCE as examples.

[zoecarver]

Yes, you're right, that would be better. Will do.

[zoecarver]

begin_access is marked as having side effects so, I still have to have this bit of logic.

[eeckstein]

That logic looks wrong. It should be sufficient to exclude begin_access in seemsUseful().
If there is a use (other than end_access), the begin_access will be marked as life in propagateLiveness().

[eeckstein]

The operand of an end_access must be a begin_access. You can use EndAccessInst::getBeginAccess() to get it.

[zoecarver]

Good to know. My thinking was that there could be a phi argument or something. Is that not valid?

[eeckstein]

No, it must be a begin_access

[eeckstein]

That logic looks wrong. It should be sufficient to exclude begin_access in seemsUseful().
If there is a use (other than end_access), the begin_access will be marked as life in propagateLiveness().

[eeckstein]

nitpick: Can you change this comment to something like: "begin_access is defined to have side effects, but this is not relevant for DCE."
Because this is the reason why you have to handle begin_access here.

[zoecarver]

Fair enough. Done.

[eeckstein]

LGTM, thanks!

[zoecarver]

@swift-ci please smoke test and merge