[LLVMdev] [RFC] Add warning capabilities in LLVM.

Eric Christopher echristo at gmail.com
Sun Jul 21 20:00:23 PDT 2013

On Sat, Jul 20, 2013 at 9:15 PM, Chris Lattner <clattner at apple.com> wrote:
> Sorry, just getting caught up on an old thread.  I haven't been involved in discussions of this.
> On Jul 17, 2013, at 8:53 AM, Bob Wilson <bob.wilson at apple.com> wrote:
>> First, let me try to clarify my proposal, in case there was any confusion about that.  LLVMContext already has a hook for diagnostics, setInlineAsmDiagnosticHandler() et al.  I was suggesting that we rename those interfaces to be more generic, add a simple enumeration of whatever diagnostics can be produced from the backend, and add support in clang for mapping those enumeration values to the corresponding clang diagnostics.  This would be a small amount of work and would also be consistent with everything you wrote above about reusing the standard and existing machinery for diagnostics in clang.
> Of all of the proposals discussed, I like this the best:
> 1) This is a really simple extension of what we already have.
> 2) The backend providing a set of enumerations for the classes of diagnostics it produces doesn't tie it to clang, and doesn't make it language specific.  Clients should be able to completely ignore the enum if they want the current (unclassified) behavior, and if an unknown enum value comes through, it is easy to handle.
> 3) I don't see how something like the stack size diagnostic can be implemented by clang calling into the backend. First, the MachineFunction (and thus, MachineFrameInfo) is a transient datastructure used by the backend when a function is compiled.  There is nothing persistent for clang to query.  Second, clang would have to know about all of the LLVM IR functions generated, which is possible, but impractical to track for things like thunks and other implicitly generated entrypoints.
> What is the specific concern with this approach?  I don't see how this couples the backend to the frontend or causes layering violation problems.

I've not talked with Chandler about this, but to sketch out the way
I'd do it (which is similar):

Have the backend vend diagnostics, this can be done either with a set
of enums and messages like you mentioned, or just have a message and
location struct ala:

struct Msg {
   const char *Message;
   Location Loc;

that the consumer of the message can use via a handler.

Alternately a handler (and we should have a default handler) can be
passed in from the printer of the message (the frontend in the case
provided) and it can be called on the error message. Absolutely this
should be done via the LLVMContext to deal with the case of parallel
function passes.

class Handler {
   void printDiagnostic(const char *Message, Location Loc);

(Note that I didn't say this was a fleshed out design ;) I think I
prefer the latter to the former and we'd just need an "diagnostic
callback handler" on the context. Though we would need to keep a set
of diagnostics that the backend handles. That said, that it provides
diagnostics based on its input language seems to make the most sense.
It can use the location metadata if it has it to produce a location,
otherwise you get the function, etc. in some sort of nicely degraded

I think this scheme could also work as a way of dealing with the
"Optimization Diary" sort of use that Hal is envisioning as well.

Keeping the separation of concerns around where the front end handles
diagnostics on what we'll call "source locations" is pretty important,
however, I agree that not every warning can be expressed this way,
i.e. the stack size diagnostic. However, leaving the printing up to
the front end is the best way to deal with this and a generic
diagnostic engine would probably help for things like llc/opt where
the backend just deals with its input language - IR.

The existing inline asm diagnostics are ... problematic and it would
definitely be nice to get a generic interface for them. Though they're
actually separated into two separate cases where, I think, we end up
with our confusion:

a) Front end diagnostics - This is an area that needs some work to be
decent, but it involves the front end querying the back end for things
like register size, valid immediates, etc and should be implemented by
the front end with an expanded set of target queries. We could use
this as a way to solidify the backend api for the MS inline asm
support as well and use some of that when parsing GNU style inline

b) Back end diagnostics - This is the stuff that the front end has no
hope of diagnosing. i.e. "ran out of registers", or "can't figure out
how to split this up into this kind of vector register". The latter
has always been a bit funny and I'm always unhappy with it, but I
don't have any better ideas. A unified scheme of communicating "help
help I'm being oppressed by the front end" in the backend would be, at
the very least, a step forward.



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