diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 92d22d1af4d..d24bfdb6719 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -1489,7 +1489,8 @@ static tree c_parser_std_attribute_specifier_sequence (c_parser *);
 static void c_parser_external_declaration (c_parser *);
 static void c_parser_asm_definition (c_parser *);
 static void c_parser_declaration_or_fndef (c_parser *, bool, bool, bool,
-					   bool, bool, tree *, vec<c_token>,
+					   bool, bool, tree * = NULL,
+					   vec<c_token> * = NULL,
 					   bool have_attrs = false,
 					   tree attrs = NULL,
 					   struct oacc_routine_data * = NULL,
@@ -1774,13 +1775,12 @@ c_parser_external_declaration (c_parser *parser)
 	 an @interface or @protocol with prefix attributes).  We can
 	 only tell which after parsing the declaration specifiers, if
 	 any, and the first declarator.  */
-      c_parser_declaration_or_fndef (parser, true, true, true, false, true,
-				     NULL, vNULL);
+      c_parser_declaration_or_fndef (parser, true, true, true, false, true);
       break;
     }
 }
 
-static void c_finish_omp_declare_simd (c_parser *, tree, tree, vec<c_token>);
+static void c_finish_omp_declare_simd (c_parser *, tree, tree, vec<c_token> *);
 static void c_finish_oacc_routine (struct oacc_routine_data *, tree, bool);
 
 /* Build and add a DEBUG_BEGIN_STMT statement with location LOC.  */
@@ -1890,11 +1890,15 @@ static void
 c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 			       bool static_assert_ok, bool empty_ok,
 			       bool nested, bool start_attr_ok,
-			       tree *objc_foreach_object_declaration,
-			       vec<c_token> omp_declare_simd_clauses,
-			       bool have_attrs, tree attrs,
-			       struct oacc_routine_data *oacc_routine_data,
-			       bool *fallthru_attr_p)
+			       tree *objc_foreach_object_declaration
+			       /* = NULL */,
+			       vec<c_token> *omp_declare_simd_clauses
+			       /* = NULL */,
+			       bool have_attrs /* = false */,
+			       tree attrs /* = NULL_TREE */,
+			       struct oacc_routine_data *oacc_routine_data
+			       /* = NULL */,
+			       bool *fallthru_attr_p /* = NULL */)
 {
   struct c_declspecs *specs;
   tree prefix_attrs;
@@ -2150,7 +2154,7 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 					C_DTR_NORMAL, &dummy);
       if (declarator == NULL)
 	{
-	  if (omp_declare_simd_clauses.exists ())
+	  if (omp_declare_simd_clauses)
 	    c_finish_omp_declare_simd (parser, NULL_TREE, NULL_TREE,
 				       omp_declare_simd_clauses);
 	  if (oacc_routine_data)
@@ -2250,7 +2254,7 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 				  chainon (postfix_attrs, all_prefix_attrs));
 		  if (!d)
 		    d = error_mark_node;
-		  if (omp_declare_simd_clauses.exists ())
+		  if (omp_declare_simd_clauses)
 		    c_finish_omp_declare_simd (parser, d, NULL_TREE,
 					       omp_declare_simd_clauses);
 		}
@@ -2262,7 +2266,7 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 				  chainon (postfix_attrs, all_prefix_attrs));
 		  if (!d)
 		    d = error_mark_node;
-		  if (omp_declare_simd_clauses.exists ())
+		  if (omp_declare_simd_clauses)
 		    c_finish_omp_declare_simd (parser, d, NULL_TREE,
 					       omp_declare_simd_clauses);
 		  init_loc = c_parser_peek_token (parser)->location;
@@ -2342,7 +2346,7 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 		      warn_parm_array_mismatch (lastloc, d, parms);
 		    }
 		}
-	      if (omp_declare_simd_clauses.exists ())
+	      if (omp_declare_simd_clauses)
 		{
 		  tree parms = NULL_TREE;
 		  if (d && TREE_CODE (d) == FUNCTION_DECL)
@@ -2496,9 +2500,9 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
       while (c_parser_next_token_is_not (parser, CPP_EOF)
 	     && c_parser_next_token_is_not (parser, CPP_OPEN_BRACE))
 	c_parser_declaration_or_fndef (parser, false, false, false,
-				       true, false, NULL, vNULL);
+				       true, false);
       store_parm_decls ();
-      if (omp_declare_simd_clauses.exists ())
+      if (omp_declare_simd_clauses)
 	c_finish_omp_declare_simd (parser, current_function_decl, NULL_TREE,
 				   omp_declare_simd_clauses);
       if (oacc_routine_data)
@@ -5699,7 +5703,7 @@ c_parser_compound_statement_nostart (c_parser *parser)
 	  bool fallthru_attr_p = false;
 	  c_parser_declaration_or_fndef (parser, true, !have_std_attrs,
 					 true, true, true, NULL,
-					 vNULL, have_std_attrs, std_attrs,
+					 NULL, have_std_attrs, std_attrs,
 					 NULL, &fallthru_attr_p);
 
 	  if (last_stmt && !fallthru_attr_p)
@@ -5731,7 +5735,7 @@ c_parser_compound_statement_nostart (c_parser *parser)
 	      last_label = false;
 	      mark_valid_location_for_stdc_pragma (false);
 	      c_parser_declaration_or_fndef (parser, true, true, true, true,
-					     true, NULL, vNULL);
+					     true);
 	      /* Following the old parser, __extension__ does not
 		 disable this diagnostic.  */
 	      restore_extension_diagnostics (ext);
@@ -6782,7 +6786,7 @@ c_parser_for_statement (c_parser *parser, bool ivdep, unsigned short unroll,
 	       || c_parser_nth_token_starts_std_attributes (parser, 1))
 	{
 	  c_parser_declaration_or_fndef (parser, true, true, true, true, true, 
-					 &object_expression, vNULL);
+					 &object_expression);
 	  parser->objc_could_be_foreach_context = false;
 	  
 	  if (c_parser_next_token_is_keyword (parser, RID_IN))
@@ -6813,7 +6817,7 @@ c_parser_for_statement (c_parser *parser, bool ivdep, unsigned short unroll,
 	      ext = disable_extension_diagnostics ();
 	      c_parser_consume_token (parser);
 	      c_parser_declaration_or_fndef (parser, true, true, true, true,
-					     true, &object_expression, vNULL);
+					     true, &object_expression);
 	      parser->objc_could_be_foreach_context = false;
 	      
 	      restore_extension_diagnostics (ext);
@@ -11277,7 +11281,7 @@ c_parser_objc_methodprotolist (c_parser *parser)
 	    }
 	  else
 	    c_parser_declaration_or_fndef (parser, false, false, true,
-					   false, true, NULL, vNULL);
+					   false, true);
 	  break;
 	}
     }
@@ -17281,12 +17285,12 @@ c_parser_oacc_routine (c_parser *parser, enum pragma_context context)
 	  while (c_parser_next_token_is (parser, CPP_KEYWORD)
 		 && c_parser_peek_token (parser)->keyword == RID_EXTENSION);
 	  c_parser_declaration_or_fndef (parser, true, true, true, false, true,
-					 NULL, vNULL, false, NULL, &data);
+					 NULL, NULL, false, NULL, &data);
 	  restore_extension_diagnostics (ext);
 	}
       else
 	c_parser_declaration_or_fndef (parser, true, true, true, false, true,
-				       NULL, vNULL, false, NULL, &data);
+				       NULL, NULL, false, NULL, &data);
     }
 }
 
@@ -18393,8 +18397,7 @@ c_parser_omp_for_loop (location_t loc, c_parser *parser, enum tree_code code,
 	    vec_safe_push (for_block, c_begin_compound_stmt (true));
 	  this_pre_body = push_stmt_list ();
 	  c_in_omp_for = true;
-	  c_parser_declaration_or_fndef (parser, true, true, true, true, true,
-					 NULL, vNULL);
+	  c_parser_declaration_or_fndef (parser, true, true, true, true, true);
 	  c_in_omp_for = false;
 	  if (this_pre_body)
 	    {
@@ -20335,12 +20338,12 @@ c_parser_omp_declare_simd (c_parser *parser, enum pragma_context context)
 	  while (c_parser_next_token_is (parser, CPP_KEYWORD)
 		 && c_parser_peek_token (parser)->keyword == RID_EXTENSION);
 	  c_parser_declaration_or_fndef (parser, true, true, true, false, true,
-					 NULL, clauses);
+					 NULL, &clauses);
 	  restore_extension_diagnostics (ext);
 	}
       else
 	c_parser_declaration_or_fndef (parser, true, true, true, false, true,
-				       NULL, clauses);
+				       NULL, &clauses);
       break;
     case pragma_struct:
     case pragma_param:
@@ -20361,7 +20364,7 @@ c_parser_omp_declare_simd (c_parser *parser, enum pragma_context context)
 	  if (c_parser_next_tokens_start_declaration (parser))
 	    {
 	      c_parser_declaration_or_fndef (parser, true, true, true, true,
-					     true, NULL, clauses);
+					     true, NULL, &clauses);
 	      restore_extension_diagnostics (ext);
 	      break;
 	    }
@@ -20370,7 +20373,7 @@ c_parser_omp_declare_simd (c_parser *parser, enum pragma_context context)
       else if (c_parser_next_tokens_start_declaration (parser))
 	{
 	  c_parser_declaration_or_fndef (parser, true, true, true, true, true,
-					 NULL, clauses);
+					 NULL, &clauses);
 	  break;
 	}
       error ("%<#pragma omp declare %s%> must be followed by "
@@ -20851,8 +20854,10 @@ c_finish_omp_declare_variant (c_parser *parser, tree fndecl, tree parms)
 
 static void
 c_finish_omp_declare_simd (c_parser *parser, tree fndecl, tree parms,
-			   vec<c_token> clauses)
+			   vec<c_token> *pclauses)
 {
+  vec<c_token> &clauses = *pclauses;
+
   /* Normally first token is CPP_NAME "simd" or "variant".  CPP_EOF there
      indicates error has been reported and CPP_PRAGMA that
      c_finish_omp_declare_simd has already processed the tokens.  */
diff --git a/gcc/dominance.c b/gcc/dominance.c
index 6a262ce8283..cc63391a39a 100644
--- a/gcc/dominance.c
+++ b/gcc/dominance.c
@@ -1227,7 +1227,7 @@ recompute_dominator (enum cdi_direction dir, basic_block bb)
    from BBS.  */
 
 static void
-prune_bbs_to_update_dominators (vec<basic_block> bbs,
+prune_bbs_to_update_dominators (vec<basic_block> &bbs,
 				bool conservative)
 {
   unsigned i;
@@ -1379,7 +1379,7 @@ determine_dominators_for_sons (struct graph *g, vec<basic_block> bbs,
    a block of BBS in the current dominance tree dominate it.  */
 
 void
-iterate_fix_dominators (enum cdi_direction dir, vec<basic_block> bbs,
+iterate_fix_dominators (enum cdi_direction dir, vec<basic_block> &bbs,
 			bool conservative)
 {
   unsigned i;
diff --git a/gcc/dominance.h b/gcc/dominance.h
index 1a8c248ee98..970da02c594 100644
--- a/gcc/dominance.h
+++ b/gcc/dominance.h
@@ -78,7 +78,7 @@ checking_verify_dominators (cdi_direction dir)
 
 basic_block recompute_dominator (enum cdi_direction, basic_block);
 extern void iterate_fix_dominators (enum cdi_direction,
-				    vec<basic_block> , bool);
+				    vec<basic_block> &, bool);
 extern void add_to_dominance_info (enum cdi_direction, basic_block);
 extern void delete_from_dominance_info (enum cdi_direction, basic_block);
 extern basic_block first_dom_son (enum cdi_direction, basic_block);
diff --git a/gcc/ipa-prop.h b/gcc/ipa-prop.h
index 19751f10974..42842d9466a 100644
--- a/gcc/ipa-prop.h
+++ b/gcc/ipa-prop.h
@@ -499,10 +499,10 @@ public:
      get reallocated, the member vectors and the underlying auto_vecs would get
      out of sync.  */
   ipa_call_arg_values (ipa_auto_call_arg_values *aavals)
-    : m_known_vals (aavals->m_known_vals),
-      m_known_contexts (aavals->m_known_contexts),
-      m_known_aggs (aavals->m_known_aggs),
-      m_known_value_ranges (aavals->m_known_value_ranges)
+    : m_known_vals (aavals->m_known_vals.to_vec_legacy ()),
+      m_known_contexts (aavals->m_known_contexts.to_vec_legacy ()),
+      m_known_aggs (aavals->m_known_aggs.to_vec_legacy ()),
+      m_known_value_ranges (aavals->m_known_value_ranges.to_vec_legacy ())
   {}
 
   /* If m_known_vals (vector of known "scalar" values) is sufficiantly long,
diff --git a/gcc/tree-data-ref.c b/gcc/tree-data-ref.c
index b6f78284885..e061baa7c20 100644
--- a/gcc/tree-data-ref.c
+++ b/gcc/tree-data-ref.c
@@ -404,8 +404,7 @@ print_dist_vectors (FILE *outf, vec<lambda_vector> dist_vects,
 /* Dump function for a DATA_DEPENDENCE_RELATION structure.  */
 
 DEBUG_FUNCTION void
-dump_data_dependence_relation (FILE *outf,
-			       struct data_dependence_relation *ddr)
+dump_data_dependence_relation (FILE *outf, const data_dependence_relation *ddr)
 {
   struct data_reference *dra, *drb;
 
@@ -479,7 +478,7 @@ dump_data_dependence_relation (FILE *outf,
 /* Debug version.  */
 
 DEBUG_FUNCTION void
-debug_data_dependence_relation (struct data_dependence_relation *ddr)
+debug_data_dependence_relation (const struct data_dependence_relation *ddr)
 {
   dump_data_dependence_relation (stderr, ddr);
 }
@@ -487,10 +486,9 @@ debug_data_dependence_relation (struct data_dependence_relation *ddr)
 /* Dump into FILE all the dependence relations from DDRS.  */
 
 DEBUG_FUNCTION void
-dump_data_dependence_relations (FILE *file,
-				vec<ddr_p> ddrs)
+dump_data_dependence_relations (FILE *file, const vec<ddr_p> &ddrs)
 {
-  for (data_dependence_relation *ddr : ddrs)
+  for (auto ddr : ddrs)
     dump_data_dependence_relation (file, ddr);
 }
 
diff --git a/gcc/tree-data-ref.h b/gcc/tree-data-ref.h
index de45f25611c..685f33d85ae 100644
--- a/gcc/tree-data-ref.h
+++ b/gcc/tree-data-ref.h
@@ -528,8 +528,8 @@ extern void debug_data_reference (struct data_reference *);
 extern void debug_data_references (vec<data_reference_p> );
 extern void debug (vec<data_reference_p> &ref);
 extern void debug (vec<data_reference_p> *ptr);
-extern void debug_data_dependence_relation (struct data_dependence_relation *);
-extern void dump_data_dependence_relations (FILE *, vec<ddr_p> );
+extern void debug_data_dependence_relation (const data_dependence_relation *);
+extern void dump_data_dependence_relations (FILE *, const vec<ddr_p> &);
 extern void debug (vec<ddr_p> &ref);
 extern void debug (vec<ddr_p> *ptr);
 extern void debug_data_dependence_relations (vec<ddr_p> );
diff --git a/gcc/tree-predcom.c b/gcc/tree-predcom.c
index bed30d2ec7a..6b195d1914f 100644
--- a/gcc/tree-predcom.c
+++ b/gcc/tree-predcom.c
@@ -639,9 +639,8 @@ dump_chain (FILE *file, chain_p chain)
 
 /* Dumps CHAINS to FILE.  */
 
-extern void dump_chains (FILE *, vec<chain_p> );
 void
-dump_chains (FILE *file, vec<chain_p> chains)
+dump_chains (FILE *file, const vec<chain_p> &chains)
 {
   chain_p chain;
   unsigned i;
@@ -2049,7 +2048,7 @@ finalize_eliminated_stores (class loop *loop, chain_p chain)
 
 static void
 initialize_root_vars_lm (class loop *loop, dref root, bool written,
-			 vec<tree> *vars, vec<tree> inits,
+			 vec<tree> *vars, const vec<tree> &inits,
 			 bitmap tmp_vars)
 {
   unsigned i;
@@ -2324,7 +2323,7 @@ pcom_worker::execute_pred_commoning_chain (chain_p chain,
    optimized.  */
 
 static unsigned
-determine_unroll_factor (vec<chain_p> chains)
+determine_unroll_factor (const vec<chain_p> &chains)
 {
   chain_p chain;
   unsigned factor = 1, af, nfactor, i;
@@ -2401,7 +2400,7 @@ pcom_worker::execute_pred_commoning (bitmap tmp_vars)
    phi node, record the ssa name that is defined by it.  */
 
 static void
-replace_phis_by_defined_names (vec<chain_p> chains)
+replace_phis_by_defined_names (vec<chain_p> &chains)
 {
   chain_p chain;
   dref a;
@@ -3276,7 +3275,7 @@ pcom_worker::prepare_finalizers ()
 /* Insert all initializing gimple stmts into LOOP's entry edge.  */
 
 static void
-insert_init_seqs (class loop *loop, vec<chain_p> chains)
+insert_init_seqs (class loop *loop, vec<chain_p> &chains)
 {
   unsigned i;
   edge entry = loop_preheader_edge (loop);
@@ -3387,7 +3386,7 @@ pcom_worker::tree_predictive_commoning_loop (bool allow_unroll_p)
 	fprintf (dump_file, "Unrolling %u times.\n", unroll_factor);
 
       dta.tmp_vars = tmp_vars;
-      dta.chains = m_chains;
+      dta.chains = m_chains.to_vec_legacy ();
       dta.worker = this;
 
       /* Cfg manipulations performed in tree_transform_and_unroll_loop before
diff --git a/gcc/tree-ssa-pre.c b/gcc/tree-ssa-pre.c
index d2a7395dd8f..ebe95cc6c73 100644
--- a/gcc/tree-ssa-pre.c
+++ b/gcc/tree-ssa-pre.c
@@ -3107,7 +3107,7 @@ create_expression_by_pieces (basic_block block, pre_expr expr,
 
 static bool
 insert_into_preds_of_block (basic_block block, unsigned int exprnum,
-			    vec<pre_expr> avail)
+			    vec<pre_expr> &avail)
 {
   pre_expr expr = expression_for_id (exprnum);
   pre_expr newphi;
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 91ce443b1b2..e237eb4ca90 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -92,7 +92,7 @@ public:
 private:
   void maybe_register_path (edge taken_edge);
   bool find_paths_to_names (basic_block bb, bitmap imports);
-  bool resolve_def (tree name, bitmap interesting, vec<tree> worklist);
+  bool resolve_def (tree name, bitmap interesting, vec<tree> &worklist);
   bool resolve_phi (gphi *phi, bitmap imports);
   edge find_taken_edge (const vec<basic_block> &path);
   edge find_taken_edge_cond (const vec<basic_block> &path, gcond *);
@@ -240,7 +240,7 @@ back_threader::find_taken_edge_cond (const vec<basic_block> &path,
 // Populate a vector of trees from a bitmap.
 
 static inline void
-populate_worklist (vec<tree> worklist, bitmap bits)
+populate_worklist (vec<tree> &worklist, bitmap bits)
 {
   bitmap_iterator bi;
   unsigned i;
@@ -317,7 +317,7 @@ back_threader::resolve_phi (gphi *phi, bitmap interesting)
 // current path to be constant, register the path, and return TRUE.
 
 bool
-back_threader::resolve_def (tree name, bitmap interesting, vec<tree> worklist)
+back_threader::resolve_def (tree name, bitmap interesting, vec<tree> &worklist)
 {
   gimple *def_stmt = SSA_NAME_DEF_STMT (name);
 
diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
index 17d24b44fc1..d594c0a1b1e 100644
--- a/gcc/tree-vect-data-refs.c
+++ b/gcc/tree-vect-data-refs.c
@@ -212,7 +212,7 @@ vect_mark_for_runtime_alias_test (ddr_p ddr, loop_vec_info loop_vinfo)
 static void
 vect_check_nonzero_value (loop_vec_info loop_vinfo, tree value)
 {
-  vec<tree> checks = LOOP_VINFO_CHECK_NONZERO (loop_vinfo);
+  const vec<tree> &checks = LOOP_VINFO_CHECK_NONZERO (loop_vinfo);
   for (unsigned int i = 0; i < checks.length(); ++i)
     if (checks[i] == value)
       return;
@@ -2349,8 +2349,8 @@ vect_enhance_data_refs_alignment (loop_vec_info loop_vinfo)
 
   if (do_versioning)
     {
-      vec<stmt_vec_info> may_misalign_stmts
-        = LOOP_VINFO_MAY_MISALIGN_STMTS (loop_vinfo);
+      const vec<stmt_vec_info> &may_misalign_stmts
+	= LOOP_VINFO_MAY_MISALIGN_STMTS (loop_vinfo);
       stmt_vec_info stmt_info;
 
       /* It can now be assumed that the data references in the statements
@@ -3364,7 +3364,8 @@ static void
 vect_check_lower_bound (loop_vec_info loop_vinfo, tree expr, bool unsigned_p,
 			poly_uint64 min_value)
 {
-  vec<vec_lower_bound> lower_bounds = LOOP_VINFO_LOWER_BOUNDS (loop_vinfo);
+  vec<vec_lower_bound> &lower_bounds
+    = LOOP_VINFO_LOWER_BOUNDS (loop_vinfo);
   for (unsigned int i = 0; i < lower_bounds.length (); ++i)
     if (operand_equal_p (lower_bounds[i].expr, expr, 0))
       {
@@ -3466,10 +3467,10 @@ vect_prune_runtime_alias_test_list (loop_vec_info loop_vinfo)
   typedef pair_hash <tree_operand_hash, tree_operand_hash> tree_pair_hash;
   hash_set <tree_pair_hash> compared_objects;
 
-  vec<ddr_p> may_alias_ddrs = LOOP_VINFO_MAY_ALIAS_DDRS (loop_vinfo);
+  const vec<ddr_p> &may_alias_ddrs = LOOP_VINFO_MAY_ALIAS_DDRS (loop_vinfo);
   vec<dr_with_seg_len_pair_t> &comp_alias_ddrs
     = LOOP_VINFO_COMP_ALIAS_DDRS (loop_vinfo);
-  vec<vec_object_pair> &check_unequal_addrs
+  const vec<vec_object_pair> &check_unequal_addrs
     = LOOP_VINFO_CHECK_UNEQUAL_ADDRS (loop_vinfo);
   poly_uint64 vect_factor = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
   tree scalar_loop_iters = LOOP_VINFO_NITERS (loop_vinfo);
@@ -5350,7 +5351,7 @@ vect_store_lanes_supported (tree vectype, unsigned HOST_WIDE_INT count,
    I4:  6 14 22 30  7 15 23 31.  */
 
 void
-vect_permute_store_chain (vec_info *vinfo, vec<tree> dr_chain,
+vect_permute_store_chain (vec_info *vinfo, vec<tree> &dr_chain,
 			  unsigned int length,
 			  stmt_vec_info stmt_info,
 			  gimple_stmt_iterator *gsi,
diff --git a/gcc/tree-vect-slp-patterns.c b/gcc/tree-vect-slp-patterns.c
index ad209deca79..b8d09b7832e 100644
--- a/gcc/tree-vect-slp-patterns.c
+++ b/gcc/tree-vect-slp-patterns.c
@@ -746,7 +746,7 @@ vect_match_call_complex_mla (slp_tree node, unsigned child,
    of the negate node.  */
 
 static inline bool
-vect_normalize_conj_loc (vec<slp_tree> args, bool *neg_first_p = NULL)
+vect_normalize_conj_loc (vec<slp_tree> &args, bool *neg_first_p = NULL)
 {
   gcc_assert (args.length () == 2);
   bool neg_found = false;
diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index e6b81a0518c..94bdb74ea8d 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -4499,7 +4499,7 @@ static void
 vect_create_vectorized_demotion_stmts (vec_info *vinfo, vec<tree> *vec_oprnds,
 				       int multi_step_cvt,
 				       stmt_vec_info stmt_info,
-				       vec<tree> vec_dsts,
+				       vec<tree> &vec_dsts,
 				       gimple_stmt_iterator *gsi,
 				       slp_tree slp_node, enum tree_code code)
 {
diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index 686644b4288..5571b3cce3b 100644
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -1990,8 +1990,8 @@ extern bool vect_grouped_store_supported (tree, unsigned HOST_WIDE_INT);
 extern bool vect_store_lanes_supported (tree, unsigned HOST_WIDE_INT, bool);
 extern bool vect_grouped_load_supported (tree, bool, unsigned HOST_WIDE_INT);
 extern bool vect_load_lanes_supported (tree, unsigned HOST_WIDE_INT, bool);
-extern void vect_permute_store_chain (vec_info *,
-				      vec<tree> ,unsigned int, stmt_vec_info,
+extern void vect_permute_store_chain (vec_info *, vec<tree> &,
+				      unsigned int, stmt_vec_info,
 				      gimple_stmt_iterator *, vec<tree> *);
 extern tree vect_setup_realignment (vec_info *,
 				    stmt_vec_info, gimple_stmt_iterator *,
diff --git a/gcc/vec.c b/gcc/vec.c
index f9dbb2cac31..6d767cc12c1 100644
--- a/gcc/vec.c
+++ b/gcc/vec.c
@@ -38,16 +38,6 @@ along with GCC; see the file COPYING3.  If not see
 #include "diagnostic-core.h"
 #endif
 
-/* vNULL is an empty type with a template cast operation that returns
-   a zero-initialized vec<T, A, L> instance.  Use this when you want
-   to assign nil values to new vec instances or pass a nil vector as
-   a function call argument.
-
-   We use this technique because vec<T, A, L> must be PODs (they are
-   stored in unions and passed in vararg functions), this means that
-   they cannot have ctors/dtors.  */
-vnull vNULL;
-
 /* Vector memory usage.  */
 class vec_usage: public mem_usage
 {
@@ -282,6 +272,42 @@ safe_push_range (vec <int>&v, int start, int limit)
     v.safe_push (i);
 }
 
+/* Verify forms of initialization.  */
+
+static void
+test_init ()
+{
+  {
+    vec<int> v1{ };
+    ASSERT_EQ (0, v1.length ());
+
+    vec<int> v2 (v1);
+    ASSERT_EQ (0, v2.length ());
+  }
+
+  {
+    vec<int> v1 = vec<int>();
+    ASSERT_EQ (0, v1.length ());
+
+    vec<int> v2 = v1;
+    ASSERT_EQ (0, v2.length ());
+  }
+
+  {
+    vec<int> v1 (vNULL);
+    ASSERT_EQ (0, v1.length ());
+    v1.safe_push (1);
+
+    vec<int> v2 (v1);
+    ASSERT_EQ (1, v1.length ());
+    v2.safe_push (1);
+
+    ASSERT_EQ (2, v1.length ());
+    ASSERT_EQ (2, v2.length ());
+    v1.release ();
+  }
+}
+
 /* Verify that vec::quick_push works correctly.  */
 
 static void
@@ -547,6 +573,7 @@ test_auto_delete_vec ()
 void
 vec_c_tests ()
 {
+  test_init ();
   test_quick_push ();
   test_safe_push ();
   test_truncate ();
diff --git a/gcc/vec.h b/gcc/vec.h
index 30ef9a69473..b3f47b1f65b 100644
--- a/gcc/vec.h
+++ b/gcc/vec.h
@@ -541,18 +541,16 @@ vec_copy_construct (T *dst, const T *src, unsigned n)
     ::new (static_cast<void*>(dst)) T (*src);
 }
 
-/* Type to provide NULL values for vec<T, A, L>.  This is used to
-   provide nil initializers for vec instances.  Since vec must be
-   a POD, we cannot have proper ctor/dtor for it.  To initialize
-   a vec instance, you can assign it the value vNULL.  This isn't
-   needed for file-scope and function-local static vectors, which
-   are zero-initialized by default.  */
-struct vnull
-{
-  template <typename T, typename A, typename L>
-  CONSTEXPR operator vec<T, A, L> () const { return vec<T, A, L>(); }
-};
-extern vnull vNULL;
+/* Type to provide zero-initialized values for vec<T, A, L>.  This is
+   used to  provide nil initializers for vec instances.  Since vec must
+   be a trivially copyable type that can be copied by memcpy and zeroed
+   out by memset, it must have defaulted default and copy ctor and copy
+   assignment.  To initialize a vec either use value initialization
+   (e.g., vec() or vec v{ };) or assign it the value vNULL.  This isn't
+   needed for file-scope and function-local static vectors, which are
+   zero-initialized by default.  */
+struct vnull { };
+constexpr vnull vNULL{ };
 
 
 /* Embeddable vector.  These vectors are suitable to be embedded
@@ -1431,10 +1429,34 @@ gt_pch_nx (vec<T, A, vl_embed> *v, gt_pointer_operator op, void *cookie)
    As long as we use C++03, we cannot have constructors nor
    destructors in classes that are stored in unions.  */
 
+template<typename T, size_t N = 0>
+class auto_vec;
+
 template<typename T>
 struct vec<T, va_heap, vl_ptr>
 {
 public:
+  /* Default ctors to ensure triviality.  Use value-initialization
+     (e.g., vec() or vec v{ };) or vNULL to create a zero-initialized
+     instance.  */
+  vec () = default;
+  vec (const vec &) = default;
+  /* Initialization from the generic vNULL.  */
+  vec (vnull): m_vec () { }
+  /* Same as default ctor: vec storage must be released manually.  */
+  ~vec () = default;
+
+  /* Defaulted same as copy ctor.  */
+  vec& operator= (const vec &) = default;
+
+  /* Prevent implicit conversion from auto_vec.  Use auto_vec::to_vec()
+     instead.  */
+  template <size_t N>
+  vec (auto_vec<T, N> &) = delete;
+
+  template <size_t N>
+  void operator= (auto_vec<T, N> &) = delete;
+
   /* Memory allocation and deallocation for the embedded vector.
      Needed because we cannot have proper ctors/dtors defined.  */
   void create (unsigned nelems CXX_MEM_STAT_INFO);
@@ -1522,7 +1544,7 @@ public:
    want to ask for internal storage for vectors on the stack because if the
    size of the vector is larger than the internal storage that space is wasted.
    */
-template<typename T, size_t N = 0>
+template<typename T, size_t N /* = 0 */>
 class auto_vec : public vec<T, va_heap>
 {
 public:
@@ -1549,6 +1571,14 @@ public:
     this->release ();
   }
 
+  /* Explicitly convert to the base class.  There is no conversion
+     from a const auto_vec because a copy of the returned vec can
+     be used to modify *THIS.
+     This is a legacy function not to be used in new code.  */
+  vec<T, va_heap> to_vec_legacy () {
+    return *static_cast<vec<T, va_heap> *>(this);
+  }
+
 private:
   vec<T, va_heap, vl_embed> m_auto;
   T m_data[MAX (N - 1, 1)];
@@ -1602,6 +1632,14 @@ public:
       return *this;
     }
 
+  /* Explicitly convert to the base class.  There is no conversion
+     from a const auto_vec because a copy of the returned vec can
+     be used to modify *THIS.
+     This is a legacy function not to be used in new code.  */
+  vec<T, va_heap> to_vec_legacy () {
+    return *static_cast<vec<T, va_heap> *>(this);
+  }
+
   // You probably don't want to copy a vector, so these are deleted to prevent
   // unintentional use.  If you really need a copy of the vectors contents you
   // can use copy ().
@@ -1781,7 +1819,7 @@ template<typename T>
 inline vec<T, va_heap, vl_ptr>
 vec<T, va_heap, vl_ptr>::copy (ALONE_MEM_STAT_DECL) const
 {
-  vec<T, va_heap, vl_ptr> new_vec = vNULL;
+  vec<T, va_heap, vl_ptr> new_vec{ };
   if (length ())
     new_vec.m_vec = m_vec->copy (ALONE_PASS_MEM_STAT);
   return new_vec;