gdb/dwarf: assume that source_cu->dies is always set in follow_die_offset

After staring at the code for a while, I got convinced that it's not
possible for cu->dies to be nullptr in follow_die_offset.  It might be a
leftover from the psymtab days.

In most cases, we see that the dwarf2_cu passedas `*ref_cu` has been
obtained by doing:

    per_objfile->get_cu (per_cu);

The only way for a dwarf2_cu to end up in the per_objfile like this is
through load_full_comp_unit or read_signatured_type.  Both of these
functions call `reader.read_all_dies ()` (which loads the DIEs in memory
and assigns dwarf2_cu::dies) before transferring the newly created
dwarf2_cu to the per_objfile.  So any dwarf2_cu obtained through

   per_objfile->get_cu (per_cu)

... will have its DIEs set.

The only case today I'm aware of of a dwarf2_cu without DIEs is in the
cooked indexer.  It creates a cutu_reader, but does not call
read_all_dies.  Instead, it gets the info_ptr from the cutu_reader and
reads the DIEs from the section buffer directly, on its own.  But this
is an entirely different code path that doesn't assign dwarf2_cu
objects to per_objfile.

So, remove the code path in follow_die_offset that tests for
`source_cu->dies == NULL`.  I added an assert at the top of the function
to verify that `source_cu->dies` is always non-nullptr, as a way to
test my hypothesis.  We could probably get rid of it, but I left it
there because it doesn't cost much to have it.

Change-Id: I97f269f092128800850aa5e64eda7032c2edec60
Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 2 insertions, 7 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index f42253b..649fe69 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -18482,7 +18482,8 @@ follow_die_offset (sect_offset sect_off, int offset_in_dwz,
   dwarf2_cu *target_cu = source_cu;
   dwarf2_per_objfile *per_objfile = source_cu->per_objfile;
 
-  gdb_assert (source_cu->per_cu != NULL);
+  gdb_assert (source_cu->per_cu != nullptr);
+  gdb_assert (source_cu->dies != nullptr);
 
   dwarf_read_debug_printf_v ("source CU offset: %s, target offset: %s, "
 			     "source CU contains target offset: %d",
@@ -18529,12 +18530,6 @@ follow_die_offset (sect_offset sect_off, int offset_in_dwz,
 	       sect_offset_str (sect_off),
 	       objfile_name (per_objfile->objfile));
     }
-  else if (source_cu->dies == NULL)
-    {
-      /* We're loading full DIEs during partial symbol reading.  */
-      load_full_comp_unit (source_cu->per_cu, per_objfile, source_cu, false,
-			   language_minimal);
-    }
 
   *ref_cu = target_cu;