[LLVMdev] First-class debug info IR: MDLocation

[Duncan P. N. Exon Smith]

> On 2014-Nov-05, at 15:47, Eric Christopher <echristo at gmail.com> wrote:
> 
> Hi Duncan,
> 
> Here's a bit of a long delayed reply, I've been thinking about this for a bit now and have a bit (I think) of a more firm idea on how I see this.
>  
> Always takes a patch to draw people in :).
> 
> 
> Agreed. I've been looking at the patches as they've been going in. I think it's mostly making me hate the Value->User hierarchy.
>  
> > Using `Value` instead of `MDNode`
> > =================================
> >
> > A number of APIs expect `MDNode` -- previously, the only referenceable
> > type of metadata -- but this patch (and the ones that will follow) have
> > referenceable metadata that *do not* inherit from `MDNode`.  Metadata
> > APIs such as `Instruction::getMetadata()` and
> > `NamedMDNode::getOperand()` need to return non-`MDNode` metadata.
> >
> > To me, this change is a red flag
> 
> This bothers me too -- which is why I highlighted it -- but I don't
> see any alternatives.  It seems like a natural fallout of the rest of
> the proposal.
> 
> 
> This hurts a lot. It's by far the worst part of this currently.

(FWIW, I think I've made all the `MDNode => Value` changes, so I don't
expect any further ugliness going forward.)

> I think ideally I'd almost rather see Metadata not inherit from Value. Metadata doesn't need to be typed, it describes typed things. It isn't or doesn't produce a value, it's just a side structure that we find worth serializing. A container of some bits that can reference IR when necessary. 
> 
> The problematic thing is the User-ness of Metadata. It definitely wants to point to Values/Users, it definitely can use a list of where it's used for things like RAUW - which we still use for temporary MDNodes right now. I think that aspect is a good thing, but it doesn't necessarily need to be blazingly fast.

I think this is an interesting direction.  More below.

Side note: currently RAUW happens *a lot* in debug info during
parsing and linking, but my plan for debug-info specific classes and
"fixed" ownership designs it away (so it's a moot point).

> `MDUser`/`CustomMD` inherits from `User` so that its subclasses can
> leverage the use-list infrastructure (8B per operand, fast RAUW).
> 
> > I also have to ask because I can't currently see it: what does debug info being metadata buy us?
> 
> I suppose it buys us:
> 
>   - the guarantee that debug info doesn't modify optimizations or code
>     generation, and
> 
>   - the flexibility for optimizations to ignore/drop it when they're
>     not smart enough to update it.
> 
> Agreed. I think this is important.
>  
> > How much code is simplified by that, and at what cost?
> 
> I think that's hard to quantify.
> 
> 
> Quite a bit IMO. If we could get rid of dbg_declare and dbg_value it'd be even more. Ideally, as we've discussed in the past, these could be metadata on instructions or <things that turn into lexical blocks>. Some work would need to be done to handle this, but we'd get rid of the instructions out of the IR.

I've thought through how to get rid of these, via !dbg.value metadata
attachments.  I don't think it would be too difficult.  However, since
dbg.value sometimes needs to reference function arguments, it requires
new IR support.

As a straw man, I was thinking something ugly like this for the
assembly:

    ; Single metadata attachment to %arg.
    void @foo(i64 %arg !dbg.value !23) {
      ret void
    }

    ; Multiple attachments to %arg.
    void @foo(i64 %arg {!dbg.value !23, !other.attachment !24}) {
      ret void
    }

I think there are non-trivial implications for SDAG and MI of removing
the intrinsics, but I haven't really dug into it yet.

>  
> I suppose the obvious alternative is to rewrite debug info from the
> ground up, without using metadata at all.  I considered this, but
> didn't find a compelling argument for it.  Main arguments against it:
> it would be harder to implement incrementally, and it would increase
> the amount of non-code IR.
> 
> Moreover, once we have specific subclasses and bitcode support for
> debug info types, moving away from metadata (or even the `Value`
> hierarchy entirely) would be an incremental step.
> 
> Do you have any specific alternatives mind?
> 
> How about the above discussion? I think it might end up being a bit more work in the short term, but it moves metadata to a separate hierarchy. I don't think a fast RAUW is too important (call it once per type - I think we RAUW much more on instructions/constants/etc and so speed is an advantage there). Module already has a separate set of iterators for named metadata so that side of things isn't too bad.
> 
> Thoughts?

This goal SGTM.  I think splitting from `Value` hierarchy designs away a
lot of problems for metadata.  I hadn't previously considered separating
`MDNode` from the `Value` hierarchy (I was thinking of just splitting
out debug info), but maybe I wasn't thinking big enough.

I have a few questions/concerns:

 1. This seems to imply dropping support for RAUW on general metadata,
    only supporting it for specific cases like `MDNode::getTemporary()`.
    Is that a good idea?

 2. This also implies dropping support for
    metadata-as-intrinsic-operands.  Is that a good idea?

 3. This is gated on replacing the debug info intrinsics with metadata
    attachments.  Is that the right sequence for staging?

Putting those questions aside, splitting metadata from `Value` seems
just as easy/hard after establishing a debug info hierarchy as before --
i.e., the assembly syntax and ownership work I've outlined is still a
reasonable incremental step.

How do you feel about moving forward roughly as I'd planned, and we can
work out design issues around cutting-the-Value-cord in parallel?