Merging debug-early work?

[Aldy Hernandez <aldyh at redhat dot com>]

Gentlemen!

I believe I have done as much as is reasonable for a merge, but I'd like 
to get your opinion before I post a huge patch to the list.

The branch bootstraps with one regression in GCC 
(gcc.dg/debug/dwarf2/stacked-qualified-types-3.c) and none for GDB.

The GCC regression is a missed optimization while merging the common 
denominator of a set of qualifiers for a type within a DIE.  For 
example, if two types share "const volatile" (say "const volatile int" 
and "const volatile char"), dwarf2out outputs things in the most 
efficient manner as to share the maximum common type DIEs.  This is not 
working in the branch as TYPE_MAIN_VARIANTs are not complete by the time 
early dwarf is run.  If it is possible, I'd like to work on this one 
regression post-merge.  Not a big deal if you disagree, but I'd prefer 
to postpone on this non crucial bit.

A few caveats...

Richi wants to play around with free-lang-data in the non LTO path.  I 
haven't not done so, and it's left as an exercise to the reader :).

Shortly after the merge I'll work on a pass to prune unused decl DIEs as 
we're presently creating more DIEs than mainline.  This was expected, 
and if I understood Jason correctly, it is ok to work on this 
post-merge.  However, even without such a pass, the .debug_info size 
difference is reasonable:

gcc/* (except testsuite):
Total .debug_info size for [debug-early]: 91081591.00
Total .debug_info size for [mainline]: 84777565.00
Total change: 7.44%

libstdc++-v3/* (except testsuite):
Total .debug_info size for [debug-early]: 5173014.00
Total .debug_info size for [mainline]: 5044971.00
Total change: 2.54%

x86_64-unknown-linux-gnu/*
Total .debug_info size for [debug-early]: 5893131.00
Total .debug_info size for [mainline]: 5694176.00
Total change: 3.49%

The above stats are for "size -A <something> | grep debug_info...".

Within gcc there were a handful of files that were significantly bigger 
(twice as much), and at least the 3-4 I investigated were all due to 
extra unused DIEs that will be handled by a DECL DIE optimization pass. 
 Specifically, there are cases where external variables have their DIEs 
generated, because we cannot look at TREE_USED within early dwarf. 
Stuff like this will get a debug info (which is not terribly bad IMO):

	struct somestruct { int somefield; };
	extern  struct somestruct *sometable;

The other common scenario is the ICF pass which will mark hunks as 
undebuggable late in the compilation process (by setting 
DECL_IGNORED_P)-- actually any pass calling expand_hunk().  This happens 
for something like c-family/stub-objc.c which have multiple identical 
stubs and get folded into one function.

So...all in all, the .debug_info increase is within what was expected 
when we started this project (3-7%).  Actually, I'm pleasantly surprised 
it's not 10-15%.  I expect to get this down significantly in short time.

Thoughts on moving forward?  Is the stacked qualifier regression a show 
stopper?  Is the .debug_info size regression acceptable?

And of course... I'm not going anywhere.  Unfortunately, I'm not even 
going on vacation... so I'm here to fix the fallout ;-).

Aldy