Re: [PATCH] Fix PR c++/70452 (regression in C++ parsing performance)

[Jason Merrill <jason@redhat.com>]

On 04/02/2016 05:18 PM, Patrick Palka wrote:
> Here's a version that uses a separate deletable table to cache the
> function copies.  For simplicity I used a hash_map instead of a
> hash_table.  Does this look OK to commit after bootstrap + regtest?

Thanks.  Minor nits:

 > +struct fundef_copies_table_t
 > +{
 > +  hash_map<tree, fun_copy *> *map;
 > +};

Why wrap the pointer in a struct?

> +	  maybe_initialize_fundef_copies_table ();
> +	  fun_copy *copy = get_fun_copy (fun);

Let's move the initialization call inside get_fun_copy.

> On a related note, I noticed that the constexpr_call_table is not marked
> deletable.  Marking it deletable speeds up the test case in the PR by
> about 10% and saves about 10MB.  Do you think doing so is a good idea?

Please.

> On another related note, I noticed that marking something as both
> GTY((deletable, cache)) doesn't work as intended, because the
> gt_cleare_cache functions run _after_ all deletable roots get
> zeroed out.  So during GC the gt_cleare_cache function of a root
> marked "deletable, cache" would always be a no-op.  Concretely I think
> this means that our cv_cache and fold_cache leak memory during GC
> because their underlying hash_map (allocated by operator new) is zeroed
> before gc_cleare_cache could clear it.

Hmm, I thought I remembered hitting the breakpoint in gt_cleare_cache 
and it being non-null.  But I guess we can get rid of the cache_map 
class and use the approach you have here, of a deletable gc-allocated 
hash_map pointer; I'd still use ->empty() for dumping the cache outside 
of GC, though.

Jason