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<p>+1, sounds like a great idea</p>
<p>And if you're volunteering to do the work, even better! :)<br>
</p>
<p>Philip</p>
<p>p.s. Any reason we can't preserve a TerminatorInst type with an
isa function which just returns true for all our terminators but
without having terminators actually inherit from it? If so, we
preserve the bit of a "type safety" for a variable which is
expected to always point to a terminator. <br>
</p>
<br>
<div class="moz-cite-prefix">On 05/17/2018 02:03 AM, Chandler
Carruth via llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAAwGriGHh=h2uQLCwjx2J=Z3JGB0YqqW4ZmcHyapTPw2L1MyLA@mail.gmail.com">
<div dir="ltr">Going to keep this RFC short and to the point:
<div><br>
</div>
<div>TerminatorInst doesn't pull its weight in the type system.
There is essentially a single relevant API -- iterating
successors. There is no other interesting aspect shared -- the
interface itself just dispatches to specific instructions to
be implemented.</div>
<div><br>
</div>
<div>On the flip side, CallInst and InvokeInst have *massive*
amounts of code shared and struggle to be effective due to
being unable to share a base class in the type system. We have
CallSite and a host of other complexity trying to cope with
this, and honestly, it isn't doing such a great job.</div>
<div><br>
</div>
<div>I propose we make "terminator-ness" a *property* of an
instruction and take it out of the type system. We can build a
handful of APIs to dispatch between instructions with this
property and expose successors. This should be really
comparable to the existing code and have nearly no down sides.</div>
<div><br>
</div>
<div>Then I propose we restructure the type system to allow
CallInst and InvokeInst to easily share logic:</div>
<div>- Create `CallBase` which is an *abstract* class derived
from Instruction that provides all of the common call logic</div>
<div>- Make `CallInst` derive from this</div>
<div>- Make `InvokeInst` derive from this, extend it for EH
aspects and successors</div>
<div>- Remove `CallSite` and all accompanying code, rewriting it
to use `CallBase`.</div>
<div><br>
</div>
<div>The end result will, IMO, be a much simpler IR type system
and implementation. The code interacting with instructions
should also be much more consistent and clear w/o the awkward
CallSite "abstraction".</div>
<div><br>
</div>
<div>Thoughts? Seem OK at a high level?</div>
<div><br>
</div>
<div>Happy to bikeshed the name `CallBase`, but I've discussed
this with several folks, including Reid and Chris and nothing
better came up really. `CallSite` might be nicer, but the
confusion with the *existing* type seems much more
problematic.</div>
<div><br>
</div>
<div><br>
</div>
<div>Assuming folks are happy with this direction, are there any
incremental patches that folks would like to see in pre-commit
review? I've only done some initial investigation of what it
takes to cut this through. Provided folks are positive about
the direction, I'll work on what this would actually look like
in practice.</div>
<div><br>
</div>
<div>-Chandler</div>
</div>
<br>
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