Re: [PATCH 1/3] Add virtual destructor to dwarf2_per_cu_data

[Tom Tromey <tromey@adacore.com>]

Tom> I'm thinking that instead I should add a custom deleter for these
Tom> objects that knows to cast to signatured_type when appropriate.  This
Tom> would avoid a vtable pointer just to hold the destructor.

Here's the patch.

Tom

commit 25e4481af0d85f3c22c824323c42bc23438c1cbf
Author: Tom Tromey <tromey@adacore.com>
Date:   Mon May 3 12:42:58 2021 -0600

    Change how dwarf2_per_cu_data is deleted
    
    Address sanitizer pointed out that the patch to use 'delete' for
    dwarf2_per_cu_data introduced a bug -- now it is possible to delete a
    signatured_type using a pointer to its base class.
    
    This patch fixes the problem by introducing a deleter and a unique_ptr
    specialization.  A virtual destructor would be more ordinary here, but
    it seemed wasteful to add a vtable just for this purpose.  If virtual
    methods are ever needed here, we can revisit this.
    
    2021-05-06  Tom Tromey  <tromey@adacore.com>
    
            * dwarf2/read.h (struct dwarf2_per_cu_data_deleter: New.
            (dwarf2_per_cu_data_up): New typedef.
            (struct dwarf2_per_bfd) <allocate_per_cu>: Change return type.
            <all_comp_units>: Use dwarf2_per_cu_data_up.
            * dwarf2/read.c (dwarf2_per_cu_data::operator()): New function.
            (dwarf2_per_bfd::allocate_per_cu): Return dwarf2_per_cu_data_up.
            (create_cu_from_index_list): Likewise.
            (create_signatured_type_table_from_index)
            (create_cus_from_debug_names_list, add_type_unit)
            (read_comp_units_from_section): Update.
            (dwarf2_find_containing_comp_unit): Change type of all_comp_units.
            (run_test): Update.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 187249db47e..78a3dadbdd5 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,18 @@
+2021-05-06  Tom Tromey  <tromey@adacore.com>
+
+	* dwarf2/read.h (struct dwarf2_per_cu_data_deleter: New.
+	(dwarf2_per_cu_data_up): New typedef.
+	(struct dwarf2_per_bfd) <allocate_per_cu>: Change return type.
+	<all_comp_units>: Use dwarf2_per_cu_data_up.
+	* dwarf2/read.c (dwarf2_per_cu_data::operator()): New function.
+	(dwarf2_per_bfd::allocate_per_cu): Return dwarf2_per_cu_data_up.
+	(create_cu_from_index_list): Likewise.
+	(create_signatured_type_table_from_index)
+	(create_cus_from_debug_names_list, add_type_unit)
+	(read_comp_units_from_section): Update.
+	(dwarf2_find_containing_comp_unit): Change type of all_comp_units.
+	(run_test): Update.
+
 2021-05-06  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* guile/scm-breakpoint.c (bpscm_print_breakpoint_smob): Only print
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 5796cf1730b..e1965c193ed 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -1760,6 +1760,17 @@ dwarf2_queue_item::~dwarf2_queue_item ()
     }
 }
 
+/* See dwarf2/read.h.  */
+
+void
+dwarf2_per_cu_data_deleter::operator() (dwarf2_per_cu_data *data)
+{
+  if (data->is_debug_types)
+    delete static_cast<signatured_type *> (data);
+  else
+    delete data;
+}
+
 /* The return type of find_file_and_directory.  Note, the enclosed
    string pointers are only valid while this object is valid.  */
 
@@ -2522,10 +2533,10 @@ dw2_instantiate_symtab (dwarf2_per_cu_data *per_cu,
 
 /* See read.h.  */
 
-std::unique_ptr<dwarf2_per_cu_data>
+dwarf2_per_cu_data_up
 dwarf2_per_bfd::allocate_per_cu ()
 {
-  std::unique_ptr<dwarf2_per_cu_data> result (new dwarf2_per_cu_data);
+  dwarf2_per_cu_data_up result (new dwarf2_per_cu_data);
   result->per_bfd = this;
   result->index = m_num_psymtabs++;
   return result;
@@ -2546,13 +2557,13 @@ dwarf2_per_bfd::allocate_signatured_type ()
 /* Return a new dwarf2_per_cu_data allocated on the per-bfd
    obstack, and constructed with the specified field values.  */
 
-static std::unique_ptr<dwarf2_per_cu_data>
+static dwarf2_per_cu_data_up
 create_cu_from_index_list (dwarf2_per_bfd *per_bfd,
 			   struct dwarf2_section_info *section,
 			   int is_dwz,
 			   sect_offset sect_off, ULONGEST length)
 {
-  std::unique_ptr<dwarf2_per_cu_data> the_cu = per_bfd->allocate_per_cu ();
+  dwarf2_per_cu_data_up the_cu = per_bfd->allocate_per_cu ();
   the_cu->sect_off = sect_off;
   the_cu->length = length;
   the_cu->section = section;
@@ -2580,7 +2591,7 @@ create_cus_from_index_list (dwarf2_per_bfd *per_bfd,
       ULONGEST length = extract_unsigned_integer (cu_list + 8, 8, BFD_ENDIAN_LITTLE);
       cu_list += 2 * 8;
 
-      std::unique_ptr<dwarf2_per_cu_data> per_cu
+      dwarf2_per_cu_data_up per_cu
 	= create_cu_from_index_list (per_bfd, section, is_dwz, sect_off,
 				     length);
       per_bfd->all_comp_units.push_back (std::move (per_cu));
@@ -2647,7 +2658,7 @@ create_signatured_type_table_from_index
       slot = htab_find_slot (sig_types_hash.get (), sig_type.get (), INSERT);
       *slot = sig_type.get ();
 
-      per_bfd->all_comp_units.push_back (std::move (sig_type));
+      per_bfd->all_comp_units.emplace_back (sig_type.release ());
     }
 
   per_bfd->signatured_types = std::move (sig_types_hash);
@@ -2699,7 +2710,7 @@ create_signatured_type_table_from_debug_names
       slot = htab_find_slot (sig_types_hash.get (), sig_type.get (), INSERT);
       *slot = sig_type.get ();
 
-      per_objfile->per_bfd->all_comp_units.push_back (std::move (sig_type));
+      per_objfile->per_bfd->all_comp_units.emplace_back (sig_type.release ());
     }
 
   per_objfile->per_bfd->signatured_types = std::move (sig_types_hash);
@@ -4930,7 +4941,7 @@ create_cus_from_debug_names_list (dwarf2_per_bfd *per_bfd,
 	     of the next CU as end of this CU.  We create the CUs here with
 	     length 0, and in cutu_reader::cutu_reader we'll fill in the
 	     actual length.  */
-	  std::unique_ptr<dwarf2_per_cu_data> per_cu
+	  dwarf2_per_cu_data_up per_cu
 	    = create_cu_from_index_list (per_bfd, &section, is_dwz,
 					 sect_off, 0);
 	  per_bfd->all_comp_units.push_back (std::move (per_cu));
@@ -4955,7 +4966,7 @@ create_cus_from_debug_names_list (dwarf2_per_bfd *per_bfd,
       if (i >= 1)
 	{
 	  const ULONGEST length = sect_off_next - sect_off_prev;
-	  std::unique_ptr<dwarf2_per_cu_data> per_cu
+	  dwarf2_per_cu_data_up per_cu
 	    = create_cu_from_index_list (per_bfd, &section, is_dwz,
 					 sect_off_prev, length);
 	  per_bfd->all_comp_units.push_back (std::move (per_cu));
@@ -6132,7 +6143,8 @@ add_type_unit (dwarf2_per_objfile *per_objfile, ULONGEST sig, void **slot)
 
   per_objfile->resize_symtabs ();
 
-  per_objfile->per_bfd->all_comp_units.push_back (std::move (sig_type_holder));
+  per_objfile->per_bfd->all_comp_units.emplace_back
+    (sig_type_holder.release ());
   sig_type->signature = sig;
   sig_type->is_debug_types = 1;
   if (per_objfile->per_bfd->using_index)
@@ -7711,7 +7723,7 @@ read_comp_units_from_section (dwarf2_per_objfile *per_objfile,
 
   while (info_ptr < section->buffer + section->size)
     {
-      std::unique_ptr<dwarf2_per_cu_data> this_cu;
+      dwarf2_per_cu_data_up this_cu;
 
       sect_offset sect_off = (sect_offset) (info_ptr - section->buffer);
 
@@ -7732,7 +7744,7 @@ read_comp_units_from_section (dwarf2_per_objfile *per_objfile,
 	  signatured_type *sig_ptr = sig_type.get ();
 	  sig_type->signature = cu_header.signature;
 	  sig_type->type_offset_in_tu = cu_header.type_cu_offset_in_tu;
-	  this_cu = std::move (sig_type);
+	  this_cu.reset (sig_type.release ());
 
 	  void **slot = htab_find_slot (types_htab.get (), sig_ptr, INSERT);
 	  gdb_assert (slot != nullptr);
@@ -24581,7 +24593,7 @@ static int
 dwarf2_find_containing_comp_unit
   (sect_offset sect_off,
    unsigned int offset_in_dwz,
-   const std::vector<std::unique_ptr<dwarf2_per_cu_data>> &all_comp_units)
+   const std::vector<dwarf2_per_cu_data_up> &all_comp_units)
 {
   int low, high;
 
@@ -24647,13 +24659,13 @@ namespace find_containing_comp_unit {
 static void
 run_test ()
 {
-  std::unique_ptr<dwarf2_per_cu_data> one (new dwarf2_per_cu_data);
+  dwarf2_per_cu_data_up one (new dwarf2_per_cu_data);
   dwarf2_per_cu_data *one_ptr = one.get ();
-  std::unique_ptr<dwarf2_per_cu_data> two (new dwarf2_per_cu_data);
+  dwarf2_per_cu_data_up two (new dwarf2_per_cu_data);
   dwarf2_per_cu_data *two_ptr = two.get ();
-  std::unique_ptr<dwarf2_per_cu_data> three (new dwarf2_per_cu_data);
+  dwarf2_per_cu_data_up three (new dwarf2_per_cu_data);
   dwarf2_per_cu_data *three_ptr = three.get ();
-  std::unique_ptr<dwarf2_per_cu_data> four (new dwarf2_per_cu_data);
+  dwarf2_per_cu_data_up four (new dwarf2_per_cu_data);
   dwarf2_per_cu_data *four_ptr = four.get ();
 
   one->length = 5;
@@ -24666,7 +24678,7 @@ run_test ()
   four->length = 7;
   four->is_dwz = 1;
 
-  std::vector<std::unique_ptr<dwarf2_per_cu_data>> units;
+  std::vector<dwarf2_per_cu_data_up> units;
   units.push_back (std::move (one));
   units.push_back (std::move (two));
   units.push_back (std::move (three));
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 80f0e3ee13b..756d1934ca0 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -74,6 +74,20 @@ struct dwarf2_queue_item
   enum language pretend_language;
 };
 
+/* A deleter for dwarf2_per_cu_data that knows to downcast to
+   signatured_type as appropriate.  This approach lets us avoid a
+   virtual destructor, which saves a bit of space.  */
+
+struct dwarf2_per_cu_data_deleter
+{
+  void operator() (dwarf2_per_cu_data *data);
+};
+
+/* A specialization of unique_ptr for dwarf2_per_cu_data and
+   subclasses.  */
+typedef std::unique_ptr<dwarf2_per_cu_data, dwarf2_per_cu_data_deleter>
+    dwarf2_per_cu_data_up;
+
 /* Some DWARF data can be shared across objfiles who share the same BFD,
    this data is stored in this object.
 
@@ -103,7 +117,7 @@ struct dwarf2_per_bfd
   /* A convenience function to allocate a dwarf2_per_cu_data.  The
      returned object has its "index" field set properly.  The object
      is allocated on the dwarf2_per_bfd obstack.  */
-  std::unique_ptr<dwarf2_per_cu_data> allocate_per_cu ();
+  dwarf2_per_cu_data_up allocate_per_cu ();
 
   /* A convenience function to allocate a signatured_type.  The
      returned object has its "index" field set properly.  The object
@@ -154,7 +168,7 @@ struct dwarf2_per_bfd
 
   /* Table of all the compilation units.  This is used to locate
      the target compilation unit of a particular reference.  */
-  std::vector<std::unique_ptr<dwarf2_per_cu_data>> all_comp_units;
+  std::vector<dwarf2_per_cu_data_up> all_comp_units;
 
   /* Table of struct type_unit_group objects.
      The hash key is the DW_AT_stmt_list value.  */