[cfe-dev] Attributes on statements and expressions

[Trevor Harmon]

Hello Clangers,

Some time ago, there was talk of extending Clang to allow attributes  
on statements and expressions [1]. By relaxing the restrictions on  
where attributes can be placed, users would be able to attach  
arbitrary info to the AST, which would then be transferred to the LLVM  
IR (presumably as LLVM 2.7 metadata [2]). Clients of the AST or the  
LLVM IR could then use this information to do whatever they want.

The clients would typically be static analysis tools such as:

* Design-by-contract checkers (to declare pre-, post-, and loop  
invariants)
* Defect finders like Clang's own Static Analyzer (to eliminate false  
positives)
* WCET analyzers (to define loop bounds)

I would like to begin work on a patch that implements this feature in  
Clang. The first order of business is to decide how the attributes  
would look in the user source code. One approach is to retain the  
existing GCC __attribute__ syntax and simply allow the __attribute__  
token to appear on statements and expressions. (Currently it is only  
allowed on variable, type, and function declarations.) For example:

   void myFunc(...) {
       __attribute__((loopbound(50))
       while (...) {
           ...
       }
   }

Any comments or questions on this? I'd also appreciate any tips on how  
to get started, as well as admonitions of obstacles I might encounter.  
Thanks,

Trevor

[1] http://lists.cs.uiuc.edu/pipermail/cfe-dev/2007-November/000473.html
[2] http://blog.llvm.org/2010/04/extensible-metadata-in-llvm-ir.html