[gdb/symtab] Work around gas PR28629

When running test-case gdb.tui/tui-layout-asm-short-prog.exp on AlmaLinux 9.2
ppc64le, I run into:
...
FAIL: gdb.tui/tui-layout-asm-short-prog.exp: check asm box contents
...

The problem is that we get:
...
    7              [ No Assembly Available ]
...
because tui_get_begin_asm_address doesn't succeed.

In more detail, tui_get_begin_asm_address calls:
...
		    find_line_pc (sal.symtab, sal.line, &addr);
...
with:
...
(gdb) p *sal.symtab
$5 = {next = 0x130393c0, m_compunit = 0x130392f0, m_linetable = 0x0,
  filename = "tui-layout-asm-short-prog.S",
  filename_for_id = "$gdb/build/gdb/testsuite/tui-layout-asm-short-prog.S",
  m_language = language_asm, fullname = 0x0}
(gdb) p sal.line
$6 = 1
...

The problem is the filename_for_id which is the source file prefixed with the
compilation dir rather than the source dir.

This is due to faulty debug info generated by gas, PR28629:
...
    <1a>   DW_AT_name        : tui-layout-asm-short-prog.S
    <1e>   DW_AT_comp_dir    : $gdb/build/gdb/testsuite
    <22>   DW_AT_producer    : GNU AS 2.35.2
...

The DW_AT_name is relative, and it's relative to the DW_AT_comp_dir entry,
making the effective name $gdb/build/gdb/testsuite/tui-layout-asm-short-prog.S.

The bug is fixed starting version 2.38, where we get instead:
...
    <1a>   DW_AT_name        :
             $gdb/src/gdb/testsuite/gdb.tui/tui-layout-asm-short-prog.S
    <1e>   DW_AT_comp_dir    : $gdb/build/gdb/testsuite
    <22>   DW_AT_producer    : GNU AS 2.38
...

Work around the faulty debug info by constructing the filename_for_id using
the second directory from the directory table in the .debug_line header:
...
 The Directory Table (offset 0x22, lines 2, columns 1):
  Entry	Name
  0	$gdb/build/gdb/testsuite
  1	$gdb/src/gdb/testsuite/gdb.tui
...

Note that the used gas contains a backport of commit 3417bfca676 ("GAS:
DWARF-5: Ensure that the 0'th entry in the directory table contains the
current working directory."), because directory 0 is correct.  With the
unpatched 2.35.2 release the directory 0 entry is incorrect: it's a copy of
entry 1.

Add a dwarf assembly test-case that reflects the debug info as generated by
unpatched gas 2.35.2.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

3 files changed, 38 insertions, 1 deletions

diff --git a/gdb/dwarf2/cu.c b/gdb/dwarf2/cu.c
index a908ec9..f9b05df 100644
--- a/gdb/dwarf2/cu.c
+++ b/gdb/dwarf2/cu.c
@@ -40,6 +40,7 @@ dwarf2_cu::dwarf2_cu (dwarf2_per_cu_data *per_cu,
     producer_is_icc_lt_14 (false),
     producer_is_codewarrior (false),
     producer_is_clang (false),
+    producer_is_gas_lt_2_38 (false),
     producer_is_gas_2_39 (false),
     processing_has_namespace_info (false),
     load_all_dies (false)
diff --git a/gdb/dwarf2/cu.h b/gdb/dwarf2/cu.h
index 6c61171..ef8db48 100644
--- a/gdb/dwarf2/cu.h
+++ b/gdb/dwarf2/cu.h
@@ -265,6 +265,7 @@ public:
   bool producer_is_icc_lt_14 : 1;
   bool producer_is_codewarrior : 1;
   bool producer_is_clang : 1;
+  bool producer_is_gas_lt_2_38 : 1;
   bool producer_is_gas_2_39 : 1;
 
   /* When true, the file that we're processing is known to have
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index bd43b5b..c494816 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -148,6 +148,8 @@ static int dwarf2_locexpr_block_index;
 static int dwarf2_loclist_block_index;
 static int ada_block_index;
 
+static bool producer_is_gas_lt_2_38 (struct dwarf2_cu *cu);
+
 /* Size of .debug_loclists section header for 32-bit DWARF format.  */
 #define LOCLIST_HEADER_SIZE32 12
 
@@ -7489,6 +7491,27 @@ read_file_scope (struct die_info *die, struct dwarf2_cu *cu)
 
   file_and_directory &fnd = find_file_and_directory (die, cu);
 
+  /* GAS supports generating dwarf-5 info starting version 2.35.  Versions
+     2.35-2.37 generate an incorrect CU name attribute: it's relative,
+     implicitly prefixing it with the compilation dir.  Work around this by
+     prefixing it with the source dir instead.  */
+  if (cu->header.version == 5 && !IS_ABSOLUTE_PATH (fnd.get_name ())
+      && producer_is_gas_lt_2_38 (cu))
+    {
+      attr = dwarf2_attr (die, DW_AT_stmt_list, cu);
+      if (attr != nullptr && attr->form_is_unsigned ())
+	{
+	  sect_offset line_offset = (sect_offset) attr->as_unsigned ();
+	  line_header_up lh = dwarf_decode_line_header (line_offset, cu,
+							fnd.get_comp_dir ());
+	  if (lh->version == 5 && lh->is_valid_file_index (1))
+	    {
+	      std::string dir = lh->include_dir_at (1);
+	      fnd.set_comp_dir (std::move (dir));
+	    }
+	}
+    }
+
   cu->start_compunit_symtab (fnd.get_name (), fnd.intern_comp_dir (objfile),
 			     lowpc);
 
@@ -11251,7 +11274,10 @@ check_producer (struct dwarf2_cu *cu)
   else if (producer_is_clang (cu->producer, &major, &minor))
     cu->producer_is_clang = true;
   else if (producer_is_gas (cu->producer, &major, &minor))
-    cu->producer_is_gas_2_39 = major == 2 && minor == 39;
+    {
+      cu->producer_is_gas_lt_2_38 = major < 2 || (major == 2 && minor < 38);
+      cu->producer_is_gas_2_39 = major == 2 && minor == 39;
+    }
   else
     {
       /* For other non-GCC compilers, expect their behavior is DWARF version
@@ -11288,6 +11314,15 @@ producer_is_codewarrior (struct dwarf2_cu *cu)
 }
 
 static bool
+producer_is_gas_lt_2_38 (struct dwarf2_cu *cu)
+{
+  if (!cu->checked_producer)
+    check_producer (cu);
+
+  return cu->producer_is_gas_lt_2_38;
+}
+
+static bool
 producer_is_gas_2_39 (struct dwarf2_cu *cu)
 {
   if (!cu->checked_producer)