ping again - Show hash table stats when -fmem-report

[Dimitrios Apostolou]

Hi,

This patch adds many nice stats about hash tables when gcc is run with 
-fmem-report. Attached patch tested on x86, no regressions.

Also attached is sample output of -fmem-report when compiling reload.c 
with -O2 -g. Especially interesting is the extreme htab usage in 
var-tracking (ofcourse we already knew that... :-) and maybe symtab's 
slightly high collision rate (given it's a very hot hash table). Moreover 
it's notable that since last year, collision rate for mem_attrs_htab has 
been reduced from around 8 to 1, still not good enough but major 
improvement.

Jan: It has negligible overhead in runtime, that's why I'm pushing it 
again as it is. Having compile-time instrumentation is something 
different, but still I'm not quite sure if it's worth the effort. IMHO 
doing separate builds to get memory info is tedious and even I've stopped 
doing them. If it has no overhead maybe it's better as a runtime option?
Any way, compile-time instrumentation can come later.

Finally I repost some of my notes I'd like to get feedback on:

* Is it OK that I included <assert.h> in libiberty's hashtab.c? What's the 
proper way to assert stuff, since gcc_assert() is not accessible?

* Many hash tables are created with htab_create_ggc(), for example 
referenced_vars and default_defs in tree-ssa.c. I collect statistics in 
delete_tree_ssa() but maybe some are not deallocated in there but 
automatically garbage collected?

* Obviously hash table sizes are inflated where two entries might 
reference the same element (for example in symtab_hash) but I don't handle 
this.

* Changes that reduce load factor have been backed out since they brought 
controversy. I still think they are good, at least for symtab. I'll 
measure numbers separately for this after this patch makes it.


Thanks,
Dimitris
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