https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105158
Bug ID: 105158
Summary: ftree-ccp drops DWARF const value attribute at
-Og/-O1/-O2/-O3
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: dc.delia at protonmail dot com
Target Milestone: ---
In this code, variable l_171 is displayed optimized out at every stepped line
when debugging, including line 10 where it is used as a call argument to an
externally defined function. We tested with -Og/O1/O2/O3 and the issue is
present at all these levels.
Apparently, -ftree-ccp drops the const value attribute in the DWARF DIE for
l_171 while the final compiled code stays the same. By turning it off, the
debugger can display the value of the variable.
We found this issue on gcc commit 500d3f0a302 using gdb 11.2 and the behavior
seems consistent with prior gcc versions (e.g., 6.4, 7.5, 8.4, 9.3, 10.3,
11.1).
$ cat a.c
int a;
void b()
{
int l_169, l_174 = 0, l_188 = 0, l_191 = 0, l_240 = 1;
short l_171 ;
l_171 = (l_169 = 0) && a;
for (; l_169 <= 9; l_169 = l_169 + 3)
{
int l_187 = 0, l_189 = 0, l_190 = 8;
test(l_169, l_171, l_174, l_188, l_191, l_240, l_187, l_189, l_190);
}
}
int main ()
{
b();
}
$ cat lib.c
#include <stdio.h>
void test(int l_169, int l_171, int l_174, int l_188, int l_191, int l_240, int
l_187, int l_189, int l_190) {
printf("%d %d %d %d %d %d %d %d %d", l_169, l_171, l_174, l_188, l_191,
l_240, l_187, l_189, l_190);
}
GDB trace:
$ gcc -O1 -g a.c lib.c -o opt
$ gdb -q opt
Reading symbols from opt...
(gdb) b 10
Breakpoint 1 at 0x4004fc: file a.c, line 10.
(gdb) r
Starting program: /tmp/opt
Breakpoint 1, b () at a.c:10
10 test(l_169, l_171, l_174, l_188, l_191, l_240, l_187, l_189,
l_190);
(gdb) info locals
l_187 = 0
l_189 = 0
l_190 = 8
l_169 = 0
l_174 = 0
l_188 = 0
l_191 = 0
l_240 = 1
l_171 = <optimized out>
Code at -O1:
00000000004004f6 <b>:
4004f6: 53 push %rbx
4004f7: bb 00 00 00 00 mov $0x0,%ebx
4004fc: 48 83 ec 08 sub $0x8,%rsp
400500: 6a 08 pushq $0x8
400502: 6a 00 pushq $0x0
400504: 6a 00 pushq $0x0
400506: 41 b9 01 00 00 00 mov $0x1,%r9d
40050c: 41 b8 00 00 00 00 mov $0x0,%r8d
400512: b9 00 00 00 00 mov $0x0,%ecx
400517: ba 00 00 00 00 mov $0x0,%edx
40051c: be 00 00 00 00 mov $0x0,%esi
400521: 89 df mov %ebx,%edi
400523: b8 00 00 00 00 mov $0x0,%eax
400528: e8 26 00 00 00 callq 400553 <test>
40052d: 83 c3 03 add $0x3,%ebx
400530: 48 83 c4 20 add $0x20,%rsp
400534: 83 fb 0c cmp $0xc,%ebx
400537: 75 c3 jne 4004fc <b+0x6>
400539: 5b pop %rbx
40053a: c3 retq
The code at higher optimization levels is very similar, besides where strength
reduction, we suppose, kicks in (e.g. mov $0, %edi => xor %edi, %edi).
DWARF at -O1:
0x000000e8: DW_TAG_variable
DW_AT_name ("l_171")
DW_AT_decl_file ("/tmp/a.c")
DW_AT_decl_line (5)
DW_AT_decl_column (0x0b)
DW_AT_type (0x00000167 "short int")
If we add -fno-tree-ccp to the compilation command line used above, the code of
function b remains unchanged but the DWARF const value information is preserved
for the variable, meaning its value 0 is visible when debugging throughout its
whole lifetime.
DWARF at -O1 with -fno-tree-ccp:
0x000000e8: DW_TAG_variable
DW_AT_name ("l_171")
DW_AT_decl_file ("/tmp/a.c")
DW_AT_decl_line (5)
DW_AT_decl_column (0x0b)
DW_AT_type (0x00000167 "short int")
