Re: [RFA/dwarf] Allocate abbrevs from a new obstack

[Jim Blandy <jimb@redhat.com>]

Daniel Jacobowitz <drow@false.org> writes:
> Another independent performance optimization from my profiling.  This one is
> good for several percent speedup on mainline.  My original description:
> 
>   - It plugs a huge memory leak.  After loading partial symbols for
>   libc, GDB uses 5.6MB memory without this patch and 1.6MB with it.
>   We weren't emptying the abbrev table.
> 
> [Specifically, we were emptying it after dwarf2_build_psymtabs_hard,
> instead of once each time through the loop.  dwarf2_read_abbrevs then
> zeroed out the allocated, unreferenced pointers.]
> 
>   - I spent a lot of time trying to find alternate data structures that
>   would be more efficient.  The expandable hash table is slower, even if
>   you initially allocate it to the right size.  The splay tree I expected
>   to be quite good in this case, but it isn't either; the overhead is simply
>   too high.  The fixed-size ad-hoc hash table appears to be the best we can
>   do.
> 
> Tested on i686-pc-linux-gnu, no regressions.  OK to commit?

Looks great --- please do.