[LLVMdev] Proposal for new Legalization framework

Chandler Carruth
Sat, 27 Apr 2013 15:10:40 -0000

On Fri, Apr 26, 2013 at 11:33 PM, Dan Gohman <dan433584 at gmail.com>
wrote:

> To all, I'm moving on and accepting what appears to be the consensus of
> the list, for now.
>

I want to point out something about this direction that hasn't really come
up, but I think deserves some better discussion. I don't think it should be
the basis of a decision one way or the other, its more a consequence of the
decision.

At the IR level, we have some great infrastructure that doesn't exist at
the MI level:

- The pass management tools.
- A verifier that can be run before and after any pass to check the basic
invariants.
- The ability to serialize and deserialize to/from a human understandable
(and authorable) form.

I think before we invest in *significantly* more complexity and logic in
the MI layer of the optimizer, we will need it to have these three things.
Without them, the work will be considerably harder, and we will continue to
be unable to do fine grained testing during the development of new
features. We might not need all of the capabilities we have in the IR, but
I think we'll need at least those used to orchestrate fine grained testing
and validation.

Of course, adding these to MI would be of great benefit to any number of
other aspects of LLVM's development. I am *not* arguing we should eschew MI
because it lacks these things. I just want people to understand that part
of the cost of deciding that MI is the right layer for this is needing to
invest in these pieces of the MI layer.
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