[gdb/symtab] Handle struct decl with DW_AT_signature

Consider a test-case with sources 36.c:
...
struct s { int i; };
extern void f (void);
int main (void) {
  struct s a;
  f ();
  return 0;
}
...
and 36b.c:
...
struct s { int j; };
void f (void) {
  struct s b;
}
...
compiled like this:
...
$ gcc 36.c 36b.c -g
...

It contains DWARF like this:
...
 <0><d2>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <d8>   DW_AT_name        : 36.c
 <1><f4>: Abbrev Number: 2 (DW_TAG_structure_type)
    <f5>   DW_AT_name        : s
 <2><fe>: Abbrev Number: 3 (DW_TAG_member)
    <ff>   DW_AT_name        : i
 <1><110>: Abbrev Number: 5 (DW_TAG_subprogram)
    <111>   DW_AT_name        : main
 <2><12d>: Abbrev Number: 6 (DW_TAG_variable)
    <12e>   DW_AT_name        : a
    <132>   DW_AT_type        : <0xf4>
 <0><146>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <14c>   DW_AT_name        : 36b.c
 <1><168>: Abbrev Number: 2 (DW_TAG_structure_type)
    <169>   DW_AT_name        : s
 <2><172>: Abbrev Number: 3 (DW_TAG_member)
    <173>   DW_AT_name        : j
 <1><184>: Abbrev Number: 5 (DW_TAG_subprogram)
    <185>   DW_AT_name        : f
 <2><19b>: Abbrev Number: 6 (DW_TAG_variable)
    <19c>   DW_AT_name        : b
    <1a0>   DW_AT_type        : <0x168>
...

And when printing "struct s", we get first a random one (with int j), and then
context-specific ones (with int i in main, and int j in f):
...
$ gdb -batch a.out \
  -ex "ptype struct s" \
  -ex start \
  -ex "ptype struct s" \
  -ex "break f" -ex continue \
  -ex "ptype struct s" \
  | grep "int [ij];"
    int j;
    int i;
    int j;
...
Same for -readnow.

However, if we use -fdebug-types-section:
...
$ gcc 36.c 36b.c -g -fdebug-types-section
...
we get:
...
$ gdb ... | grep "int [ij];"
    int j;
    int i;
    int i;
$ gdb -readnow ... | grep "int [ij];"
    int j;
    int j;
    int j;
...

This is due to the fact that both "struct s" DIEs have been moved to the
.debug_types section:
...
  Compilation Unit @ offset 0x0:
   Signature:     0xfd1462823bb6f7b7
 <0><17>: Abbrev Number: 1 (DW_TAG_type_unit)
 <1><1d>: Abbrev Number: 2 (DW_TAG_structure_type)
    <1e>   DW_AT_name        : s
 <2><27>: Abbrev Number: 3 (DW_TAG_member)
    <28>   DW_AT_name        : i
  Compilation Unit @ offset 0x3a:
   Signature:     0x534310fbefba324d
 <0><51>: Abbrev Number: 1 (DW_TAG_type_unit)
 <1><57>: Abbrev Number: 2 (DW_TAG_structure_type)
    <58>   DW_AT_name        : s
 <2><61>: Abbrev Number: 3 (DW_TAG_member)
    <62>   DW_AT_name        : j
...
and there's no longer a "struct s" DIE in the 36.c and
and 36b.c CUs to specify which "struct s" belongs in the CU.  This is gcc
PR90232.

However, using a tentative patch for gcc that adds these DIEs (according to
DWARF standard: If the complete declaration of a type has been placed in a
separate type unit, an incomplete declaration of that type in the compilation
unit may provide the unique 64-bit signature of the type using a
DW_AT_signature attribute):
...
  <0><d2>: Abbrev Number: 5 (DW_TAG_compile_unit)
     <d8>   DW_AT_name        : 36.c
+ <1><f4>: Abbrev Number: 6 (DW_TAG_structure_type)
+    <f5>   DW_AT_name        : s
+    <f7>   DW_AT_signature   : signature: 0xfd1462823bb6f7b7
+    <ff>   DW_AT_declaration : 1
  <0><13c>: Abbrev Number: 5 (DW_TAG_compile_unit)
     <142>   DW_AT_name        : 36b.c
+ <1><15e>: Abbrev Number: 6 (DW_TAG_structure_type)
+    <15f>   DW_AT_name        : s
+    <161>   DW_AT_signature   : signature: 0x534310fbefba324d
+    <169>   DW_AT_declaration : 1
...
still does not help, because they're declarations, so new_symbol is not called
for them in process_structure_scope.

Fix this by calling new_symbol for these decls.

Build and tested on x86_64-linux.

Also tested with target board enabling by default -fdebug-types-section
-gdwarf-4, and with gcc with aforementioned tentative patch.  In this
configuration, the patch reduces number of FAILs from 2888 to 238.

gdb/ChangeLog:

2020-04-28  Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c (process_structure_scope): Add symbol for struct decl
	with DW_AT_signature.

gdb/testsuite/ChangeLog:

2020-04-28  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/main-foo.c: New test.
	* gdb.dwarf2/struct-with-sig.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-28  Tom de Vries  <tdevries@suse.de>
+
+	* dwarf2/read.c (process_structure_scope): Add symbol for struct decl
+	with DW_AT_signature.
+
 2020-04-27  Simon Marchi  <simon.marchi@efficios.com>
 
 	* configure.ac: Remove check for fs_base/gs_base in

gdb/dwarf2/read.c

@@ -15748,7 +15748,8 @@ process_structure_scope (struct die_info *die, struct dwarf2_cu *cu)
      these DIEs are identified by the fact that they have no byte_size
      attribute, and a declaration attribute.  */
   if (dwarf2_attr (die, DW_AT_byte_size, cu) != NULL
-      || !die_is_declaration (die, cu))
+      || !die_is_declaration (die, cu)
+      || dwarf2_attr (die, DW_AT_signature, cu) != NULL)
     {
       struct symbol *sym = new_symbol (die, type, cu);
 

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-28  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.dwarf2/main-foo.c: New test.
+	* gdb.dwarf2/struct-with-sig.exp: New file.
+
 2020-04-25  Tom de Vries  <tdevries@suse.de>
 
 	* boards/debug-types.exp: New file.

gdb/testsuite/gdb.dwarf2/main-foo.c

@@ -0,0 +1,34 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Dummy foo function.  */
+
+void
+foo (void)
+{
+  asm ("foo_label: .globl foo_label");
+}
+
+/* Dummy main function.  */
+
+int
+main()
+{
+  asm ("main_label: .globl main_label");
+  foo ();
+  return 0;
+}

gdb/testsuite/gdb.dwarf2/struct-with-sig.exp

@@ -0,0 +1,141 @@
+# Copyright 2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile main-foo.c .S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+
+    lassign [function_range main ${srcdir}/${subdir}/${srcfile}] \
+	main_start main_length
+
+    lassign [function_range foo ${srcdir}/${subdir}/${srcfile}] \
+	foo_start foo_length
+
+    cu {} {
+	compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name main.c}
+	} {
+	    structure_type {
+		{name s}
+		{signature 0x0000000000000001 ref_sig8}
+		{declaration 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{name "main"}
+		{low_pc $main_start addr}
+		{high_pc "$main_start + $main_length" addr}
+	    }
+	}
+    }
+
+    cu {} {
+	compile_unit {
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name     foo.c}
+	} {
+	    structure_type {
+		{name s}
+		{signature 0x0000000000000002 ref_sig8}
+		{declaration 1 flag}
+	    }
+	    DW_TAG_subprogram {
+		{name "foo"}
+		{low_pc $foo_start addr}
+		{high_pc "$foo_start + $foo_length" addr}
+	    }
+	}
+    }
+
+    tu {} 0x0000000000000001 the_type_i {
+	type_unit {} {
+	    declare_labels int_type
+
+	    the_type_i: structure_type {
+		{name s}
+		{byte_size 4 sdata}
+	    } {
+		member {
+		    {name i}
+		    {type :$int_type}
+		}
+	    }
+	    int_type: base_type {
+		{name int}
+		{encoding @DW_ATE_signed}
+		{byte_size 4 sdata}
+	    }
+	}
+    }
+
+    tu {} 0x0000000000000002 the_type_j {
+	type_unit {} {
+	    declare_labels int_type
+
+	    the_type_j: structure_type {
+		{name s}
+		{byte_size 4 sdata}
+	    } {
+		member {
+		    {name j}
+		    {type :$int_type}
+		}
+	    }
+	    int_type: base_type {
+		{name int}
+		{encoding @DW_ATE_signed}
+		{byte_size 4 sdata}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+set struct_s_i_re \
+    [multi_line \
+	 "type = struct s {" \
+	 "    int i;" \
+	 "}"]
+set struct_s_j_re \
+    [multi_line \
+	 "type = struct s {" \
+	 "    int j;" \
+	 "}"]
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test "ptype struct s" $struct_s_i_re \
+    "struct s with int i"
+
+gdb_breakpoint "foo"
+gdb_continue_to_breakpoint "foo"
+
+gdb_test "ptype struct s" $struct_s_j_re \
+    "struct s with int j"