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Re: [PATCH 14/14] Share DWARF partial symtabs


On 2/15/20 1:54 PM, Tom Tromey wrote:
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".

"Don't delete..."

	(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);
  }

Spurious line removal, but it doesn't matter much.

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.  */


I've gone through the series and didn't see anything obviously wrong. I suppose this will get more thoroughly exercised when running in multi-process/multi-target mode with quite a few objfiles and libraries.

Is it currently possible to exercise such a scenario automatically? I suppose manually loading something like a web browser will be a good test as well.


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