[PATCH 0/3] Make allocate_on_obstack a bit safer

[PATCH 0/3] Make allocate_on_obstack a bit safer

[Tom Tromey]

This series makes the allocate_on_obstack mixin a little safer by
having it enforce the rule that an object allocated on an obstack must
have a trivial destructor.

---
Tom Tromey (3):
      Use addrmap_fixed in a few spots
      Don't use virtual destructor in addrmap
      Require trivial destructor in allocate_on_obstack

 gdb/addrmap.h             |  7 ++++---
 gdb/block.c               |  2 +-
 gdb/block.h               | 12 ++++++------
 gdb/dwarf2/cooked-index.h |  4 ++--
 gdb/dwarf2/read.h         |  2 +-
 gdb/gdbtypes.h            |  4 ++--
 gdb/symtab.h              |  2 +-
 gdbsupport/gdb_obstack.h  |  6 +++++-
 8 files changed, 22 insertions(+), 17 deletions(-)
---
base-commit: 407ca654547b100903f7eab44d078a2440736f13
change-id: 20240227-obstac-alloc-deabe0f935c3

Best regards,
-- 
Tom Tromey <tromey@adacore.com>

[PATCH 1/3] Use addrmap_fixed in a few spots

[Tom Tromey]

There are a few spots in the tree that use 'addrmap' where only an
addrmap_fixed will ever really be seen.  This patch changes this code
to use the more specific type.
---
 gdb/block.h               | 10 +++++-----
 gdb/dwarf2/cooked-index.h |  2 +-
 gdb/dwarf2/read.h         |  2 +-
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/gdb/block.h b/gdb/block.h
index 00630123120..4c29f6599ef 100644
--- a/gdb/block.h
+++ b/gdb/block.h
@@ -30,7 +30,7 @@ struct compunit_symtab;
 struct block_namespace_info;
 struct using_direct;
 struct obstack;
-struct addrmap;
+struct addrmap_fixed;
 
 /* Blocks can occupy non-contiguous address ranges.  When this occurs,
    startaddr and endaddr within struct block (still) specify the lowest
@@ -410,22 +410,22 @@ struct blockvector
   { return this->block (STATIC_BLOCK); }
 
   /* Return the address -> block map of this blockvector.  */
-  addrmap *map ()
+  addrmap_fixed *map ()
   { return m_map; }
 
   /* Const version of the above.  */
-  const addrmap *map () const
+  const addrmap_fixed *map () const
   { return m_map; }
 
   /* Set this blockvector's address -> block map.  */
-  void set_map (addrmap *map)
+  void set_map (addrmap_fixed *map)
   { m_map = map; }
 
 private:
   /* An address map mapping addresses to blocks in this blockvector.
      This pointer is zero if the blocks' start and end addresses are
      enough.  */
-  struct addrmap *m_map;
+  addrmap_fixed *m_map;
 
   /* Number of blocks in the list.  */
   int m_num_blocks;
diff --git a/gdb/dwarf2/cooked-index.h b/gdb/dwarf2/cooked-index.h
index 629a5b6b9ee..928c4ef655c 100644
--- a/gdb/dwarf2/cooked-index.h
+++ b/gdb/dwarf2/cooked-index.h
@@ -362,7 +362,7 @@ class cooked_index_shard
   cooked_index_entry *m_main = nullptr;
   /* The addrmap.  This maps address ranges to dwarf2_per_cu_data
      objects.  */
-  addrmap *m_addrmap = nullptr;
+  addrmap_fixed *m_addrmap = nullptr;
   /* Storage for canonical names.  */
   std::vector<gdb::unique_xmalloc_ptr<char>> m_names;
 };
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 5ee7e09a743..73def88c4c0 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -536,7 +536,7 @@ struct dwarf2_per_bfd
     abstract_to_concrete;
 
   /* The address map that is used by the DWARF index code.  */
-  struct addrmap *index_addrmap = nullptr;
+  addrmap_fixed *index_addrmap = nullptr;
 };
 
 /* An iterator for all_units that is based on index.  This

-- 
2.43.0

[PATCH 2/3] Don't use virtual destructor in addrmap

[Tom Tromey]

The addrmap polymorphism is sort of "phony" in that there isn't really
code in the tree that can be presented with either type.  I haven't
tried to fix this (though perhaps I may); but meanwhile it's handy for
the next patch if addrmap_fixed has a trivial destructor.  This patch
achieves this by making the addrmap destructor non-virtual, and also
making it protected so that objects of any of these types cannot be
destroyed when only the base class is known.
---
 gdb/addrmap.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gdb/addrmap.h b/gdb/addrmap.h
index ba83607ad8c..0d61046cebd 100644
--- a/gdb/addrmap.h
+++ b/gdb/addrmap.h
@@ -44,8 +44,6 @@ using addrmap_foreach_const_fn
 /* The base class for addrmaps.  */
 struct addrmap
 {
-  virtual ~addrmap () = default;
-
   /* Return the object associated with ADDR in MAP.  */
   const void *find (CORE_ADDR addr) const
   { return this->do_find (addr); }
@@ -68,6 +66,9 @@ struct addrmap
   { return this->do_foreach (fn); }
 
 
+protected:
+  ~addrmap () = default;
+
 private:
   /* Worker for find, implemented by sub-classes.  */
   virtual void *do_find (CORE_ADDR addr) const = 0;

-- 
2.43.0

[PATCH 3/3] Require trivial destructor in allocate_on_obstack

[Tom Tromey]

This patch makes allocate_on_obstack a little bit safer, by enforcing
the rule that objects allocated on an obstack must have a trivial
destructor.

The static assert is done in a method -- doing it inside the class
itself won't work because the class is incomplete at that point.
---
 gdb/addrmap.h             | 2 +-
 gdb/block.c               | 2 +-
 gdb/block.h               | 2 +-
 gdb/dwarf2/cooked-index.h | 2 +-
 gdb/gdbtypes.h            | 4 ++--
 gdb/symtab.h              | 2 +-
 gdbsupport/gdb_obstack.h  | 6 +++++-
 7 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/gdb/addrmap.h b/gdb/addrmap.h
index 0d61046cebd..55dea36b877 100644
--- a/gdb/addrmap.h
+++ b/gdb/addrmap.h
@@ -81,7 +81,7 @@ struct addrmap_mutable;
 
 /* Fixed address maps.  */
 struct addrmap_fixed : public addrmap,
-		       public allocate_on_obstack
+		       public allocate_on_obstack<addrmap_fixed>
 {
 public:
 
diff --git a/gdb/block.c b/gdb/block.c
index 079053cd79e..6d0d33fa85e 100644
--- a/gdb/block.c
+++ b/gdb/block.c
@@ -31,7 +31,7 @@
    C++ files, namely using declarations and the current namespace in
    scope.  */
 
-struct block_namespace_info : public allocate_on_obstack
+struct block_namespace_info : public allocate_on_obstack<block_namespace_info>
 {
   const char *scope = nullptr;
   struct using_direct *using_decl = nullptr;
diff --git a/gdb/block.h b/gdb/block.h
index 4c29f6599ef..ae676c4ba67 100644
--- a/gdb/block.h
+++ b/gdb/block.h
@@ -105,7 +105,7 @@ struct blockranges
    This implies that within the body of one function
    the blocks appear in the order of a depth-first tree walk.  */
 
-struct block : public allocate_on_obstack
+struct block : public allocate_on_obstack<block>
 {
   /* Return this block's start address.  */
   CORE_ADDR start () const
diff --git a/gdb/dwarf2/cooked-index.h b/gdb/dwarf2/cooked-index.h
index 928c4ef655c..0d4fcb8538e 100644
--- a/gdb/dwarf2/cooked-index.h
+++ b/gdb/dwarf2/cooked-index.h
@@ -104,7 +104,7 @@ extern bool language_requires_canonicalization (enum language lang);
    This is an "open" class and the members are all directly
    accessible.  It is read-only after the index has been fully read
    and processed.  */
-struct cooked_index_entry : public allocate_on_obstack
+struct cooked_index_entry : public allocate_on_obstack<cooked_index_entry>
 {
   cooked_index_entry (sect_offset die_offset_, enum dwarf_tag tag_,
 		      cooked_index_flag flags_,
diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index 435a040544a..f80bd7e071a 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -211,7 +211,7 @@ struct variant_part;
    control other variant parts as well.  This struct corresponds to
    DW_TAG_variant in DWARF.  */
 
-struct variant : allocate_on_obstack
+struct variant : allocate_on_obstack<variant>
 {
   /* * The discriminant ranges for this variant.  */
   gdb::array_view<discriminant_range> discriminants;
@@ -243,7 +243,7 @@ struct variant : allocate_on_obstack
    and holds an array of variants.  This struct corresponds to
    DW_TAG_variant_part in DWARF.  */
 
-struct variant_part : allocate_on_obstack
+struct variant_part : allocate_on_obstack<variant_part>
 {
   /* * The index of the discriminant field in the outer type.  This is
      an index into the type's array of fields.  If this is -1, there
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 5bd63979132..d49b7066e3a 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1244,7 +1244,7 @@ extern gdb::array_view<const struct symbol_impl> symbol_impls;
 
 /* This structure is space critical.  See space comments at the top.  */
 
-struct symbol : public general_symbol_info, public allocate_on_obstack
+struct symbol : public general_symbol_info, public allocate_on_obstack<symbol>
 {
   symbol ()
     /* Class-initialization of bitfields is only allowed in C++20.  */
diff --git a/gdbsupport/gdb_obstack.h b/gdbsupport/gdb_obstack.h
index 20f888660f9..7b3bb05bc00 100644
--- a/gdbsupport/gdb_obstack.h
+++ b/gdbsupport/gdb_obstack.h
@@ -133,14 +133,18 @@ struct auto_obstack : obstack
   { obstack_free (this, obstack_base (this)); }
 };
 
-/* Objects are allocated on obstack instead of heap.  */
+/* Objects are allocated on obstack instead of heap.  This is a mixin
+   that uses CRTP to ensure that the type in question is trivially
+   destructible.  */
 
+template<typename T>
 struct allocate_on_obstack
 {
   allocate_on_obstack () = default;
 
   void* operator new (size_t size, struct obstack *obstack)
   {
+    static_assert (IsFreeable<T>::value);
     return obstack_alloc (obstack, size);
   }
 

-- 
2.43.0

Re: [PATCH 1/3] Use addrmap_fixed in a few spots

[Alexandra Petlanova Hajkova]

[-- Attachment #1: Type: text/plain, Size: 561 bytes --]

On Tue, Feb 27, 2024 at 10:42 PM Tom Tromey <tromey@adacore.com> wrote:

> There are a few spots in the tree that use 'addrmap' where only an
> addrmap_fixed will ever really be seen.  This patch changes this code
> to use the more specific type.
> ---
>  gdb/block.h               | 10 +++++-----
>  gdb/dwarf2/cooked-index.h |  2 +-
>  gdb/dwarf2/read.h         |  2 +-
>  3 files changed, 7 insertions(+), 7 deletions(-)
>
>
>
A reasonable change. I can confirm this change does not introduce any
regressions on aarch64 with Fedora Rawhide.

Re: [PATCH 2/3] Don't use virtual destructor in addrmap

[Alexandra Petlanova Hajkova]

[-- Attachment #1: Type: text/plain, Size: 702 bytes --]

On Tue, Feb 27, 2024 at 10:43 PM Tom Tromey <tromey@adacore.com> wrote:

> The addrmap polymorphism is sort of "phony" in that there isn't really
> code in the tree that can be presented with either type.  I haven't
> tried to fix this (though perhaps I may); but meanwhile it's handy for
> the next patch if addrmap_fixed has a trivial destructor.  This patch
> achieves this by making the addrmap destructor non-virtual, and also
> making it protected so that objects of any of these types cannot be
> destroyed when only the base class is known.
> ---
>
I think it's a reasonable change and I can confirm this change does not
introduce any regressions on aarch64 with Fedora Rawhide.

Re: [PATCH 3/3] Require trivial destructor in allocate_on_obstack

[Alexandra Petlanova Hajkova]

[-- Attachment #1: Type: text/plain, Size: 487 bytes --]

On Tue, Feb 27, 2024 at 10:43 PM Tom Tromey <tromey@adacore.com> wrote:

> This patch makes allocate_on_obstack a little bit safer, by enforcing
> the rule that objects allocated on an obstack must have a trivial
> destructor.
>
> The static assert is done in a method -- doing it inside the class
> itself won't work because the class is incomplete at that point.
> ---
>
> Looks great. I can confirm this change does not cause any regressions for
aarch64 Fedora Rawhide.

Re: [PATCH 0/3] Make allocate_on_obstack a bit safer

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tromey@adacore.com> writes:

Tom> This series makes the allocate_on_obstack mixin a little safer by
Tom> having it enforce the rule that an object allocated on an obstack must
Tom> have a trivial destructor.

I'm checking this in.

Tom