[Bug c++/70001] [5 regression] Infinity compilation time

["ppalka at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70001

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
struct X
{
  int a;

  constexpr X () : a (0) { }
};

const int N = 1 << 19;

struct A
{
  X elems[N];
} F;


Just playing around, but naively sharing the constructor of each array element
in the case of default- or value-initialization lowers compile time of the
original test case down to a few seconds.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index d3b04b1..2ce44a5 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2328,6 +2328,7 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree
atype, tree init,
   vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor);
   vec_alloc (*p, max + 1);
   bool pre_init = false;
+  tree pre_init_elt = NULL_TREE;
   unsigned HOST_WIDE_INT i;

   /* For the default constructor, build up a call to the default
@@ -2377,9 +2378,12 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree
atype, tree init,
        {
          /* Initializing an element using value or default initialization
             we just pre-built above.  */
-         eltinit = (cxx_eval_constant_expression
-                    (&new_ctx, init,
-                     lval, non_constant_p, overflow_p));
+         if (pre_init_elt == NULL_TREE)
+           pre_init_elt = (cxx_eval_constant_expression
+                           (&new_ctx, init,
+                            lval, non_constant_p, overflow_p));
+
+          eltinit = pre_init_elt;
        }
       else
        {