[cfe-commits] [PATCH] Allow PCHReader to be used without having a pre-initialized Preprocessor

[Chris Lattner]

On Jun 18, 2009, at 8:04 AM, Douglas Gregor wrote:
>> Ah, okay, that makes sense.  Although, what specific uses do you have
>> in mind?  I can only think of a couple off the top of my head: a
>> dist-cc-like thing and an inter-procedural analysis tool.
>
> Another use of "load an AST and send it to codegen" is implement  
> implicit template instantiation via prelinking. In this scheme,  
> function templates are not implicitly instantiated in .o files.  
> Instead, when the linker sees that it needs a symbol that  
> corresponds to a template instantiation, it spawns the compiler to  
> perform the template instantiation and give it a .o file containing  
> that template instantiation.
>
> Prelinking can save a lot of disk I/O and a bunch of computation,  
> since many templates end up being instantiated in several  
> different .o files. With prelinking, these instantiations only  
> happen once, and we only instantiate what the linker actually needs.  
> However, prelinking has never been a huge win because "spawning the  
> compiler" meant re-parsing all of the source code. With AST files,  
> we only bring in the code that we need to perform the template  
> instantiation itself.

That would be very nice.  We'd only want to do that at -O0, because  
this would prevent inlining of these templates etc.  However it could  
be a pretty huge compile-time performance win for -O0.

-Chris