[cfe-dev] [RFC] Adding API Notes

[Dean Michael Berris via cfe-dev]

> On 18 May 2017, at 01:29, Gábor Horváth via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Hi!
> 
> I wanted to revive the discussion on adding API notes feature to Clang. 
> 
> The description of the feature from the original discussion
> 
> "API notes solve a not-uncommon problem: we invent some new Clang attribute that would be beneficial to add to some declarations in system headers (e.g., adding a ‘noreturn’ attribute to the C ‘exit’ function), but we can’t go around and fix all of the system headers everywhere. With API notes, we can write a separate YAML file that states that we want to add ‘noreturn’ to the ‘exit’ function: when we feed that YAML file into Clang as part of normal compilation (via a command-line option), Clang will add ‘noreturn’ to the ‘exit’ function when it parses the declaration of ‘exit’."
> 
> The old discussion can be found here: http://lists.llvm.org/pipermail/cfe-dev/2015-December/046335.html <http://lists.llvm.org/pipermail/cfe-dev/2015-December/046335.html>
> 
> The summary of the discussion
> 
> * It would be useful for the static analyzer to attach additional info to the functions
> * There were already other trials to get similar feature working, see https://reviews.llvm.org/D13731 <https://reviews.llvm.org/D13731>
> * Anna has a nice summary what is the problem with augmented declarations: http://lists.llvm.org/pipermail/cfe-dev/2015-December/046378.html <http://lists.llvm.org/pipermail/cfe-dev/2015-December/046378.html>
> * C++ support is requested by the community, this is missing right now. 
> * Wider range of annotation support is missing. It is also requested by the community. 
> * Parameter annotations are supported
> * One of the concerns is the performance
> 
> The case for adding API Notes
> 
> * Importing annotations from external source looks to be an interesting feature for the community
> * Sean presented some API checkers which works based on special annotations: http://llvm.org/devmtg/2017-03//2017/02/20/accepted-sessions.html#29 <http://llvm.org/devmtg/2017-03//2017/02/20/accepted-sessions.html#29>
> * Similar feature can be used to provide fake implementations for functions. It could help to finish a half done feature of the Static Analyzer that was implemented during a GSoC project: https://www.google-melange.com/archive/gsoc/2014/orgs/llvm/projects/xazax.html <https://www.google-melange.com/archive/gsoc/2014/orgs/llvm/projects/xazax.html>
> * Performance might be crucial for regular compilation, but less of a problem for the Clang Static Analyzer which tends to be slower than compilations, so it is less likely to be bottlenecked by this phase
> * It would also be possible to import sanitizer/optimizer/codegen related annotations
> 
> Proposed roadmap
> 
> * Commit the feature as is
> * Extend it with C++ support (namespaces, overloading, templates..)
> * Extend it with additional annotations, and attaching custom data (like fake function bodies)
> 
> What do you think? What are the main concerns with this feature?
> 

I'm interested in having something like API notes available in Clang, definitely. I have a few questions though:

- Is YAML absolutely necessary? What I think I don't understand is what the actual data structure is that it will represent. Is it a configuration for something that will actually imbue the attributes into the IR? Or is it a dump of an in-memory data structure, that specifies a side-table of attributes for the generated IR?

- What is the overlap with the sanitiser special case list, which provides regular expressions and a simple configuration language? Can one be ported to be built on top of the other? Is there a roadmap/plan for unifying that features?

- What is the programmatic API for imbuing the attributes before IR generation? Say I'm using clang as a library, is there a data structure (in-memory) that I can programmatically define so that the attributes are applied to the IR as a transformation? If so, should this be part of the LLVM back-end instead? Or should it be a stand-alone library that clang uses in the static analysis phase? I can certainly imagine this being useful in LTO or PGO, not necessarily tied to Clang.

Cheers

-- Dean

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