[PATCH 14/14] Share DWARF partial symtabs

[Tom Tromey <tom at tromey dot com>]

This changes the DWARF reader to share partial symtabs (or indices if
they are available) across objfiles.  This has a few parts.

* The dwarf2_unshareable object is created per-objfile and is stored
  on the objfile using a registry key.  dwarf2_enter_objfile is
  modified to save and restore this pointer in the dwarf2_per_objfile.

* objfile::partial_symtabs is changed to be a shared_ptr again.  This
  lets us stash a second reference in dwarf2_per_objfile; if the DWARF
  data is being shared, we can simply copy this value to the new
  objfile.

* The dwarf2_per_objfile itself may be shared via the BFD -- using a
  new per-BFD registry key -- or not.  This depends both on whether
  some other symbol reader has found symbols, and whether any BFD
  sections require relocation.

2020-02-15  Tom Tromey  <tom@tromey.com>

	* objfiles.h (struct objfile) <partial_symtabs>: Now a
	shared_ptr.
	* dwarf2/read.h (struct dwarf2_per_objfile) <partial_symtabs>: New
	member.
	(class dwarf2_enter_objfile): Constructor no longer inline.
	<m_restore_unshared>: New member.
	* dwarf2/read.c (dwarf2_unshared_data_key, dwarf2_bfd_data_key):
	New globals.
	(dwarf2_objfile_data_key): Add comment.
	(get_dwarf2_per_objfile): Rewrite.
	(dwarf2_enter_objfile::dwarf2_enter_objfile): Move from read.h.
	Initialize m_restore_unshared.
	(dwarf2_per_objfile::dwarf2_per_objfile): Add "unshared"
	parameter.  Don't initialize objfile member.
	(dwarf2_per_objfile::~dwarf2_per_objfile): Don't deleted
	"unshareable".
	(dwarf2_has_info): Check dwarf2_bfd_data_key and
	dwarf2_unshared_data_key.
	(dwarf2_get_section_info): Use get_dwarf2_per_objfile.
	(dwarf2_build_psymtabs): Set objfile::partial_symtabs and
	short-circuit when sharing.
	(dwarf2_build_psymtabs): Set dwarf2_per_objfile::partial_symtabs.
	(dwarf2_psymtab::expand_psymtab): Use free_cached_comp_units.
---
 gdb/ChangeLog     | 26 ++++++++++++++++++
 gdb/dwarf2/read.c | 70 +++++++++++++++++++++++++++++++++++++----------
 gdb/dwarf2/read.h | 12 ++++----
 gdb/objfiles.h    |  2 +-
 4 files changed, 89 insertions(+), 21 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 15813b72005..7be57661296 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -100,7 +100,15 @@ static bool check_physname = false;
 /* When true, do not reject deprecated .gdb_index sections.  */
 static bool use_deprecated_index_sections = false;
 
-static const struct objfile_key<dwarf2_per_objfile> dwarf2_objfile_data_key;
+/* This is used to store the data that is always per objfile.  */
+static const objfile_key<dwarf2_unshareable> dwarf2_unshared_data_key;
+
+/* These are used to store the dwarf2_per_objfile; either on the
+   objfile or on the BFD.  */
+static const struct objfile_key<dwarf2_per_objfile>
+  dwarf2_objfile_data_key;
+static const struct bfd_key<dwarf2_per_objfile>
+  dwarf2_bfd_data_key;
 
 /* The "aclass" indices for various kinds of computed DWARF symbols.  */
 
@@ -274,7 +282,18 @@ struct mapped_debug_names final : public mapped_index_base
 dwarf2_per_objfile *
 get_dwarf2_per_objfile (struct objfile *objfile)
 {
-  return dwarf2_objfile_data_key.get (objfile);
+  dwarf2_per_objfile *result = dwarf2_bfd_data_key.get (objfile->obfd);
+  if (result == nullptr)
+    result = dwarf2_objfile_data_key.get (objfile);
+  return result;
+}
+
+dwarf2_enter_objfile::dwarf2_enter_objfile (struct objfile *objfile)
+  : m_per_objfile (get_dwarf2_per_objfile (objfile)),
+    m_restore_objfile (&m_per_objfile->objfile, objfile),
+    m_restore_unshared (&m_per_objfile->unshareable,
+			dwarf2_unshared_data_key.get (objfile))
+{
 }
 
 /* Default names of the debugging sections.  */
@@ -1754,16 +1773,12 @@ line_header_eq_voidp (const void *item_lhs, const void *item_rhs)
 dwarf2_per_objfile::dwarf2_per_objfile (struct objfile *objfile_,
 					const dwarf2_debug_sections *names,
 					bool can_copy_)
-  : objfile (objfile_),
-    can_copy (can_copy_),
-    /* Temporarily just allocate one per objfile -- we don't have
-       sharing yet.  */
-    unshareable (new dwarf2_unshareable)
+  : can_copy (can_copy_)
 {
   if (names == NULL)
     names = &dwarf2_elf_names;
 
-  bfd *obfd = objfile->obfd;
+  bfd *obfd = objfile_->obfd;
 
   for (asection *sec = obfd->sections; sec != NULL; sec = sec->next)
     locate_sections (obfd, sec, *names);
@@ -1780,8 +1795,6 @@ dwarf2_per_objfile::~dwarf2_per_objfile ()
   for (signatured_type *sig_type : all_type_units)
     sig_type->per_cu.imported_symtabs_free ();
 
-  delete unshareable;
-
   /* Everything else should be on the objfile obstack.  */
 }
 
@@ -1841,13 +1854,26 @@ dwarf2_has_info (struct objfile *objfile,
   if (objfile->flags & OBJF_READNEVER)
     return 0;
 
+  struct dwarf2_unshareable *unshared = dwarf2_unshared_data_key.get (objfile);
+  if (unshared == nullptr)
+    unshared = dwarf2_unshared_data_key.emplace (objfile);
+
   struct dwarf2_per_objfile *dwarf2_per_objfile
     = get_dwarf2_per_objfile (objfile);
 
   if (dwarf2_per_objfile == NULL)
-    dwarf2_per_objfile = dwarf2_objfile_data_key.emplace (objfile, objfile,
-							  names,
-							  can_copy);
+    {
+      dwarf2_per_objfile = new ::dwarf2_per_objfile (objfile, names,
+						     can_copy);
+      /* We can share this if the objfile doesn't require relocations
+	 and if there aren't partial symbols from some other
+	 reader.  */
+      if (!objfile_has_partial_symbols (objfile)
+	  && !gdb_bfd_requires_relocations (objfile->obfd))
+	dwarf2_bfd_data_key.set (objfile->obfd, dwarf2_per_objfile);
+      else
+	dwarf2_objfile_data_key.set (objfile, dwarf2_per_objfile);
+    }
 
   return (!dwarf2_per_objfile->info.is_virtual
 	  && dwarf2_per_objfile->info.s.section != NULL
@@ -2006,7 +2032,7 @@ dwarf2_get_section_info (struct objfile *objfile,
                          asection **sectp, const gdb_byte **bufp,
                          bfd_size_type *sizep)
 {
-  struct dwarf2_per_objfile *data = dwarf2_objfile_data_key.get (objfile);
+  struct dwarf2_per_objfile *data = get_dwarf2_per_objfile (objfile);
   struct dwarf2_section_info *info;
 
   /* We may see an objfile without any DWARF, in which case we just
@@ -5862,6 +5888,15 @@ dwarf2_build_psymtabs (struct objfile *objfile)
   struct dwarf2_per_objfile *dwarf2_per_objfile
     = get_dwarf2_per_objfile (objfile);
 
+  if (dwarf2_per_objfile->partial_symtabs != nullptr)
+    {
+      /* Partial symbols were already read, so now we can simply
+	 attach them.  */
+      objfile->partial_symtabs = dwarf2_per_objfile->partial_symtabs;
+      dwarf2_resize_unshareable (dwarf2_per_objfile);
+      return;
+    }
+
   init_psymbol_list (objfile, 1024);
 
   try
@@ -5882,6 +5917,11 @@ dwarf2_build_psymtabs (struct objfile *objfile)
     {
       exception_print (gdb_stderr, except);
     }
+
+  /* Finish by setting the local reference to partial symtabs, so that
+     we don't try to read again.  If we can't in fact share, this
+     won't make a difference anyway.  */
+  dwarf2_per_objfile->partial_symtabs = objfile->partial_symtabs;
 }
 
 /* Find the base address of the compilation unit for range lists and
@@ -8915,8 +8955,8 @@ dwarf2_psymtab::expand_psymtab (struct objfile *objfile)
   if (symtab.has_value ())
     return;
 
+  free_cached_comp_units freer (dwarf2_per_objfile);
   read_dependencies (objfile);
-
   dw2_do_instantiate_symtab (per_cu_data, false);
 }
 
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 241f3c8e71e..891a844a673 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -299,6 +299,11 @@ public:
   /* CUs that are queued to be read.  */
   std::queue<dwarf2_queue_item> queue;
 
+  /* We keep a separate reference to the partial symtabs, in case we
+     are sharing them between objfiles.  This is only set after
+     partial symbols have been read the first time.  */
+  std::shared_ptr<psymtab_storage> partial_symtabs;
+
   /* The total number of per_cu and signatured_type objects that have
      been created for this reader.  */
   size_t num_psymtabs = 0;
@@ -321,11 +326,7 @@ class dwarf2_enter_objfile
 {
 public:
 
-  dwarf2_enter_objfile (struct objfile *objfile)
-    : m_per_objfile (get_dwarf2_per_objfile (objfile)),
-      m_restore_objfile (&m_per_objfile->objfile, objfile)
-  {
-  }
+  dwarf2_enter_objfile (struct objfile *objfile);
 
   ~dwarf2_enter_objfile () = default;
 
@@ -335,6 +336,7 @@ private:
 
   dwarf2_per_objfile *m_per_objfile;
   scoped_restore_tmpl<struct objfile *> m_restore_objfile;
+  scoped_restore_tmpl<dwarf2_unshareable *> m_restore_unshared;
 };
 
 /* A partial symtab specialized for DWARF.  */
diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index b71a8a9edb8..12892f80f71 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -558,7 +558,7 @@ public:
 
   /* The partial symbol tables.  */
 
-  std::unique_ptr<psymtab_storage> partial_symtabs;
+  std::shared_ptr<psymtab_storage> partial_symtabs;
 
   /* The object file's BFD.  Can be null if the objfile contains only
      minimal symbols, e.g. the run time common symbols for SunOS4.  */
-- 
2.17.2