[PATCH][C++] Save memory/time when folding CONSTRUCTORs

[Jakub Jelinek]

On Wed, Feb 01, 2017 at 04:18:49PM +0100, Richard Biener wrote:
> 2017-02-01  Richard Biener  <rguenther@suse.de>
> 
> 	PR cp/14179
> 	cp/
> 	* cp-gimplify.c (cp_fold): When folding a CONSTRUCTOR copy
> 	it lazily on the first changed element only and copy it
> 	fully upfront, only storing changed elements.
> 
> Index: gcc/cp/cp-gimplify.c
> ===================================================================
> --- gcc/cp/cp-gimplify.c	(revision 245096)
> +++ gcc/cp/cp-gimplify.c	(working copy)
> @@ -2358,15 +2358,11 @@ cp_fold (tree x)
>        {
>  	unsigned i;
>  	constructor_elt *p;
> -	bool changed = false;
>  	vec<constructor_elt, va_gc> *elts = CONSTRUCTOR_ELTS (x);
>  	vec<constructor_elt, va_gc> *nelts = NULL;
> -	vec_safe_reserve (nelts, vec_safe_length (elts));
>  	FOR_EACH_VEC_SAFE_ELT (elts, i, p)
>  	  {
>  	    tree op = cp_fold (p->value);
> -	    constructor_elt e = { p->index, op };
> -	    nelts->quick_push (e);
>  	    if (op != p->value)
>  	      {
>  		if (op == error_mark_node)
> @@ -2375,10 +2371,12 @@ cp_fold (tree x)
>  		    changed = false;
>  		    break;
>  		  }
> -		changed = true;
> +		if (nelts == NULL)
> +		  nelts = elts->copy ();
> +		(*nelts)[i].value = op;
>  	      }
>  	  }
> -	if (changed)
> +	if (nelts)
>  	  x = build_constructor (TREE_TYPE (x), nelts);
>  	else
>  	  vec_free (nelts);

vec_free (nelts); doesn't make sense in this case though,
so I'd also remove the last 2 lines.

	Jakub