[C++] Fix compile time hog in replace_placeholders (PR sanitizer/81929)

Hi!

When the expression replace_placeholders is called on contains
many SAVE_EXPRs that appear more than once in the tree, we hang walking them
over and over again, while it is sufficient to just walk it without
duplicates (not using cp_walk_tree_without_duplicates, because the callback
can cp_walk_tree* again and we want to use the same pointer set between
those).

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2017-09-14  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/81929
	* tree.c (struct replace_placeholders_t): Add pset field.
	(replace_placeholders_r): Call cp_walk_tree with d->pset as
	last argument instead of NULL.  Formatting fix.
	(replace_placeholders): Add pset variable, add its address
	into data.  Pass &pset instead of NULL to cp_walk_tree.

	* g++.dg/ubsan/pr81929.C: New test.


	Jakub

OK.

On Thu, Sep 14, 2017 at 4:12 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> Hi!
>
> When the expression replace_placeholders is called on contains
> many SAVE_EXPRs that appear more than once in the tree, we hang walking them
> over and over again, while it is sufficient to just walk it without
> duplicates (not using cp_walk_tree_without_duplicates, because the callback
> can cp_walk_tree* again and we want to use the same pointer set between
> those).
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2017-09-14  Jakub Jelinek  <jakub@redhat.com>
>
>         PR sanitizer/81929
>         * tree.c (struct replace_placeholders_t): Add pset field.
>         (replace_placeholders_r): Call cp_walk_tree with d->pset as
>         last argument instead of NULL.  Formatting fix.
>         (replace_placeholders): Add pset variable, add its address
>         into data.  Pass &pset instead of NULL to cp_walk_tree.
>
>         * g++.dg/ubsan/pr81929.C: New test.
>
> --- gcc/cp/tree.c.jj    2017-09-12 09:35:47.000000000 +0200
> +++ gcc/cp/tree.c       2017-09-14 17:38:07.717064412 +0200
> @@ -3063,6 +3063,7 @@ struct replace_placeholders_t
>  {
>    tree obj;        /* The object to be substituted for a PLACEHOLDER_EXPR.  */
>    bool seen;       /* Whether we've encountered a PLACEHOLDER_EXPR.  */
> +  hash_set<tree> *pset;        /* To avoid walking same trees multiple times.  */
>  };
>
>  /* Like substitute_placeholder_in_expr, but handle C++ tree codes and
> @@ -3085,8 +3086,8 @@ replace_placeholders_r (tree* t, int* wa
>      case PLACEHOLDER_EXPR:
>        {
>         tree x = obj;
> -       for (; !(same_type_ignoring_top_level_qualifiers_p
> -                (TREE_TYPE (*t), TREE_TYPE (x)));
> +       for (; !same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (*t),
> +                                                          TREE_TYPE (x));
>              x = TREE_OPERAND (x, 0))
>           gcc_assert (TREE_CODE (x) == COMPONENT_REF);
>         *t = x;
> @@ -3118,8 +3119,7 @@ replace_placeholders_r (tree* t, int* wa
>                   valp = &TARGET_EXPR_INITIAL (*valp);
>               }
>             d->obj = subob;
> -           cp_walk_tree (valp, replace_placeholders_r,
> -                         data_, NULL);
> +           cp_walk_tree (valp, replace_placeholders_r, data_, d->pset);
>             d->obj = obj;
>           }
>         *walk_subtrees = false;
> @@ -3151,10 +3151,11 @@ replace_placeholders (tree exp, tree obj
>      return exp;
>
>    tree *tp = &exp;
> -  replace_placeholders_t data = { obj, false };
> +  hash_set<tree> pset;
> +  replace_placeholders_t data = { obj, false, &pset };
>    if (TREE_CODE (exp) == TARGET_EXPR)
>      tp = &TARGET_EXPR_INITIAL (exp);
> -  cp_walk_tree (tp, replace_placeholders_r, &data, NULL);
> +  cp_walk_tree (tp, replace_placeholders_r, &data, &pset);
>    if (seen_p)
>      *seen_p = data.seen;
>    return exp;
> --- gcc/testsuite/g++.dg/ubsan/pr81929.C.jj     2017-09-14 17:48:09.052611540 +0200
> +++ gcc/testsuite/g++.dg/ubsan/pr81929.C        2017-09-14 17:49:21.644711332 +0200
> @@ -0,0 +1,14 @@
> +// PR sanitizer/81929
> +// { dg-do compile }
> +// { dg-options "-std=c++14 -fsanitize=undefined" }
> +
> +struct S { S &operator<< (long); S foo (); S (); };
> +
> +void
> +bar ()
> +{
> +  static_cast<S&>(S () << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0
> +                      << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0
> +                      << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0
> +                      << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0 << 0).foo ();
> +}
>
>         Jakub