Specialize partial_symtab for DWARF include files

Include files are represented by a partial symtab, but don't expand to
anything.  From dwarf2_psymtab::expand_psymtab:

  if (per_cu == NULL)
    {
      /* It's an include file, no symbols to read for it.
         Everything is in the parent symtab.  */
      readin = true;
      return;
    }

This patch introduces a new specialization of partial_symtab to handle
this case.  In addition to being slightly smaller, I believe an
include file is the only situation where a DWARF psymtab can result in
a null compunit_symtab.  This adds an assert to that effect as well.
This change will simplify one of the psymtab sharing patches.

gdb/ChangeLog
2020-02-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (struct dwarf2_include_psymtab): New.
	(dwarf2_create_include_psymtab): Use dwarf2_include_psymtab.
	(dwarf2_psymtab::expand_psymtab, dwarf2_psymtab::readin_p)
	(dwarf2_psymtab::get_compunit_symtab): Remove null checks for
	per_cu_data.

2 files changed, 49 insertions, 19 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 2bb65e7..2f92b0a 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,13 @@
 2020-02-26  Tom Tromey  <tom@tromey.com>
 
+	* dwarf2/read.c (struct dwarf2_include_psymtab): New.
+	(dwarf2_create_include_psymtab): Use dwarf2_include_psymtab.
+	(dwarf2_psymtab::expand_psymtab, dwarf2_psymtab::readin_p)
+	(dwarf2_psymtab::get_compunit_symtab): Remove null checks for
+	per_cu_data.
+
+2020-02-26  Tom Tromey  <tom@tromey.com>
+
 	* dwarf2/index-write.c (psym_index_map): Change type.
 	(add_address_entry_worker, write_one_signatured_type)
 	(recursively_count_psymbols, recursively_write_psymbols)
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index d6b34d8..c5f0ffa 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -5918,6 +5918,44 @@ read_abbrev_offset (struct dwarf2_per_objfile *dwarf2_per_objfile,
   return (sect_offset) read_offset (abfd, info_ptr, offset_size);
 }
 
+/* A partial symtab that is used only for include files.  */
+struct dwarf2_include_psymtab : public partial_symtab
+{
+  dwarf2_include_psymtab (const char *filename, struct objfile *objfile)
+    : partial_symtab (filename, objfile)
+  {
+  }
+
+  void read_symtab (struct objfile *objfile) override
+  {
+    expand_psymtab (objfile);
+  }
+
+  void expand_psymtab (struct objfile *objfile) override
+  {
+    if (m_readin)
+      return;
+    /* It's an include file, no symbols to read for it.
+       Everything is in the parent symtab.  */
+    read_dependencies (objfile);
+    m_readin = true;
+  }
+
+  bool readin_p () const override
+  {
+    return m_readin;
+  }
+
+  struct compunit_symtab *get_compunit_symtab () const override
+  {
+    return nullptr;
+  }
+
+private:
+
+  bool m_readin = false;
+};
+
 /* Allocate a new partial symtab for file named NAME and mark this new
    partial symtab as being an include of PST.  */
 
@@ -5925,7 +5963,7 @@ static void
 dwarf2_create_include_psymtab (const char *name, dwarf2_psymtab *pst,
                                struct objfile *objfile)
 {
-  dwarf2_psymtab *subpst = new dwarf2_psymtab (name, objfile);
+  dwarf2_include_psymtab *subpst = new dwarf2_include_psymtab (name, objfile);
 
   if (!IS_ABSOLUTE_PATH (subpst->filename))
     {
@@ -5936,11 +5974,6 @@ dwarf2_create_include_psymtab (const char *name, dwarf2_psymtab *pst,
   subpst->dependencies = objfile->partial_symtabs->allocate_dependencies (1);
   subpst->dependencies[0] = pst;
   subpst->number_of_dependencies = 1;
-
-  /* No private part is necessary for include psymtabs.  This property
-     can be used to differentiate between such include psymtabs and
-     the regular ones.  */
-  subpst->per_cu_data = nullptr;
 }
 
 /* Read the Line Number Program data and extract the list of files
@@ -8851,24 +8884,13 @@ process_queue (struct dwarf2_per_objfile *dwarf2_per_objfile)
 void
 dwarf2_psymtab::expand_psymtab (struct objfile *objfile)
 {
-  struct dwarf2_per_cu_data *per_cu;
-
   if (readin)
     return;
 
   read_dependencies (objfile);
 
-  per_cu = per_cu_data;
-
-  if (per_cu == NULL)
-    {
-      /* It's an include file, no symbols to read for it.
-         Everything is in the parent symtab.  */
-      readin = true;
-      return;
-    }
-
-  dw2_do_instantiate_symtab (per_cu, false);
+  dw2_do_instantiate_symtab (per_cu_data, false);
+  gdb_assert (get_compunit_symtab () != nullptr);
 }
 
 /* Trivial hash function for die_info: the hash value of a DIE