[cfe-dev] LLVM Dev meeting: Slides & Minutes from the Static Analyzer BoF

[Manuel Klimek via cfe-dev]

On Tue, Nov 3, 2015 at 10:19 PM Chris Lattner <clattner at apple.com> wrote:

> On Nov 3, 2015, at 9:26 AM, Manuel Klimek <klimek at google.com> wrote:
>
> I’m sorry I missed this part of the discussion, but IMO, but the right
> answer is to build a “CIL” analog to “SIL”.  The problems with the existing
> Clang CFG are that:
>
>>
>> a) it is not tested as part of IRGen, so it falls out of date.
>> b) it is not a proper IR, which can be serialized/deserialized etc.  This
>> makes it very difficult to write tests for.
>> c) its “operations” or “instructions" are defined as AST nodes, so its
>> “CILGen” stage doesn’t allow any lowering of operations.
>>
>
> Those are all arguments for not using the current clang CFG (which I agree
> with).
>
> What are your arguments against implementing a type system on top of llvm
> IR (that lives on a similar level as debug info) that is not language
> specific per se, but allows frontends to model their language semantics and
> have pointers back to their AST? Do you believe such a type system would
> inherently be coupled to language semantics, and thus not possible to build
> in a generic enough (and still useful) way? Or are there other problems?
>
>
> Two problems: it doesn’t solve the problem I think need to be solved, and
> would end up with a really awkward/inelegant solution if it could be made
> to work.
>
> The problems that need to be solved:
>
> 1) You need an augmented source level type system to do the
> transformations that are interesting at this level.  It is the full
> complexity of the AST represented by Clang, as well as some minor
> extensions for things that get exposed by the process of lowering.  It is
> extremely unclear to me how you’d handle this. Using debug information
> doesn’t work well given that you’ll need multiple types associated with
> some operations.  Debug info and MDNodes in general would also be an
> extremely awkward way to express things.
>

I agree this is needed, and that it would need a well thought out design.
I'd also think some explorative coding would be necessary to identify how
we'd do that (or whether it's even possible).


> 2) You need a full suite of [lowered] source level operations that have
> little to do with the LLVM IR operations like getelementptr.  Even
> operations that are common (e.g. load and store) need to be expressed in
> the source level type system, not the IR type system, so they probably
> can’t be used (depending on the approach you use for #1).  This can be
> expressed in LLVM IR as intrinsics, but all the intrinsics would be
> language specific, so you wouldn’t achieve your language agnostic goal.
>

I'd expect the LLVM IR to be annotated with the types; I agree that we'd
need to annotate basically all generated IR with the types, and that that
would be a substantial effort.


> 3) The only good way I know of to generate good source-level diagnostics
> (which should include source ranges, etc) is to point back to an AST node
> (Expr*, Decl*, etc) that it came from.  This mapping is obviously highly
> frontend-specific, and the lifetime issues managing this are also
> interesting.  I don’t know how this would be expressed in LLVM IR.
>

I'd expect to have a mechanism in the new higher level type system to point
back at frontend specific nodes, if the frontend choses to do so. I agree
that lifetime would be interesting, but that seems not insurmountable, but
rather a straight forward engineering problem (I'm much more concerned
about whether it's possible to define a higher level type system that is
language agnostic enough to fit LLVM, but is expressive enough to fit the
use cases of the frontends).


> 4) In terms of layering of the library stack, Clang should depend on LLVM,
> and LLVM IR is intentionally very low in the stack.
>

Agreed, and it should stay that way. I'd expect 2 pieces to live in LLVM:
- a higher level type system that we can annotate IR with, and which will
be kept sound by a subset of the passes (especially the early ones)
- interfaces in that type system for frontend specific callbacks, so
frontend can store backrefs to their AST nodes for accurate diagnostics if
they chose to do so
Do you think that would already contradict the layering requirements?


> 5) Almost all clients of this data structure would be source-language
> specific (keep in mind that the type system and operations are all language
> specific) so there would be little reuse anyway.  You’re right that you’d
> be able to reuse things like “class BasicBlock”, but that isn’t where the
> complexity is: things like ilist already do the interesting stuff for it,
> and is shared.
>
> When thinking about this, it is important to consider the specific clients
> that you’d want to support.
>

Agreed.


>  Even things like the -Wunreachable diagnostic in clang is totally
> language specific (what kind of crazy language defaults variables to being
> uninitialized memory in the first place??).  The most interesting
> diagnostics in this space that Clang (and the static analyzer) want to
> reason about tend to be language specific as well (e.g. the objc
> retain/release checker).
>

On the other hand, we have some evidence for checks that are less language
specific (or have very generic components). Thread safety analysis comes to
mind. We're basically building the same things for all languages; there are
language specific pieces we need the frontends to generate, but the gist of
the issue is a lower level type checking system.
Another thing we'd like is that we can use more static analysis for
optimization (for example, devirtualization; iirc in your talk you
mentioned you use SIL for that). I am not an expert here, so I'll believe
you if you say this is not possible :) On the other hand, some of the ideas
behind dependent type systems look to me like they could be useful in llvm
(dependent type parameters, for example), and seem non-trivial enough, so
that re-implementing them in a new IR for each targeted language would be
very costly, when instead it could make LLVM as a platform interesting to a
new set of applications.


> While it is obvious that the typestate engine for a checker like a nil
> dereference check can be shared, this is true regardless of the IR.
>  These sorts of state machines are quite simple, the complexity is in the
> analyses they depend on (e.g. alias analysis, which is pretty language
> specific) and in the code that deals with each kind of AST node/IR
> operation.
>

Yep, I think the main challenge will be to come up with a type system on
top of LLVM IR that is expressive enough so frontends can map their types
to it in a way that we can write language independent analysis passes. The
proof is in the pudding, of course, but I'm not (yet) convinced it's
impossible :D


> I’ve heard many proposed approaches to encode source level information in
> IR, but they all have major disadvantages, which is why none of them have
> been successful.  That said, it could be that you have a specific approach
> in mind that I haven’t envisioned.  What are you thinking of?
>

I agree that encoding source level information is probably not enough; I'd
expect that we actually need to encode a language independent type system
on top of LLVM IR; so far I'm not aware somebody has tried that - if that's
the case, I'd be interested to learn more about the attempts and their
shortcomings.

Cheers,
/Manuel


>
> -Chris
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20151104/399910bb/attachment.html>