[C++] Improve memory use for PR12245

On Wed, 1 Feb 2017, Richard Biener wrote:

> On Wed, 1 Feb 2017, Jakub Jelinek wrote:
> 
> > On Wed, Feb 01, 2017 at 02:14:20PM +0100, Richard Biener wrote:
> > > 
> > > Looks like we cache the answer to maybe_constant_value (INTEGER_CST)
> > > which results in (-fmem-report):
> > > 
> > > cp/constexpr.c:4814 (maybe_constant_value)         67108816:100.0% 
> > > 100663104        17:  0.0%       ggc
> > > 
> > > this can be improved trivially to
> > > 
> > > cp/constexpr.c:4817 (maybe_constant_value)             2032: 13.6%      
> > > 2144         2:  0.0%       ggc
> > > 
> > > with the following patch which I am testing right now.
> > > 
> > > Ok for trunk?
> > > 
> > > (just in case it causes some fallout because, err, some tcc_constant
> > > is not really constant, what's the subset we can cheaply check here?
> > > basically we want to avoid caching all INTEGER_CSTs we use for
> > > CONSTRUCTOR_INDEX in large initializers)
> > 
> > I'm worried that we don't want to handle all the constants that way.
> > As I wrote on IRC, I see some problematic constants:
> > 1) not sure if constants can't be
> >    potential_nondependent_constant_expression, then we don't want to return
> >    them
> > 2) cxx_eval_outermost_constant_expr has some special handling of
> >    trees with vector type (and array type)
> > 3) constants with TREE_OVERFLOW should go through maybe_constant_value_1
> > 4) INTEGER_CSTs with POINTER_TYPE (if they aren't 0) likewise
> > 
> > For 3) and 4) I believe maybe_constant_value is supposed to wrap the
> > constants into a NOP_EXPR or something.
> 
> Just to mention, bootstrap & regtest completed successfully without
> regressions on x86_64-unknown-linux-gnu so we at least have zero
> testing coverage for the cases that break.
> 
> I'll wait for Jason to suggest specific things to avoid, TREE_OVERFLOW
> and pointer types are easy (no need to special case zero, it's just
> one entry per pointer type).

Oh, and just to mention the same issue of course plagues 
maybe_constant_init which ends up allocating a hash_map 1630776 times
(fixing that doesn't fix any memory-hog but would avoid some needless
cycles spent on this).  Similar "simple" patch would be

	* constexpr.c (maybe_constant_init): Bail out early
	for CONSTANT_CLASS_P.


which is even eventually safer because it's after 
the !potential_nondependent_static_init_expression (if that can
be ever true for CONSTANT_CLASS_P t).

Then the other issue noticed is that we always copy every
CONSTRUCTOR at least once via reshape_init_array.

I think both maybe_constant_value and maybe_constant_init are
low-hanging fruit to fix at this point so waiting for some
guidance on Jakubs concerns (or just take it yourself from here).

Thanks,
Richard.

> > > 2017-02-01  Richard Biener  <rguenther@suse.de>
> > > 
> > > 	cp/
> > > 	* constexpr.c (maybe_constant_value): Do not cache
> > > 	CONSTANT_CLASS_P nodes.
> > > 
> > > Index: gcc/cp/constexpr.c
> > > ===================================================================
> > > --- gcc/cp/constexpr.c	(revision 245094)
> > > +++ gcc/cp/constexpr.c	(working copy)
> > > @@ -4810,6 +4810,9 @@ static GTY((deletable)) hash_map<tree, t
> > >  tree
> > >  maybe_constant_value (tree t, tree decl)
> > >  {
> > > +  if (CONSTANT_CLASS_P (t))
> > > +    return t;
> > > +
> > >    if (cv_cache == NULL)
> > >      cv_cache = hash_map<tree, tree>::create_ggc (101);
> > >