diagnostics: Make line-ending logic consistent with libcpp [PR91733]

diff --git a/gcc/diagnostic-show-locus.cc b/gcc/diagnostic-show-locus.cc
index 6eafe19785f..d267d2c258d 100644
--- a/gcc/diagnostic-show-locus.cc
+++ b/gcc/diagnostic-show-locus.cc
@@ -5508,7 +5508,7 @@  test_tab_expansion (const line_table_case &case_)
 static void
 test_escaping_bytes_1 (const line_table_case &case_)
 {
-  const char content[] = "before\0\1\2\3\r\x80\xff""after\n";
+  const char content[] = "before\0\1\2\3\v\x80\xff""after\n";
   const size_t sz = sizeof (content);
   temp_source_file tmp (SELFTEST_LOCATION, ".c", content, sz);
   line_table_test ltt (case_);
@@ -5523,18 +5523,18 @@  test_escaping_bytes_1 (const line_table_case &case_)
   if (finish > LINE_MAP_MAX_LOCATION_WITH_COLS)
     return;
 
-  /* Locations of the NUL and \r bytes.  */
+  /* Locations of the NUL and \v bytes.  */
   location_t nul_loc
     = linemap_position_for_line_and_column (line_table, ord_map, 1, 7);
-  location_t r_loc
+  location_t v_loc
     = linemap_position_for_line_and_column (line_table, ord_map, 1, 11);
   gcc_rich_location richloc (nul_loc);
-  richloc.add_range (r_loc);
+  richloc.add_range (v_loc);
 
   {
     test_diagnostic_context dc;
     diagnostic_show_locus (&dc, &richloc, DK_ERROR);
-    ASSERT_STREQ (" before \1\2\3 \x80\xff""after\n"
+    ASSERT_STREQ (" before \1\2\3\v\x80\xff""after\n"
 		  "       ^   ~\n",
 		  pp_formatted_text (dc.printer));
   }
@@ -5544,7 +5544,7 @@  test_escaping_bytes_1 (const line_table_case &case_)
     dc.escape_format = DIAGNOSTICS_ESCAPE_FORMAT_UNICODE;
     diagnostic_show_locus (&dc, &richloc, DK_ERROR);
     ASSERT_STREQ
-      (" before<U+0000><U+0001><U+0002><U+0003><U+000D><80><ff>after\n"
+      (" before<U+0000><U+0001><U+0002><U+0003><U+000B><80><ff>after\n"
        "       ^~~~~~~~                        ~~~~~~~~\n",
        pp_formatted_text (dc.printer));
   }
@@ -5552,7 +5552,7 @@  test_escaping_bytes_1 (const line_table_case &case_)
     test_diagnostic_context dc;
     dc.escape_format = DIAGNOSTICS_ESCAPE_FORMAT_BYTES;
     diagnostic_show_locus (&dc, &richloc, DK_ERROR);
-    ASSERT_STREQ (" before<00><01><02><03><0d><80><ff>after\n"
+    ASSERT_STREQ (" before<00><01><02><03><0b><80><ff>after\n"
 		  "       ^~~~            ~~~~\n",
 		  pp_formatted_text (dc.printer));
   }
diff --git a/gcc/input.cc b/gcc/input.cc
index 2acbfdea4f8..060ca160126 100644
--- a/gcc/input.cc
+++ b/gcc/input.cc
@@ -646,6 +646,37 @@  file_cache_slot::maybe_read_data ()
   return read_data ();
 }
 
+/* Helper function for file_cache_slot::get_next_line (), to find the end of
+   the next line.  Returns with the memchr convention, i.e. nullptr if a line
+   terminator was not found.  We need to determine line endings in the same
+   manner that libcpp does: any of \n, \r\n, or \r is a line ending.  */
+
+static char *
+find_end_of_line (char *s, size_t len)
+{
+  for (const auto end = s + len; s != end; ++s)
+    {
+      if (*s == '\n')
+	return s;
+      if (*s == '\r')
+	{
+	  const auto next = s + 1;
+	  if (next == end)
+	    {
+	      /* Don't find the line ending if \r is the very last character
+		 in the buffer; we do not know if it's the end of the file or
+		 just the end of what has been read so far, and we wouldn't
+		 want to break in the middle of what's actually a \r\n
+		 sequence.  Instead, we will handle the case of a file ending
+		 in a \r later.  */
+	      break;
+	    }
+	  return (*next == '\n' ? next : s);
+	}
+    }
+  return nullptr;
+}
+
 /* Read a new line from file FP, using C as a cache for the data
    coming from the file.  Upon successful completion, *LINE is set to
    the beginning of the line found.  *LINE points directly in the
@@ -671,17 +702,16 @@  file_cache_slot::get_next_line (char **line, ssize_t *line_len)
 
   char *next_line_start = NULL;
   size_t len = 0;
-  char *line_end = (char *) memchr (line_start, '\n', remaining_size);
+  char *line_end = find_end_of_line (line_start, remaining_size);
   if (line_end == NULL)
     {
-      /* We haven't found the end-of-line delimiter in the cache.
-	 Fill the cache with more data from the file and look for the
-	 '\n'.  */
+      /* We haven't found an end-of-line delimiter in the cache.
+	 Fill the cache with more data from the file and look again.  */
       while (maybe_read_data ())
 	{
 	  line_start = m_data + m_line_start_idx;
 	  remaining_size = m_nb_read - m_line_start_idx;
-	  line_end = (char *) memchr (line_start, '\n', remaining_size);
+	  line_end = find_end_of_line (line_start, remaining_size);
 	  if (line_end != NULL)
 	    {
 	      next_line_start = line_end + 1;
@@ -690,14 +720,22 @@  file_cache_slot::get_next_line (char **line, ssize_t *line_len)
 	}
       if (line_end == NULL)
 	{
-	  /* We've loadded all the file into the cache and still no
-	     '\n'.  Let's say the line ends up at one byte passed the
+	  /* We've loaded all the file into the cache and still no
+	     terminator.  Let's say the line ends up at one byte past the
 	     end of the file.  This is to stay consistent with the case
-	     of when the line ends up with a '\n' and line_end points to
-	     that terminal '\n'.  That consistency is useful below in
-	     the len calculation.  */
-	  line_end = m_data + m_nb_read ;
-	  m_missing_trailing_newline = true;
+	     of when the line ends up with a terminator and line_end points to
+	     that.  That consistency is useful below in the len calculation.
+
+	     If the file ends in a \r, we didn't identify it as a line
+	     terminator above, so do that now instead.  */
+	  line_end = m_data + m_nb_read;
+	  if (m_nb_read && line_end[-1] == '\r')
+	    {
+	      --line_end;
+	      m_missing_trailing_newline = false;
+	    }
+	  else
+	    m_missing_trailing_newline = true;
 	}
       else
 	m_missing_trailing_newline = false;
@@ -711,9 +749,8 @@  file_cache_slot::get_next_line (char **line, ssize_t *line_len)
   if (m_fp && ferror (m_fp))
     return false;
 
-  /* At this point, we've found the end of the of line.  It either
-     points to the '\n' or to one byte after the last byte of the
-     file.  */
+  /* At this point, we've found the end of the of line.  It either points to
+     the line terminator or to one byte after the last byte of the file.  */
   gcc_assert (line_end != NULL);
 
   len = line_end - line_start;
diff --git a/gcc/testsuite/c-c++-common/pr91733.c b/gcc/testsuite/c-c++-common/pr91733.c
new file mode 100644
index 00000000000..1539bb4f386
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pr91733.c
@@ -0,0 +1,17 @@ 
+/* { dg-do preprocess } */
+/* { dg-additional-options "-fdiagnostics-show-caret" } */
+
+const char *s = "
";
+
+/* { dg-warning "missing terminating \"" "test1" { target *-*-* } 4 } */
+/* { dg-warning "missing terminating \"" "test2" { target *-*-* } 5 } */
+
+/* { dg-begin-multiline-output "test3" }
+ const char *s = "
+                 ^
+{ dg-end-multiline-output "test3" } */
+
+/* { dg-begin-multiline-output "test4" }
+ ";
+ ^