Faster compilation speed

[Theodore Papadopoulo]

OK, since this is a brainstorming about speeding up gcc, and since 
silly ideas are at least discussed, let me try one.

Why not make incremental compilation a standard for gcc...

This would mean storing some information into the object files. 
Things I can see are:

- Compilation flags (defines, optimization, code generation 
  and debugging flags at least).
- A signature (eg MD5 or other) for each data_type/function/global (decl ?)
  allowing for a quick check for a change. We may even differentiate
  between visible/invisible changes. Eg if a function body changes 
  but not its interface there is no need to recompile the functions
  calling it. More generally name changes could be detected as 
  non-changes, but I suspect that this will mess up with debugging information.

Then generate code only for the relevant symbols (ie the new ones or 
those that have been changed or affected indirectly by a change ie 
depending on a function or variable that changed) and do a replacing 
of these in the .o file (is there an gas option like --replace ?).

In some way this is like PCH but pushed one step further. I can 
understand that making it work reliably is quite difficult, but the 
perspective of having a fast incremental compiler is tempting...
The information to store is certainly one of the trickiest part so a 
first step could be to add a flag stating recompile only this symbol and 
what depends on it. Not very user friendly, but maybe an interesting 
first step...

Is this a totally remote/stupid idea, or can it be done in some 
eventually not too distant future ??

	Theo.

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Theodore Papadopoulo
Email: Theodore.Papadopoulo@sophia.inria.fr Tel: (33) 04 92 38 76 01
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