http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46724
Summary: Wrong debug info: Invalid variable location
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: debug
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: kreb...@gcc.gnu.org
The guality/nrv-1.c testcase currently fails on s390x when compiled with -O2:
Breakpoint 1, f () at /build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:20
20 a2.i[4] = 7; /* { dg-final { gdb-test 20 "a2.i\[0\]" "42" } } */
Missing separate debuginfos, use: debuginfo-install glibc-2.12.1-2.s390x
(gdb) p a2.i[0]
$1 = -336596880
The memory location of 'a2' is given as being based on r1:
readelf --debug-dump=info
...
<2><85>: Abbrev Number: 9 (DW_TAG_variable)
<86> DW_AT_name : a2
<89> DW_AT_decl_file : 1
<8a> DW_AT_decl_line : 16
<8b> DW_AT_type : <0x2d>
<8f> DW_AT_location : 2 byte block: 71 0 (DW_OP_breg1: 0)
...
But in fact r1 is not used at all. Instead the location is in r2:
f:
.LFB0:
.file 1 "/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c"
.loc 1 15 0
.cfi_startproc
.loc 1 17 0
lhi %r3,42
st %r3,0(%r2) <-- a2.i[0] = 42;
...
a2 is returned by f. The address of the location where the return value should
reside is passed to f as implicit first parameter in r2.
Right before cprop_hardreg the code looks like this. r2 is copied into r1 and
r1 is used to reference a2. DECL_RTL (<retval>) is (mem/s:SI (reg/f:DI 1 %r1
[orig:45 D.1986 ] [45]).
nrv-1.c.201r.ce3:
(insn 2 8 47 2 (set (reg/f:DI 1 %r1 [orig:45 D.1986 ] [45])
(reg:DI 2 %r2 [ D.1986 ]))
/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:15 62 {*movdi_64}
(nil))
(insn 47 2 6 2 (set (reg:SI 3 %r3)
(const_int 42 [0x2a]))
/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:17 66 {*movsi_zarch}
(nil))
(insn 6 47 45 2 (set (mem/s:SI (reg/f:DI 1 %r1 [orig:45 D.1986 ] [45]) [2
<retval>.i+0 S4 A32])
(reg:SI 3 %r3))
/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:17 66 {*movsi_zarch}
(expr_list:REG_DEAD (reg:SI 3 %r3)
(nil)))
Then r1 is propagated into insn 6 and insn 2 is removed in DCE while DECL_RTL
(<retval>) still references r1.
nrv-1.c.203r.cprop_hardreg:
(insn 2 8 47 2 (set (reg/f:DI 1 %r1 [orig:45 D.1986 ] [45])
(reg:DI 2 %r2 [ D.1986 ]))
/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:15 62 {*movdi_64}
(nil))
(insn 47 2 6 2 (set (reg:SI 3 %r3)
(const_int 42 [0x2a]))
/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:17 66 {*movsi_zarch}
(nil))
(insn 6 47 45 2 (set (mem/s:SI (reg/f:DI 2 %r2 [orig:45 D.1986 ] [45]) [2
<retval>.i+0 S4 A32])
(reg:SI 3 %r3))
/build3/gcc-head/gcc/testsuite/gcc.dg/guality/nrv-1.c:17 66 {*movsi_zarch}
(expr_list:REG_DEAD (reg:SI 3 %r3)
(nil)))
When generating the debug info for a2 add_location_or_const_value_attribute
first tries to find an RTX for a2 directly by invoking rtl_for_decl_location p
on the VAR_DECL of a2 what returns NULL. It then invokes loc_list_from_tree
which uses the DECL_VALUE_EXPR of the a2 decl - this is <retval>.
rtl_for_decl_location then returns (mem/s:SI (reg/f:DI 1 %r1 [orig:45 D.1986 ]
[45]) for <retval>.
