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

[Elena Zannoni <ezannoni at redhat dot com>]

Jim Blandy writes:
 > 
 > 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.

Jim, there was agreement to not touch these central files until after
the branch.