[C++] Fix PR84281

The following patch optimizes the equal element case of 
cxx_eval_vec_init_1 to use a RANGE_EXPR in the built CONSTRUCTOR
instead of repeating the same element over and over.  This
cuts down memory use of the invalid testcase in the PR from
tens of GB to nothing and makes us rejct it instantanously
instead of by going OOM.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

Ok for trunk?

Thanks,
Richard.

2018-02-09  Richard Biener  <rguenther@suse.de>

	PR c++/84281
	* constexpr.c (cxx_eval_vec_init_1): Use a RANGE_EXPR to compact
	uniform constructors and delay allocating them fully.

OK.

On Fri, Feb 9, 2018 at 7:44 AM, Richard Biener <rguenther@suse.de> wrote:
>
> The following patch optimizes the equal element case of
> cxx_eval_vec_init_1 to use a RANGE_EXPR in the built CONSTRUCTOR
> instead of repeating the same element over and over.  This
> cuts down memory use of the invalid testcase in the PR from
> tens of GB to nothing and makes us rejct it instantanously
> instead of by going OOM.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>
> Ok for trunk?
>
> Thanks,
> Richard.
>
> 2018-02-09  Richard Biener  <rguenther@suse.de>
>
>         PR c++/84281
>         * constexpr.c (cxx_eval_vec_init_1): Use a RANGE_EXPR to compact
>         uniform constructors and delay allocating them fully.
>
> Index: gcc/cp/constexpr.c
> ===================================================================
> --- gcc/cp/constexpr.c  (revision 257491)
> +++ gcc/cp/constexpr.c  (working copy)
> @@ -2885,7 +2885,6 @@ cxx_eval_vec_init_1 (const constexpr_ctx
>    unsigned HOST_WIDE_INT max = tree_to_uhwi (array_type_nelts_top (atype));
>    verify_ctor_sanity (ctx, atype);
>    vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor);
> -  vec_alloc (*p, max + 1);
>    bool pre_init = false;
>    unsigned HOST_WIDE_INT i;
>
> @@ -2978,13 +2977,14 @@ cxx_eval_vec_init_1 (const constexpr_ctx
>         {
>           if (new_ctx.ctor != ctx->ctor)
>             eltinit = new_ctx.ctor;
> -         for (i = 1; i < max; ++i)
> -           {
> -             idx = build_int_cst (size_type_node, i);
> -             CONSTRUCTOR_APPEND_ELT (*p, idx, unshare_constructor (eltinit));
> -           }
> +         tree range = build2 (RANGE_EXPR, size_type_node,
> +                              build_int_cst (size_type_node, 1),
> +                              build_int_cst (size_type_node, max - 1));
> +         CONSTRUCTOR_APPEND_ELT (*p, range, unshare_constructor (eltinit));
>           break;
>         }
> +      else
> +       vec_safe_reserve (*p, max + 1);
>      }
>
>    if (!*non_constant_p)

On Fri, 9 Feb 2018, Jason Merrill wrote:

> OK.

Failed to notice that vec_safe_reserve reserves additional space
so applied as follows restricting it to the very first iteration.

Richard.

2018-02-12  Richard Biener  <rguenther@suse.de>

	PR c++/84281
	* constexpr.c (cxx_eval_vec_init_1): Use a RANGE_EXPR to compact
	uniform constructors and delay allocating them fully.

Index: gcc/cp/constexpr.c
===================================================================
--- gcc/cp/constexpr.c	(revision 257525)
+++ gcc/cp/constexpr.c	(working copy)
@@ -2885,7 +2885,6 @@ cxx_eval_vec_init_1 (const constexpr_ctx
   unsigned HOST_WIDE_INT max = tree_to_uhwi (array_type_nelts_top (atype));
   verify_ctor_sanity (ctx, atype);
   vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor);
-  vec_alloc (*p, max + 1);
   bool pre_init = false;
   unsigned HOST_WIDE_INT i;
 
@@ -2978,13 +2977,14 @@ cxx_eval_vec_init_1 (const constexpr_ctx
 	{
 	  if (new_ctx.ctor != ctx->ctor)
 	    eltinit = new_ctx.ctor;
-	  for (i = 1; i < max; ++i)
-	    {
-	      idx = build_int_cst (size_type_node, i);
-	      CONSTRUCTOR_APPEND_ELT (*p, idx, unshare_constructor (eltinit));
-	    }
+	  tree range = build2 (RANGE_EXPR, size_type_node,
+			       build_int_cst (size_type_node, 1),
+			       build_int_cst (size_type_node, max - 1));
+	  CONSTRUCTOR_APPEND_ELT (*p, range, unshare_constructor (eltinit));
 	  break;
 	}
+      else if (i == 0)
+	vec_safe_reserve (*p, max);
     }
 
   if (!*non_constant_p)