[PATCH 7/7] gdb: generate dwarf-5 index identically as worker-thread count changes

[Andrew Burgess <aburgess@redhat.com>]

Similar to the previous commit, this commit ensures that the dwarf-5
index files are generated identically as the number of worker-threads
changes.

Building the dwarf-5 index makes use of a closed hash table, the
bucket_hash local within debug_names::build().  Entries are added to
bucket_hash from m_name_to_value_set, which, in turn, is populated
by calls to debug_names::insert() in write_debug_names.  The insert
calls are ordered based on the entries within the cooked_index, and
the ordering within cooked_index depends on the number of worker
threads that GDB is using.

My proposal is to sort each chain within the bucket_hash closed hash
table prior to using this to build the dwarf-5 index.

The buckets within bucket_hash will always have the same ordering (for
a given GDB build with a given executable), and by sorting the chains
within each bucket, we can be sure that GDB will see each entry in a
deterministic order.

I've extended the index creation test to cover this case.
---
 gdb/dwarf2/index-write.c             | 17 +++++++++++++++--
 gdb/testsuite/gdb.gdb/index-file.exp |  8 +++++++-
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index bc07bcb8f4c..1bc0c8ab57e 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -454,6 +454,11 @@ class c_str_view
     return strcmp (m_cstr, other.m_cstr) == 0;
   }
 
+  bool operator< (const c_str_view &other) const
+  {
+    return strcmp (m_cstr, other.m_cstr) < 0;
+  }
+
   /* Return the underlying C string.  Note, the returned string is
      only a reference with lifetime of this object.  */
   const char *c_str () const
@@ -773,10 +778,18 @@ class debug_names
       }
     for (size_t bucket_ix = 0; bucket_ix < bucket_hash.size (); ++bucket_ix)
       {
-	const std::forward_list<hash_it_pair> &hashitlist
-	  = bucket_hash[bucket_ix];
+	std::forward_list<hash_it_pair> &hashitlist = bucket_hash[bucket_ix];
 	if (hashitlist.empty ())
 	  continue;
+
+	/* Sort the items within each bucket.  This ensures that the
+	   generated index files will be the same no matter the order in
+	   which symbols were added into the index.  */
+	hashitlist.sort ([] (const hash_it_pair &a, const hash_it_pair &b)
+	{
+	  return a.it->first < b.it->first;
+	});
+
 	uint32_t &bucket_slot = m_bucket_table[bucket_ix];
 	/* The hashes array is indexed starting at 1.  */
 	store_unsigned_integer (reinterpret_cast<gdb_byte *> (&bucket_slot),
diff --git a/gdb/testsuite/gdb.gdb/index-file.exp b/gdb/testsuite/gdb.gdb/index-file.exp
index 08415920061..7154d234dd5 100644
--- a/gdb/testsuite/gdb.gdb/index-file.exp
+++ b/gdb/testsuite/gdb.gdb/index-file.exp
@@ -44,6 +44,9 @@ remote_exec host "mkdir -p ${dir1}"
 with_timeout_factor $timeout_factor {
     gdb_test_no_output "save gdb-index $dir1" \
 	"create gdb-index file"
+
+    gdb_test_no_output "save gdb-index -dwarf-5 $dir1" \
+	"create dwarf-index files"
 }
 
 # Close GDB.
@@ -140,13 +143,16 @@ if { $worker_threads > 1 } {
     with_timeout_factor $timeout_factor {
 	gdb_test_no_output "save gdb-index $dir2" \
 	    "create second gdb-index file"
+
+	gdb_test_no_output "save gdb-index -dwarf-5 $dir2" \
+	    "create second dwarf-index files"
     }
 
     # Close GDB.
     gdb_exit
 
     # Now check that the index files are identical.
-    foreach suffix { gdb-index  } {
+    foreach suffix { gdb-index debug_names debug_str } {
 	set result \
 	    [remote_exec host \
 		 "cmp -s \"$dir1/${index_filename_base}.${suffix}\" \"$dir2/${index_filename_base}.${suffix}\""]
-- 
2.25.4