[PATCH 02/11] [gdb/macros] Handle 64-bit dwarf in gdb.base/fission-macro.exp

[Tom de Vries <tdevries@suse.de>]

When enabling 64-bit dwarf in the test-case gdb.base/fission-macro.exp, we run
into:
...
(gdb) break -qualified main^M
DW_MACRO_define_strx pointing outside of .debug_str.dwo section \
  [in module fission-macro-5-640.dwo]^M
(gdb) FAIL: gdb.base/fission-macro.exp: dwarf_version=5: dwarf_bits=64: \
  gdb_breakpoint: set breakpoint at main
...

A dwarf-5 .debug_str_offsets section starts with a header consisting of:
- an initial length (4 bytes for 32-bit and 12 bytes for 64-bit),
- a 2 byte version string, and
- 2 bytes padding
so in total 8 bytes for 32-bit and 16 bytes for 64-bit.

This offset is calculated here in dwarf_decode_macros:
...
      str_offsets_base = cu->header.addr_size;
...
and in both cases this evaluates to 8.

So in the 64-bit case, we interpret the last 8 bytes of the header as an offset.

Fix this by computing str_offsets_base correctly for dwarf-5, for both the
32-bit and 64-bit case.

Tested on x86_64-linux.

Tested test-case using a current gcc trunk build, and gcc 14.
---
 gdb/dwarf2/read.c                        | 11 ++++++++++-
 gdb/testsuite/gdb.base/fission-macro.exp |  2 +-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index e8416905163..4884553000a 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -21232,7 +21232,16 @@ dwarf_decode_macros (struct dwarf2_cu *cu, unsigned int offset,
       str_offsets_section = &cu->dwo_unit->dwo_file
 			       ->sections.str_offsets;
       str_section = &cu->dwo_unit->dwo_file->sections.str;
-      str_offsets_base = cu->header.addr_size;
+      if (cu->per_cu->version () <= 4)
+	str_offsets_base = cu->header.addr_size;
+      else
+	{
+	  bfd *abfd = str_offsets_section->get_bfd_owner ();
+	  unsigned int bytes_read = 0;
+	  read_initial_length (abfd, str_offsets_section->buffer, &bytes_read, false);
+	  const bool is_dwarf64 = bytes_read != 4;
+	  str_offsets_base = is_dwarf64 ? 16 : 8;
+	}
     }
   else
     {
diff --git a/gdb/testsuite/gdb.base/fission-macro.exp b/gdb/testsuite/gdb.base/fission-macro.exp
index 705e3dd0c4b..4eb9c0efe21 100644
--- a/gdb/testsuite/gdb.base/fission-macro.exp
+++ b/gdb/testsuite/gdb.base/fission-macro.exp
@@ -67,7 +67,7 @@ proc do_tests { dwarf_version dwarf_bits strict_dwarf } {
 }
 
 foreach_with_prefix dwarf_version {5} {
-    foreach_with_prefix dwarf_bits {32} {
+    foreach_with_prefix dwarf_bits {32 64} {
 	foreach_with_prefix strict_dwarf {0 1} {
 	    do_tests $dwarf_version $dwarf_bits $strict_dwarf
 	}
-- 
2.35.3