[PATCH][C++] Improve memory use for PR12245

[Richard Biener]

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)

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);