Re: [RFC] WHOPR - A whole program optimizer framework for GCC

[Chris Lattner <clattner at apple dot com>]

On Dec 19, 2007, at 2:19 PM, Tim Josling wrote:

> > > ...
> > >  http://gcc.gnu.org/projects/lto/lto.pdf
> > > ...
>
> Was there any more about this?
>
> I have restarted work on my COBOL front end. Based on my previous
> experiences writing a GCC front end I want to have as little code as
> possible in the same process as the GCC back end.
>
> This means passing over a file. So I would like to understand how to
> avoid getting into political/legal trouble when doing this.

While it is possible to make this work once LTO is finished, it seems  
unlikely that it will be pleasant.  Doing so will basically mean  
reimplementing a 'writer' for the LTO format to interoperate with the  
GCC code.  This seems to be a hard task, as there is no document on  
the structure and contents of the LTO file.  OTOH, you can get this   
by reverse engineering the code to find out what it does.  Further,  
it has been publicly stated that the format will evolve and is not  
going to be stable (though I don't recall where).  Unless you want to  
fight to keep up with the format, this sounds like a major pain.

You might be interested in checking out LLVM: http://llvm.org/ which  
has a well defined and well specified file formats (one text and one  
binary), and preserves backwards compatibility with them across major  
release (i.e. 1.0 -> 1.9 and 2.0 -> 2.x).  http://llvm.org/docs/ 
LangRef.html

I'd be interested to hear if keeping the LTO format stable is  
something the GCC community plans to do,

-Chris