dwarf2out: Always emit required 0 entries for DWARF 5 in *.debug_line [PR98796]

When GCC is emitting .debug_line or .gnu.debuglto_.debug_line section by
itself (happens either with too old or non-GNU assembler, with
-gno-as-loc-support or with -flto) on empty translation units, it violates
the DWARF 5 requirements.
The standard says:
"The first entry is the current directory of the compilation."
and a few lines later:
"The first entry in the sequence is the primary source file whose file name
exactly matches that given in the DW_AT_name attribute in the compilation
unit debugging information entry."
GCC emits 4 zeros (directory entry format count, directories count,
filename entry format count and filename count), which would be ok if the
spec said The first entry may be rather than is.

I had a brief look at whether I could just fall through into the rest of the
function, but there are too many assumptions that there is at least one
normal file that it can't be done that way easily.

So this patch instead extends the early out code to emit the required
minimum, which is 15 bytes more than we used to emit before.

2021-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR debug/98796
	* dwarf2out.c (output_file_names): For -gdwarf-5, if there are no
	filenames to emit, still emit the required 0 index directory and
	filename entries that match DW_AT_comp_dir and DW_AT_name of the
	compilation unit.

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 77ea4948d5ea84d62dab52474f6800c83f89bc6e..2ec3449a19062b18fc19a957e1b027c1e0dfc80e 100644 (file)
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -12252,10 +12252,50 @@ output_file_names (void)
     {
       if (dwarf_version >= 5)
        {
-         dw2_asm_output_data (1, 0, "Directory entry format count");
-         dw2_asm_output_data_uleb128 (0, "Directories count");
-         dw2_asm_output_data (1, 0, "File name entry format count");
-         dw2_asm_output_data_uleb128 (0, "File names count");
+         const char *comp_dir = comp_dir_string ();
+         if (comp_dir == NULL)
+           comp_dir = "";
+         dw2_asm_output_data (1, 1, "Directory entry format count");
+         enum dwarf_form str_form = DW_FORM_string;
+         if (DWARF5_USE_DEBUG_LINE_STR)
+           str_form = DW_FORM_line_strp;
+         dw2_asm_output_data_uleb128 (DW_LNCT_path, "DW_LNCT_path");
+         dw2_asm_output_data_uleb128 (str_form, "%s",
+                                      get_DW_FORM_name (str_form));
+         dw2_asm_output_data_uleb128 (1, "Directories count");
+         if (str_form == DW_FORM_string)
+           dw2_asm_output_nstring (comp_dir, -1, "Directory Entry: %#x", 0);
+         else
+           output_line_string (str_form, comp_dir, "Directory Entry", 0);
+         const char *filename0 = get_AT_string (comp_unit_die (), DW_AT_name);
+         if (filename0 == NULL)
+           filename0 = "";
+#ifdef VMS_DEBUGGING_INFO
+         dw2_asm_output_data (1, 4, "File name entry format count");
+#else
+         dw2_asm_output_data (1, 2, "File name entry format count");
+#endif
+         dw2_asm_output_data_uleb128 (DW_LNCT_path, "DW_LNCT_path");
+         dw2_asm_output_data_uleb128 (str_form, "%s",
+                                      get_DW_FORM_name (str_form));
+         dw2_asm_output_data_uleb128 (DW_LNCT_directory_index,
+                                      "DW_LNCT_directory_index");
+         dw2_asm_output_data_uleb128 (DW_FORM_data1, "%s",
+                                      get_DW_FORM_name (DW_FORM_data1));
+#ifdef VMS_DEBUGGING_INFO
+         dw2_asm_output_data_uleb128 (DW_LNCT_timestamp, "DW_LNCT_timestamp");
+         dw2_asm_output_data_uleb128 (DW_FORM_udata, "DW_FORM_udata");
+         dw2_asm_output_data_uleb128 (DW_LNCT_size, "DW_LNCT_size");
+         dw2_asm_output_data_uleb128 (DW_FORM_udata, "DW_FORM_udata");
+#endif
+         dw2_asm_output_data_uleb128 (1, "File names count");
+
+         output_line_string (str_form, filename0, "File Entry", 0);
+         dw2_asm_output_data (1, 0, NULL);
+#ifdef VMS_DEBUGGING_INFO
+         dw2_asm_output_data_uleb128 (0, NULL);
+         dw2_asm_output_data_uleb128 (0, NULL);
+#endif
        }
       else
        {