[PATCH v2 1/7] gdb: implement readnow_functions::find_pc_sect_compunit_symtab

[Jan Vrany <jan.vrany@labware.com>]

This commit implements readnow_functions::find_pc_sect_compunit_symtab.
As comment in read.h states, index_table member dwarf2_per_bfd is null
when -readnow is used and thus this method would always return null
when using -readnow.

This issue did not manifest until now because:

 1) readnow_functions::find_pc_sect_compunit_symtab is called from
    find_compunit_symtab_for_pc_sect *after* all CUs expanded so far
    are searched and,
 2) it happens that required CUs were expanded by other "quick function"
    called prior find_pc_sect_compunit_symtab.

This is a preparation for simplifying find_compunit_symtab_for_pc_sect
by removing the code that walks all existing CUs.

In order to handle interleaved CUs with "holes" this implementation walks
over all CUs and looks for a CU that contains the "closest" (smallest)
block that contain given PC. An example of such a case is test
dw2-entry-points.exp.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33829
---
 gdb/dwarf2/read.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index a6244319f71..f7f39d75c16 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -1525,6 +1525,10 @@ struct readnow_functions : public dwarf2_base_index_functions
     return true;
   }
 
+  struct compunit_symtab *find_pc_sect_compunit_symtab (
+    struct objfile *objfile, bound_minimal_symbol msymbol, CORE_ADDR pc,
+    struct obj_section *section, int warn_if_readin) override;
+
   struct symbol *find_symbol_by_address (struct objfile *objfile,
 					 CORE_ADDR address) override
   {
@@ -2166,6 +2170,57 @@ dwarf2_base_index_functions::find_pc_sect_compunit_symtab
   return result;
 }
 
+struct compunit_symtab *
+readnow_functions::find_pc_sect_compunit_symtab
+     (struct objfile *objfile,
+      bound_minimal_symbol msymbol,
+      CORE_ADDR pc,
+      struct obj_section *section,
+      int warn_if_readin)
+{
+  dwarf2_per_objfile *per_objfile = get_dwarf2_per_objfile (objfile);
+  dwarf2_per_bfd *per_bfd = per_objfile->per_bfd;
+
+  /* This invariant is documented in read.h  */
+  gdb_assert (per_bfd->index_table == nullptr);
+
+  /* Since we have no index, we have to walk all CUs and find one with the
+     "closest" block that contains PC.  For the purpose of this method,
+     a block B1 is considerent closer matching than block B2 if B1 is not null
+     and B2 is either null or has larger total span than B1.  In other words,
+     smaller blocks are "closer".  */
+
+  auto closer = [&] (const block *b1, const block *b2)
+    {
+      if (b1 == nullptr)
+	return false;
+      if (b2 == nullptr)
+	return true;
+      return (b1->end () - b1->start ()) < (b2->end () - b2->start ());
+    };
+
+  compunit_symtab *best_cu = nullptr;
+  const block *best_bl = nullptr;
+
+  for (int i = 0; i < per_bfd->all_units.size (); i++)
+    {
+      dwarf2_per_cu *data = per_bfd->all_units[i].get ();
+
+      if (per_objfile->symtab_set_p (data))
+	{
+	  compunit_symtab *this_cu = per_objfile->get_symtab (data);
+	  const block *this_bl = this_cu->blockvector ()->lookup (pc);
+
+	  if (closer (this_bl, best_bl))
+	    {
+	      best_cu = this_cu;
+	      best_bl = this_bl;
+	    }
+	}
+    }
+  return best_cu;
+}
+
 void
 dwarf2_base_index_functions::map_symbol_filenames (objfile *objfile,
 						   symbol_filename_listener fun,
-- 
2.51.0