Pre Compiled Headers

[Mike Stump]

On Feb 12, 2007, at 10:42 PM, Basile STARYNKEVITCH wrote:
> A big thanks for the explanation, which is something I partly  
> guessed, but which leaves me in a deep trouble :-)

Glad to turn on the flashlight so that you may see the hole.  I hope  
you know which way to dig now.  :-)

> I can't figure out why every GTY()-ed global data (extern or  
> static) should be overwritten at each PCH inclusion, but then maybe  
> I am wrong.

overwritten?  No, that'd be wrong.  If we supported it, and we don't,  
because it is well past the point of diminishing returns, it would be  
an unmap followed by an mmap.  This should happen so that the state  
is the state post the second include.  I can't fathom why you'd think  
that would be wrong, or slow, or anything else.

> It seems to me that some such GTY-ed global data might explicily  
> want not to be reloaded,

You've not understood the basic design of PCH.  It isn't there.  If  
you think it is there, you're thinking too hard.  Data can't want to  
stay around due to pch, because it can't know about pch.  By not  
knowing about pch, it can't fail to work, and it doesn't need any  
testing (or much testing)  to ensure it works well.  There can't be  
any obscure bugs in pch due in part to the design.  Now, those that  
follow it closely understand just how it deviates from this idealized  
goal.

> or maybe want to be always cleeared at each pch reload. However, I  
> am not sure there is any GTY tag for this.

There is no such rope, you can't hang yourself.  :-)

> I also don't understand at which point of the compilation is the  
> PCH dumped.

It is as if all the work that was done during

   #include <foo.h>

wasn't done again and all that work just magically appeared done.   
There is no limit on what that work is.  For example, you want to run  
an early optimization pass on the inline functions found in header  
files and do inlining, have fun.  You want  to build a data flow  
graph, have at it.

If you want to be at one with the code, set a breakpoint on the pch  
writer and type up a few times.  You can see just where it is called  
from and that it has done.  For C, it is done in pop_file_scope.   
Everything done before pop_file_scope (after all of c_parse_file is  
done) hits the pch file.

The idea is to front load as much processing as possible before that  
point.  This way, the cost is saved in every unit that uses the pch.

So, for example, let say there is an inline definition that we know  
we will need and external version of, logically, it should be written  
out before that point, so that we can just squirt the .s for it out  
into the file .s file from the pch file.

> For example, do PCH file contain Gimple-SSA trees (I would guess  
> yes) or
> only Generic trees?