Re: Save 6% of GGC memory (consumed in tree-ssa-pre)

[Daniel Berlin <dberlin at dberlin dot org>]

Jan Hubicka wrote:
> Hi,
> memory statistics for combine.c at -O2 shows 13% of memory to be taken
> by create_tree_ann function:
> tree-phinodes.c:156 (allocate_phi_node)             3813160: 4.6% 0: 0.0%          0: 0.0%      22568: 0.3%      12114
> genrtl.c:17 (gen_rtx_fmt_ee)                        7518920: 9.1% 0: 0.0%       3080: 0.0%    1504400:17.1%     188050
> tree-dfa.c:209 (create_tree_ann)                   10927080:13.3% 0: 0.0%       7440: 0.1%     728968: 8.3%      91121
> Total                                              82223861 12694004          6318427          8805836          1369076
> 
> This is the same for my now favorite testcase and other common sources.
> I always accounted it to annotations being bloated, but I unstrumented
> create_tree_ann for statistics and found out that this all memory is in
> fact allocated all by tree-vn and it is all there just to hold one
> pointer value_handle.  I would say that a hashtable would do a lot
> better job here (or direct use of the annotation pointer on
> expressions), but I am leaving it for Danny 

If you use a hashtable, your PRE times will jump about 4 or 5x (my
recollection but feel free to try it yourself :) )
It makes *very* heavy use of getting the value handle, and it can't
really be avoided.