linemap_line_start: protect against location_t overflow (PR lto/88147)

PR lto/88147 reports an assertion failure due to a bogus location_t value
when adding a line to a pre-existing line map, when there's a large
difference between the two line numbers.

For some "large differences", this leads to a location_t value that exceeds
LINE_MAP_MAX_LOCATION, in which case linemap_line_start returns 0.  This
isn't ideal, but at least should lead to safe degradation of location
information.

However, if the difference is very large, it's possible for the line
number offset (relative to the start of the map) to be sufficiently large
that overflow occurs when left-shifted by the column-bits, and hence
the check against the LINE_MAP_MAX_LOCATION limit fails, leading to
a seemingly-valid location_t value, but encoding the wrong location.  This
triggers the assertion failure:
  linemap_assert (SOURCE_LINE (map, r) == to_line);

The fix (thanks to Martin) is to check for overflow when determining
whether to reuse an existing map, and to not reuse it if it would occur.

gcc/ChangeLog: David Malcolm  <dmalcolm@redhat.com>
	PR lto/88147
	* input.c (selftest::test_line_offset_overflow): New selftest.
	(selftest::input_c_tests): Call it.

libcpp/ChangeLog: Martin Liska  <mliska@suse.cz>
	PR lto/88147
	* line-map.c (linemap_line_start): Don't reuse the existing line
	map if the line offset is sufficiently large to cause overflow
	when computing location_t values.

From-SVN: r268789

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2019-02-11  David Malcolm  <dmalcolm@redhat.com>
+
+	PR lto/88147
+	* input.c (selftest::test_line_offset_overflow): New selftest.
+	(selftest::input_c_tests): Call it.
+
 2019-02-11  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/88771

gcc/input.c

@@ -3557,6 +3557,34 @@ for_each_line_table_case (void (*testcase) (const line_table_case &))
   ASSERT_EQ (num_cases_tested, 2 * 12);
 }
 
+/* Verify that when presented with a consecutive pair of locations with
+   a very large line offset, we don't attempt to consolidate them into
+   a single ordinary linemap where the line offsets within the line map
+   would lead to overflow (PR lto/88147).  */
+
+static void
+test_line_offset_overflow ()
+{
+  line_table_test ltt (line_table_case (5, 0));
+
+  linemap_add (line_table, LC_ENTER, false, "foo.c", 0);
+  linemap_line_start (line_table, 1, 100);
+  location_t loc_a = linemap_line_start (line_table, 2578, 255);
+  assert_loceq ("foo.c", 2578, 0, loc_a);
+
+  const line_map_ordinary *ordmap_a = LINEMAPS_LAST_ORDINARY_MAP (line_table);
+  ASSERT_EQ (ordmap_a->m_column_and_range_bits, 13);
+  ASSERT_EQ (ordmap_a->m_range_bits, 5);
+
+  location_t loc_b = linemap_line_start (line_table, 404198, 512);
+  assert_loceq ("foo.c", 404198, 0, loc_b);
+
+  /* We should have started a new linemap, rather than attempting to store
+     a very large line offset.  */
+  const line_map_ordinary *ordmap_b = LINEMAPS_LAST_ORDINARY_MAP (line_table);
+  ASSERT_NE (ordmap_a, ordmap_b);
+}
+
 /* Run all of the selftests within this file.  */
 
 void
@@ -3596,6 +3624,8 @@ input_c_tests ()
   for_each_line_table_case (test_lexer_char_constants);
 
   test_reading_source_line ();
+
+  test_line_offset_overflow ();
 }
 
 } // namespace selftest

libcpp/ChangeLog

@@ -1,3 +1,10 @@
+2019-02-11  Martin Liska  <mliska@suse.cz>
+
+	PR lto/88147
+	* line-map.c (linemap_line_start): Don't reuse the existing line
+	map if the line offset is sufficiently large to cause overflow
+	when computing location_t values.
+
 2019-01-26  Jakub Jelinek  <jakub@redhat.com>
 
 	PR preprocessor/88974

libcpp/line-map.c

@@ -742,6 +742,10 @@ linemap_line_start (struct line_maps *set, linenum_type to_line,
       if (line_delta < 0
 	  || last_line != ORDINARY_MAP_STARTING_LINE_NUMBER (map)
 	  || SOURCE_COLUMN (map, highest) >= (1U << (column_bits - range_bits))
+	  || ( /* We can't reuse the map if the line offset is sufficiently
+		  large to cause overflow when computing location_t values.  */
+	      (to_line - ORDINARY_MAP_STARTING_LINE_NUMBER (map))
+	      >= (1U << (CHAR_BIT * sizeof (linenum_type) - column_bits)))
 	  || range_bits < map->m_range_bits)
 	map = linemap_check_ordinary
 	        (const_cast <line_map *>