Multi-threaded dwarf parsing

[Simon Marchi <simon.marchi@polymtl.ca>]

Hi all,

When debugging large programs, simply loading the binary in gdb can take 
a significant amount of time.  I was wondering if the dwarf parsing 
(building partial and/or full symtabs, I suppose) could be a good 
candidate for parallelization.  I did some quick checks to determine 
that, at least when reading from my SSD drive, the operation is not 
IO-bound.  Also, according to my limited understanding of the Dwarf 
format, it seems like the compilation units DIEs are entities that could 
be processed independently.  These two facts, if we assume they are 
true, suggest that there is a good potential for performance gain here.

I couldn't find anything on the mailing list about that, please point 
out any discussion I might have missed.

I found (and it was a very good surprise) this branch by Tom Tromey:

https://github.com/tromey/gdb/tree/threaded-dwarf-reader

According to his description (from https://github.com/tromey/gdb/wiki): 
"I think it doesn't help any real-world case".  I'd like to ask you 
directly, Tom: now that you debug Firefox (i.e. a quite large program) 
daily with gdb, are you still of the same opinion?  Of course, I'm also 
interested in what others have to say about that.  Is it something that 
would have value, you think?

Also, since not so long ago, LLDB does it.  Apparently, it "can 
drastically incrase the speed of loading debug info" (sic).  If it's 
good for LLDB, I don't see why it wouldn't be good for GDB.
Ref: http://blog.llvm.org/2015/10/llvm-weekly-95-oct-26th-2015.html

So, in a word, are there any gotchas or good reasons not do take this 
path?

Simon