[LLVMdev] pragmas

[Dan Gohman]

On Apr 1, 2009, at 7:25 AM, Torvald Riegel wrote:

> On Wednesday 25 March 2009, Luke Dalessandro wrote:
>> You could encode this information as simple library function calls  
>> and
>> then find them again in the generated LLVM IR. The client then just
>> needs a header declaring the functions and information on what they
>> mean. Since there are never any definitions of them they won't end up
>> going anywhere.
>
> I've been using this approach now for almost two years in a project  
> and it
> _seems_ to work fine. However, I'd like to get some feedback from  
> the experts
> on which kinds of code could be moved by the standard passes over  
> the points
> in code marked by such calls.
> For me, loads, stores, and calls to other functions are most  
> interesting. Do
> the current optimization passes do that? (For example, by checking  
> whether a
> global's address would escape to external entities like the dummy  
> function
> that is the marker and allowing accesses to the global to be moved  
> if it
> doesn't escape.)

Yes, there are current optimizations which do things like this.  Global
variables are implicitly escaping, because they can be referenced by
name, but in other cases LLVM does determine variables that don't
escape and performs optimizations accordingly.

Dan