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

[Evan Cheng]

Sent from my iPad

On Jul 17, 2013, at 8:53 AM, Bob Wilson <bob.wilson at apple.com> wrote:

> 
> On Jul 17, 2013, at 2:12 AM, Chandler Carruth <chandlerc at google.com> wrote:
> 
>> On Tue, Jul 16, 2013 at 9:34 PM, Bob Wilson <bob.wilson at apple.com> wrote:
>>> 
>>> On Jul 16, 2013, at 5:51 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> 
>>>> On Tue, Jul 16, 2013 at 5:21 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>>>>> ** Advices Needed **
>>>>> 
>>>>> 1. Decide whether or not we want such capabilities (if we do not we may just
>>>>> add sporadically the support for a new warning/group of warning/error).
>>>>> 2. Come up with a plan to implement that (assuming we want it).
>>>> 
>>>> The frontend should be presenting warnings, not the backend; adding a
>>>> hook which provides the appropriate information shouldn't be too hard.
>>>> Warnings coming out of the backend are very difficult to design well,
>>>> so I don't expect we will add many.  Also, keep in mind that the
>>>> information coming out of the backend could be used in other ways; it
>>>> might not make sense for the backend to decide that some piece of
>>>> information should be presented as a warning.  (Consider, for example,
>>>> IDE integration to provide additional information about functions and
>>>> loops on demand.)
>>> 
>>> I think we definitely need this.  In fact, I tried adding something simple earlier this year but gave up when I realized that the task was bigger than I expected.  We already have a hook for diagnostics that can be easily extended to handle warnings as well as errors (which is what I tried earlier), but the problem is that it is hardwired for inline assembly errors.  To do this right, new warnings really need to be associated with warning groups so that can be controlled from the front-end.
>>> 
>>> I agree with Eli that there probably won’t be too many of these.  Adding a few new entries to clang’s diagnostic .td files would be fine, except that the backend doesn’t see those.  It seems like we will need something in llvm that defines a set of “backend diagnostics”, along with a table in the frontend to correlate those with the corresponding clang diagnostics.  That seems awkward at best but maybe it’s tolerable as long as there aren’t many of them.
>> 
>> I actually think this is the wrong approach, and I don't think it's quite what Eli or I am suggestion (of course, Eli may want to clarify, I'm only really clarifying what *I'm* suggesting.
>> 
>> I think all of the warnings should be in the frontend, using the standard and existing machinery for generating, controlling, and displaying a warning. We already know how to do that well. The difference is that these warnings will need to query the LLVM layer for detailed information through some defined API, and base the warning on this information. This accomplishes two things:
>> 
>> 1) It ensures the warning machinery is simple, predictable, and integrates cleanly with everything else in Clang. It does so in the best way by simply being the existing machinery.
>> 
>> 2) It forces us to design reasonable APIs in LLVM to expose to a FE for this information. A consequence of this will be to sort out the layering issues, etc. Another consequence will be a strong chance of finding general purpose APIs in LLVM that can serve many purposes, not just a warning. Consider JITs and other systems that might benefit from having good APIs for querying the size and makeup (at a high level) of a generated function.
>> 
>> A nice side-effect is that it simplifies the complexity involved for simple warnings -- now it merely is the complexity of exposing the commensurately simple API in LLVM. If instead we go the route of threading a FE interface for *reporting* warnings into LLVM, we have to thread an interface with sufficient power to express many different concepts.
> 
> I don't understand what you are proposing.
> 
> 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.  For the record, I had started down that path in svn commits 171041 and 171047, but I reverted those changes in 174748 and 174860, since they didn't go far enough to make it work properly and it wasn't clear at the time whether we really needed it.
> 
> Now let me try to understand what you're suggesting….  You refer several times to having clang query the LLVM layer.  Is this to determine whether to emit a diagnostic for some condition?  How would this work?  Would you have clang insert extra passes to check for various conditions that might require diagnostics?  I don't see how else you would do it, since clang's interface to the backend just sets up the PerFunctionPasses, PerModulePasses and CodeGenPasses pass managers and then runs them.  Assuming you did add some special passes to check for problems, wouldn't those passes have to duplicate a lot of effort in some cases to find the answers?  Take for example the existing warnings in IntrinsicLowering::LowerIntrinsicCall.  Those badly need to be cleaned up.  Would clang run a special pass to check for intrinsics that are not supported by the target?  That pass would need to be implemented as part of clang so that it would have access to clang's diagnostic machinery, but it would also need to know details about what intrinsics are supported by the target.  Interesting layering problems there….  Apologies if I'm misinterpreting your proposal.

We can't assume clang is the frontend or design a system that only works with clang. There are many systems that use llvm which are not even c compilers. 

Evan

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