https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105239
Bug ID: 105239
Summary: Presence of unrelated function call makes a variable
optimized out only at -Og
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, the value of variable l_31 from function e is available when
debugging on line 14, which re-assigns it, and optimized out at line 27, which
uses it as a call argument for test() from an external module.
We found this at -Og. Looking at DWARF info, the update of l_31 seems not
tracked with l_31’s DIE location defined only for instructions that precede the
reassignment. At -O1/-O2/-O3 the reassignment appears to be tracked correctly
as the updated value is available when debugging.
Interestingly, by removing the call to external function nop() located right
before the call to test(..., l_31, ...), the issue disappears and the DWARF
info becomes identical to the one from -O1 for the original program.
We found this issue on x64 gcc commit 500d3f0a302 using gdb 11.2; as for gcc
releases, the issue is found in every tested version (6.4, 7.5, 8.4, 9.3, 10.3,
11.1). We were unable to pinpoint a specific optimization behind it.
$ cat a.c
short a;
int c;
short b()
{
return a;
}
void e(char f)
{
short g;
int l_57 = 0 ;
for (; c <= 0; c++)
{
int l_30 = 4073709551615, l_31 = 2, l_80 = 2;
char l_81 = l_31 = f;
g = b();
nop();
test(l_57, l_30, l_31, l_80, l_81);
}
}
int main ()
{
e(8);
}
$ cat lib.c
#include <stdio.h>
void nop() {
printf("\n");
}
void test(int l_51, int l_57, int l_30, int l_31, int l_80, int l_81) {
printf("%d %d %d %d %d %d", l_51, l_57, l_30, l_31, l_80, l_81);
}
GDB trace:
$ gcc -Og -g a.c lib.c -o opt
$ gdb -q opt
Reading symbols from opt...
(gdb) b 17
Breakpoint 1 at 0x400566: file a.c, line 17.
(gdb) r
Starting program: /tmp/opt
Breakpoint 1, e (f=f@entry=8 '\b') at a.c:17
17 test(l_57, l_30, l_31, l_80, l_81);
(gdb) info loc
l_30 = 2080555007
l_31 = <optimized out>
l_80 = 2
l_81 = 8 '\b'
g = <optimized out>
l_57 = 0
Code at -Og:
000000000040054e <e>:
40054e: 55 push %rbp
40054f: 53 push %rbx
400550: 48 83 ec 08 sub $0x8,%rsp
400554: 89 fd mov %edi,%ebp
400556: eb 33 jmp 40058b <e+0x3d>
400558: 40 0f be dd movsbl %bpl,%ebx
40055c: b8 00 00 00 00 mov $0x0,%eax
400561: e8 4d 00 00 00 callq 4005b3 <nop>
400566: 89 da mov %ebx,%edx
400568: 41 89 d8 mov %ebx,%r8d
40056b: b9 02 00 00 00 mov $0x2,%ecx
400570: be ff bf 02 7c mov $0x7c02bfff,%esi
400575: bf 00 00 00 00 mov $0x0,%edi
40057a: b8 00 00 00 00 mov $0x0,%eax
40057f: e8 42 00 00 00 callq 4005c6 <test>
400584: 83 05 b1 0a 20 00 01 addl $0x1,0x200ab1(%rip) #
60103c <c>
40058b: 83 3d aa 0a 20 00 00 cmpl $0x0,0x200aaa(%rip) #
60103c <c>
400592: 7e c4 jle 400558 <e+0xa>
400594: 48 83 c4 08 add $0x8,%rsp
400598: 5b pop %rbx
400599: 5d pop %rbp
40059a: c3 retq
DWARF at -Og:
0x0000011f: DW_TAG_variable
DW_AT_name ("l_31")
DW_AT_decl_line (13)
DW_AT_decl_column (0x24)
DW_AT_type (0x0000005b "int")
DW_AT_location (0x00000034:
[0x0000000000400558, 0x000000000040055c): DW_OP_lit2,
DW_OP_stack_value)
DW_AT_GNU_locviews (0x00000032)
Code at -O1:
000000000040054e <e>:
40054e: 83 3d e7 0a 20 00 00 cmpl $0x0,0x200ae7(%rip) #
60103c <c>
400555: 7f 42 jg 400599 <e+0x4b>
400557: 53 push %rbx
400558: 40 0f be df movsbl %dil,%ebx
40055c: b8 00 00 00 00 mov $0x0,%eax
400561: e8 4c 00 00 00 callq 4005b2 <nop>
400566: 41 89 d8 mov %ebx,%r8d
400569: b9 02 00 00 00 mov $0x2,%ecx
40056e: 89 da mov %ebx,%edx
400570: be ff bf 02 7c mov $0x7c02bfff,%esi
400575: bf 00 00 00 00 mov $0x0,%edi
40057a: b8 00 00 00 00 mov $0x0,%eax
40057f: e8 41 00 00 00 callq 4005c5 <test>
400584: 8b 05 b2 0a 20 00 mov 0x200ab2(%rip),%eax #
60103c <c>
40058a: 83 c0 01 add $0x1,%eax
40058d: 89 05 a9 0a 20 00 mov %eax,0x200aa9(%rip) #
60103c <c>
400593: 85 c0 test %eax,%eax
400595: 7e c5 jle 40055c <e+0xe>
400597: 5b pop %rbx
400598: c3 retq
DWARF at -O1:
0x0000011e: DW_TAG_variable
DW_AT_name ("l_31")
DW_AT_decl_column (0x24)
DW_AT_type (0x0000005b "int")
DW_AT_location (0x0000003f:
[0x000000000040055c, 0x000000000040055c): DW_OP_lit2,
DW_OP_stack_value
[0x000000000040055c, 0x0000000000400598): DW_OP_reg3 RBX
[0x0000000000400598, 0x0000000000400599):
DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_const1u 0x38, DW_OP_shl, DW_OP_const1u
0x38, DW_OP_shra, DW_OP_stack_value)
DW_AT_GNU_locviews (0x00000039)
