Re: [PATCH] c++: use auto_vec in cp_parser_template_argument_list

[Patrick Palka <ppalka@redhat.com>]

On Tue, 9 Nov 2021, Jason Merrill wrote:

> On 11/9/21 11:02, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> 
> OK, though I wonder about using releasing_vec instead of auto_vec; reusing a
> previously allocated vec vs. building one on the stack.

Thanks a lot.  And hmm, I think using reusing a previously allocated vec
here would be tricky since cp_parser_template_argument_list can be
called recursively, and so only the outermost call would be able to
benefit from reuse unless we perhaps maintain a freelist of such vecs
IIUC.  Seems like that would complicate the code enough to not be worth
it.

> > gcc/cp/ChangeLog:
> > 
> > 	* parser.c (cp_parser_template_argument_list): Use auto_vec
> > 	instead of manual memory management.
> > ---
> >   gcc/cp/parser.c | 35 ++++++++---------------------------
> >   1 file changed, 8 insertions(+), 27 deletions(-)
> > 
> > diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> > index 32de97b08bd..8823399529e 100644
> > --- a/gcc/cp/parser.c
> > +++ b/gcc/cp/parser.c
> > @@ -18558,11 +18558,6 @@ cp_parser_template_name (cp_parser* parser,
> >   static tree
> >   cp_parser_template_argument_list (cp_parser* parser)
> >   {
> > -  tree fixed_args[10];
> > -  unsigned n_args = 0;
> > -  unsigned alloced = 10;
> > -  tree *arg_ary = fixed_args;
> > -  tree vec;
> >     bool saved_in_template_argument_list_p;
> >     bool saved_ice_p;
> >     bool saved_non_ice_p;
> > @@ -18581,16 +18576,15 @@ cp_parser_template_argument_list (cp_parser*
> > parser)
> >     parser->non_integral_constant_expression_p = false;
> >       /* Parse the arguments.  */
> > +  auto_vec<tree, 10> args;
> >     do
> >       {
> > -      tree argument;
> > -
> > -      if (n_args)
> > +      if (!args.is_empty ())
> >   	/* Consume the comma.  */
> >   	cp_lexer_consume_token (parser->lexer);
> >           /* Parse the template-argument.  */
> > -      argument = cp_parser_template_argument (parser);
> > +      tree argument = cp_parser_template_argument (parser);
> >           /* If the next token is an ellipsis, we're expanding a template
> >            argument pack. */
> > @@ -18610,29 +18604,16 @@ cp_parser_template_argument_list (cp_parser*
> > parser)
> >             argument = make_pack_expansion (argument);
> >           }
> >   -      if (n_args == alloced)
> > -	{
> > -	  alloced *= 2;
> > -
> > -	  if (arg_ary == fixed_args)
> > -	    {
> > -	      arg_ary = XNEWVEC (tree, alloced);
> > -	      memcpy (arg_ary, fixed_args, sizeof (tree) * n_args);
> > -	    }
> > -	  else
> > -	    arg_ary = XRESIZEVEC (tree, arg_ary, alloced);
> > -	}
> > -      arg_ary[n_args++] = argument;
> > +      args.safe_push (argument);
> >       }
> >     while (cp_lexer_next_token_is (parser->lexer, CPP_COMMA));
> >   -  vec = make_tree_vec (n_args);
> > +  int n_args = args.length ();
> > +  tree vec = make_tree_vec (n_args);
> >   -  while (n_args--)
> > -    TREE_VEC_ELT (vec, n_args) = arg_ary[n_args];
> > +  for (int i = 0; i < n_args; i++)
> > +    TREE_VEC_ELT (vec, i) = args[i];
> >   -  if (arg_ary != fixed_args)
> > -    free (arg_ary);
> >     parser->non_integral_constant_expression_p = saved_non_ice_p;
> >     parser->integral_constant_expression_p = saved_ice_p;
> >     parser->in_template_argument_list_p = saved_in_template_argument_list_p;
> > 
> 
>