diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 4eeea5c12fb..32fe0bbe814 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2019-01-10  Tom Tromey  <tom@tromey.com>
+
+	* objfiles.h (objfile::reset_psymtabs): Update.
+	* objfiles.c (objfile::objfile): Update.
+	* psymtab.h (psymtab_storage::obstack): Update.
+	(psymtab_storage::m_obstack): Use gdb::optional.
+	(class psymtab_storage): Update comment.  Remove objfile
+	parameter.
+	* psymtab.c (psymtab_storage::psymtab_storage): Update.
+
 2019-01-10  Tom Tromey  <tom@tromey.com>
 
 	* psymtab.h (psymtab_storage::allocate_psymtab): New method.
diff --git a/gdb/objfiles.c b/gdb/objfiles.c
index 34154482124..255a3d636d1 100644
--- a/gdb/objfiles.c
+++ b/gdb/objfiles.c
@@ -369,7 +369,7 @@ build_objfile_section_table (struct objfile *objfile)
 objfile::objfile (bfd *abfd, const char *name, objfile_flags flags_)
   : flags (flags_),
     pspace (current_program_space),
-    partial_symtabs (new psymtab_storage (this)),
+    partial_symtabs (new psymtab_storage ()),
     obfd (abfd)
 {
   const char *expanded_name;
diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index cde025d9d91..f7d08c43b3b 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -298,7 +298,7 @@ struct objfile
   void reset_psymtabs ()
   {
     psymbol_map.clear ();
-    partial_symtabs.reset (new psymtab_storage (this));
+    partial_symtabs.reset (new psymtab_storage ());
   }
 
 
diff --git a/gdb/psymtab.c b/gdb/psymtab.c
index 7ead0eb81ad..e034fda3402 100644
--- a/gdb/psymtab.c
+++ b/gdb/psymtab.c
@@ -67,9 +67,8 @@ static struct compunit_symtab *psymtab_to_symtab (struct objfile *objfile,
 
 
 
-psymtab_storage::psymtab_storage (struct objfile *objfile)
-  : psymbol_cache (psymbol_bcache_init ()),
-    m_obstack (&objfile->objfile_obstack)
+psymtab_storage::psymtab_storage ()
+  : psymbol_cache (psymbol_bcache_init ())
 {
 }
 
diff --git a/gdb/psymtab.h b/gdb/psymtab.h
index a21e88fd0a0..26aeb29646e 100644
--- a/gdb/psymtab.h
+++ b/gdb/psymtab.h
@@ -31,13 +31,26 @@ struct partial_symbol;
 struct psymbol_bcache;
 
 /* An instance of this class manages the partial symbol tables and
-   partial symbols for a given objfile.  */
+   partial symbols for a given objfile.
+
+   The core psymtab functions -- those in psymtab.c -- arrange for
+   nearly all psymtab- and psymbol-related allocations to happen
+   either in the psymtab_storage object (either on its obstack or in
+   other memory managed by this class), or on the per-BFD object.  The
+   only link from the psymtab storage object back to the objfile (or
+   objfile_obstack) that is made by the core psymtab code is the
+   compunit_symtab member in the psymtab.
+
+   However, it is up to each symbol reader to maintain this invariant
+   in other ways, if it wants to reuse psymtabs across multiple
+   objfiles.  The main issue here is ensuring that read_symtab_private
+   does not point into objfile_obstack.  */
 
 class psymtab_storage
 {
 public:
 
-  explicit psymtab_storage (struct objfile *objfile);
+  psymtab_storage ();
 
   ~psymtab_storage ();
 
@@ -60,7 +73,9 @@ public:
 
   struct obstack *obstack ()
   {
-    return m_obstack;
+    if (!m_obstack.has_value ())
+      m_obstack.emplace ();
+    return &*m_obstack;
   }
 
   /* Allocate storage for the "dependencies" field of a psymtab.
@@ -108,9 +123,10 @@ private:
 
   struct partial_symtab *free_psymtabs = nullptr;
 
-  /* The obstack where allocations are made.  */
+  /* The obstack where allocations are made.  This is lazily allocated
+     so that we don't waste memory when there are no psymtabs.  */
 
-  struct obstack *m_obstack;
+  gdb::optional<auto_obstack> m_obstack;
 };