[RFA/DWARF] variables with no DW_AT_location

[Joel Brobecker <brobecker@adacore.com>]

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Hello,

We came across a case where GNAT generated some debugging info for
a local variable, but no DW_AT_location. As a result, the debugger
ended up not "seeing" that variable.

        (gdb) p subp
        No definition of "subp" in current context.

After investigation, it appeared that the variable was actually optimized
away by the compiler.  Although we were later able to link it to one of
its temporary location, so the symptoms disappeared.

But I had a look at the DWARF reference, and it says that the missing
attribute means that the variable was optimized away. I propose to change
GDB to print "optimized out" in this case.

The simplest way of doing this was to take advantage of the fact that
the dwarf2read module sets the symbol class of such variables to
LOC_UNRESOLVED, and then modify the code in findvar to return an
"optimized out" value rather than no value at all.

To be honest, I find the logic in read_var_value for LOC_UNRESOLVED
types to be a bit questionable: We revert to minimal symbols if the
debugging info does not provide the location. But if the variable was
indeed optimized out, what really proves that we are picking up the
same symbol? The one we're interested in could be a local variable
that was optimized away while the one found by GDB in the symbol
table could be another global variable that happens to have the same
name.

However, I tried adjusting dwarf2read to avoid setting the class
to LOC_UNRESOLVED, but then starting hitting other problems (one was
a dereference of a NULL type). I'm doing this on my own time, so
I'm wondering what you guys think of the approach is suggesting in
this patch.

2007-03-28  Joel Brobecker  <brobecker@adacore.com>

        * findvar.c (read_var_value): Return optimized-out values for
        LOC_UNRESOLVED symbols when we cannot find an associated
        minimal symbol.

I have create a new testcase for this. But the change also affects
one of the tests in gdb.cp. I think it's OK, although GDB is a bit
less verbose. I have attached a proposed update of this testcase.

2007-03-28  Joel Brobecker  <brobecker@adacore.com>

        * gdb.dwarf2/dw2-noloc.S: New file.
        * gdb.dwarf2/dw2-noloc.exp: New file.
        * gdb.cp/m-static.exp: Minor adjustment to new GDB output.

Tested on x86-linux.

OK to apply?
-- 
Joel

[-- Attachment #2: no_at_loc.diff --]
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Index: findvar.c
===================================================================
RCS file: /cvs/src/src/gdb/findvar.c,v
retrieving revision 1.100
diff -u -p -r1.100 findvar.c
--- findvar.c	27 Feb 2007 19:46:04 -0000	1.100
+++ findvar.c	28 Mar 2007 21:52:06 -0000
@@ -575,7 +575,11 @@ addresses have not been bound by the dyn
 
 	msym = lookup_minimal_symbol (DEPRECATED_SYMBOL_NAME (var), NULL, NULL);
 	if (msym == NULL)
-	  return 0;
+	  {
+            VALUE_LVAL (v) = not_lval;
+            set_value_optimized_out (v, 1);
+            return v;
+          }
 	if (overlay_debugging)
 	  addr = symbol_overlayed_address (SYMBOL_VALUE_ADDRESS (msym),
 					   SYMBOL_BFD_SECTION (msym));
Index: testsuite/gdb.cp/m-static.exp
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.cp/m-static.exp,v
retrieving revision 1.5
diff -u -p -r1.5 m-static.exp
--- testsuite/gdb.cp/m-static.exp	9 Jan 2007 17:59:12 -0000	1.5
+++ testsuite/gdb.cp/m-static.exp	28 Mar 2007 22:18:23 -0000
@@ -126,7 +126,7 @@ gdb_test "print test3.data.value_derived
 gdb_test "print test4.elsewhere" "\\$\[0-9\].* = 221" "static const int initialized elsewhere"
 
 # static const int that nobody initializes.  From PR gdb/635.
-gdb_test "print test4.nowhere" "field nowhere is nonexistent or has been optimised out" "static const int initialized nowhere"
+gdb_test "print test4.nowhere" "<value optimized out>" "static const int initialized nowhere"
 
 # Perhaps at some point test4 should also include a test for a static
 # const int that was initialized in the header file.  But I'm not sure

[-- Attachment #3: dw2-noloc.S --]
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/* This testcase is part of GDB, the GNU debugger.

   Copyright 2007 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 2 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, write to the Free Software
   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
   USA.  */

	.text
.Lbegin_text1:
	.globl func_cu1
	.type func_cu1, %function
func_cu1:
.Lbegin_func_cu1:
	.int 0
.Lend_func_cu1:
	.size func_cu1, .-func_cu1
.Lend_text1:

/* Debug information */

	.section .debug_info
.Lcu1_begin:
	/* CU header */
	.4byte	.Lcu1_end - .Lcu1_start		/* Length of Compilation Unit */
.Lcu1_start:
	.2byte	2				/* DWARF Version */
	.4byte	.Labbrev1_begin			/* Offset into abbrev section */
	.byte	4				/* Pointer size */

	/* CU die */
	.uleb128 1				/* Abbrev: DW_TAG_compile_unit */
	.4byte	.Lline1_begin			/* DW_AT_stmt_list */
	.4byte	.Lend_text1			/* DW_AT_high_pc */
	.4byte	.Lbegin_text1			/* DW_AT_low_pc */
	.ascii	"file1.txt\0"			/* DW_AT_name */
	.ascii	"GNU C 3.3.3\0"			/* DW_AT_producer */
	.byte	1				/* DW_AT_language (C) */

	/* func_cu1 */
	.uleb128	2			/* Abbrev: DW_TAG_subprogram */
	.byte		1			/* DW_AT_external */
	.byte		1			/* DW_AT_decl_file */
	.byte		2			/* DW_AT_decl_line */
	.ascii		"func_cu1\0"		/* DW_AT_name */
	.4byte		.Ltype_int-.Lcu1_begin	/* DW_AT_type */
	.4byte		.Lbegin_func_cu1	/* DW_AT_low_pc */
	.4byte		.Lend_func_cu1		/* DW_AT_high_pc */
	.byte		1			/* DW_AT_frame_base: length */
	.byte		0x55			/* DW_AT_frame_base: DW_OP_reg5 */

.Ltype_int:
	.uleb128	3			/* Abbrev: DW_TAG_base_type */
	.ascii		"int\0"			/* DW_AT_name */
	.byte		4			/* DW_AT_byte_size */
	.byte		5			/* DW_AT_encoding */

	.uleb128	4			/* Abbrev: DW_TAG_variable */
	.ascii		"noloc\0"		/* DW_AT_name */
	.4byte		.Ltype_int-.Lcu1_begin	/* DW_AT_type */
	.byte		1			/* DW_AT_external */

	.byte		0			/* End of children of CU */

.Lcu1_end:

/* Abbrev table */
	.section .debug_abbrev
.Labbrev1_begin:
	.uleb128	1			/* Abbrev code */
	.uleb128	0x11			/* DW_TAG_compile_unit */
	.byte		1			/* has_children */
	.uleb128	0x10			/* DW_AT_stmt_list */
	.uleb128	0x6			/* DW_FORM_data4 */
	.uleb128	0x12			/* DW_AT_high_pc */
	.uleb128	0x1			/* DW_FORM_addr */
	.uleb128	0x11			/* DW_AT_low_pc */
	.uleb128	0x1			/* DW_FORM_addr */
	.uleb128	0x3			/* DW_AT_name */
	.uleb128	0x8			/* DW_FORM_string */
	.uleb128	0x25			/* DW_AT_producer */
	.uleb128	0x8			/* DW_FORM_string */
	.uleb128	0x13			/* DW_AT_language */
	.uleb128	0xb			/* DW_FORM_data1 */
	.byte		0x0			/* Terminator */
	.byte		0x0			/* Terminator */

	.uleb128	2			/* Abbrev code */
	.uleb128	0x2e			/* DW_TAG_subprogram */
	.byte		0			/* has_children */
	.uleb128	0x3f			/* DW_AT_external */
	.uleb128	0xc			/* DW_FORM_flag */
	.uleb128	0x3a			/* DW_AT_decl_file */
	.uleb128	0xb			/* DW_FORM_data1 */
	.uleb128	0x3b			/* DW_AT_decl_line */
	.uleb128	0xb			/* DW_FORM_data1 */
	.uleb128	0x3			/* DW_AT_name */
	.uleb128	0x8			/* DW_FORM_string */
	.uleb128	0x49			/* DW_AT_type */
	.uleb128	0x13			/* DW_FORM_ref4 */
	.uleb128	0x11			/* DW_AT_low_pc */
	.uleb128	0x1			/* DW_FORM_addr */
	.uleb128	0x12			/* DW_AT_high_pc */
	.uleb128	0x1			/* DW_FORM_addr */
	.uleb128	0x40			/* DW_AT_frame_base */
	.uleb128	0xa			/* DW_FORM_block1 */
	.byte		0x0			/* Terminator */
	.byte		0x0			/* Terminator */

	.uleb128	3			/* Abbrev code */
	.uleb128	0x24			/* DW_TAG_base_type */
	.byte		0			/* has_children */
	.uleb128	0x3			/* DW_AT_name */
	.uleb128	0x8			/* DW_FORM_string */
	.uleb128	0xb			/* DW_AT_byte_size */
	.uleb128	0xb			/* DW_FORM_data1 */
	.uleb128	0x3e			/* DW_AT_encoding */
	.uleb128	0xb			/* DW_FORM_data1 */
	.byte		0x0			/* Terminator */
	.byte		0x0			/* Terminator */

	.uleb128	4			/* Abbrev code */
	.uleb128	0x34			/* DW_TAG_variable */
	.byte		0			/* has_children */
	.uleb128	0x3			/* DW_AT_name */
	.uleb128	0x8			/* DW_FORM_string */
	.uleb128	0x49			/* DW_AT_type */
	.uleb128	0x13			/* DW_FORM_ref4 */
	.uleb128	0x3f			/* DW_AT_external */
	.uleb128	0xc			/* DW_FORM_flag */
	.byte		0x0			/* Terminator */
	.byte		0x0			/* Terminator */

	.byte		0x0			/* Terminator */
	.byte		0x0			/* Terminator */

/* Line table */
	.section .debug_line
.Lline1_begin:
	.4byte		.Lline1_end - .Lline1_start	/* Initial length */
.Lline1_start:
	.2byte		2			/* Version */
	.4byte		.Lline1_lines - .Lline1_hdr	/* header_length */
.Lline1_hdr:
	.byte		1			/* Minimum insn length */
	.byte		1			/* default_is_stmt */
	.byte		1			/* line_base */
 	.byte		1			/* line_range */
	.byte		0x10			/* opcode_base */

	/* Standard lengths */
	.byte		0
	.byte		1
	.byte		1
	.byte		1
	.byte		1
	.byte		0
	.byte		0
	.byte		0
	.byte		1
	.byte		0
	.byte		0
	.byte		1
	.byte		0
	.byte		0
	.byte		0

	/* Include directories */
	.byte		0

	/* File names */
	.ascii		"file1.txt\0"
	.uleb128	0
	.uleb128	0
	.uleb128	0

	.byte		0

.Lline1_lines:
	.byte		0	/* DW_LNE_set_address */
	.uleb128	5
	.byte		2
	.4byte		.Lbegin_func_cu1

	.byte		3	/* DW_LNS_advance_line */
	.sleb128	3	/* ... to 4 */

	.byte		1	/* DW_LNS_copy */

	.byte		1	/* DW_LNS_copy (second time as an end-of-prologue marker) */

	.byte		0	/* DW_LNE_set_address */
	.uleb128	5
	.byte		2
	.4byte		.Lend_func_cu1

	.byte		0	/* DW_LNE_end_of_sequence */
	.uleb128	1
	.byte		1

.Lline1_end:

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# Copyright 2007 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 2 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, write to the Free Software
# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

# This test can only be run on targets which support DWARF-2 and use gas.
# For now pick a sampling of likely targets.
if {![istarget *-*-linux*]
    && ![istarget *-*-gnu*]
    && ![istarget *-*-elf*]
    && ![istarget *-*-openbsd*]
    && ![istarget arm-*-eabi*]
    && ![istarget powerpc-*-eabi*]} {
    return 0  
}

set testfile "dw2-noloc"
set srcfile ${testfile}.S
set binfile ${objdir}/${subdir}/${testfile}.x

if  { [gdb_compile "${srcdir}/${subdir}/main.c" "main.o" object {debug}] != "" } {
    return -1
}

if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${testfile}.o" object {nodebug}] != "" } {
    return -1
}

if  { [gdb_compile "${testfile}.o main.o" "${binfile}" executable {debug}] != "" } {
    return -1
}

gdb_exit
gdb_start
gdb_reinitialize_dir $srcdir/$subdir
gdb_load ${binfile}

gdb_test "print noloc" "<value optimized out>" "print noloc"