[Bug debug/105248] New: Missing value or location attribute for constant live variable likely caused by tree-dse at -O1/-O2/-O3

["assaiante at diag dot uniroma1.it" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105248

            Bug ID: 105248
           Summary: Missing value or location attribute for constant live
                    variable likely caused by tree-dse at -O1/-O2/-O3
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: assaiante at diag dot uniroma1.it
  Target Milestone: ---

In this minimized C example, variable l_56 is marked as optimized out when
stepping on a call to an external function that takes it as an argument. This
happens then the optimization level is -O1/-O2/-O3 while at -Og the debug
information is correctly defined. The variable has constant value.

Apparently, tree-dse optimization is responsible for a missing location
attribute in the DWARF DIE associated with variable l_56. If we prevent the
compiler from performing such optimization, the generated ASM will be identical
but the DWARF location info will be complete, making the variable available in
its uses.

Please find below a detailed analysis for -O1 on x64 and a quick assessment on
past gcc versions where the issue is sometimes not present.

$ cat a.c
void a()
{
    int *b;
    short l_47 = 5;
    int l_56 = 6;
    int **c = &b;
    test(l_47, l_56);
    *c = &l_56;
}
int main ()
{
    a();
}

$ cat lib.c
#include <stdio.h>

void test(int l_47, int l_56) {
   printf("%d %d", l_47, l_56);
}

GCC and GDB version:
- gcc (GCC) 12.0.0 20211227 (experimental) - commit id: 500d3f0a302
- GNU gdb (GDB) 11.2

GDB trace:
$ gcc -O1 -g a.c lib.c -o opt
$ gdb -q opt
Reading symbols from opt...
(gdb) b 7
Breakpoint 1 at 0x40054a: file a.c, line 7.
(gdb) r
Starting program: /tmp/opt 

Breakpoint 1, a () at a.c:7
7           test(l_47, l_56);
(gdb) info locals
b = <optimized out>
l_47 = 5
l_56 = <optimized out>
c = <synthetic pointer>

ASM at -O1:
0000000000400546 <a>:
  400546:       48 83 ec 08             sub    $0x8,%rsp
  40054a:       be 06 00 00 00          mov    $0x6,%esi
  40054f:       bf 05 00 00 00          mov    $0x5,%edi
  400554:       b8 00 00 00 00          mov    $0x0,%eax
  400559:       e8 30 00 00 00          callq  40058e <test>
  40055e:       48 83 c4 08             add    $0x8,%rsp
  400562:       c3                      retq

DWARF info at -O1:
0x000000a7:     DW_TAG_variable
                  DW_AT_name    ("l_56")
                  DW_AT_decl_file       ("/tmp/a.c")
                  DW_AT_decl_line       (5)
                  DW_AT_decl_column     (0x09)
                  DW_AT_type    (0x0000003d "int")

>From DWARF info we can see how the variable l_56 DIE is completely missing
either a const value or a location attribute and this lack of information
causes the unavailability of its value during debugging.

Through some testing we found out that the optimization that makes the variable
not available and that introduces the location overlapping is -ftree-dse. If we
add -fno-tree-dse to the previous compilation command line, the issue is not
present anymore since the DWARF location is correctly computed with the correct
value associated with it.


ASM at -O1 with -fno-tree-dse:
0000000000400546 <a>:
  400546:       48 83 ec 08             sub    $0x8,%rsp
  40054a:       be 06 00 00 00          mov    $0x6,%esi
  40054f:       bf 05 00 00 00          mov    $0x5,%edi
  400554:       b8 00 00 00 00          mov    $0x0,%eax
  400559:       e8 30 00 00 00          callq  40058e <test>
  40055e:       48 83 c4 08             add    $0x8,%rsp
  400562:       c3                      retq

DWARF info at -O1 with -fno-tree-dse:
0x000000a7:     DW_TAG_variable
                  DW_AT_name    ("l_56")
                  DW_AT_decl_file       ("/tmp/a.c")
                  DW_AT_decl_line       (5)
                  DW_AT_decl_column     (0x09)
                  DW_AT_type    (0x0000003d "int")
                  DW_AT_location        (0x0000000e: 
                     [0x000000000040054a, 0x000000000040055e): DW_OP_lit6,
DW_OP_stack_value)
                  DW_AT_GNU_locviews    (0x0000000c)


We have also tested older gcc versions (6.4, 7.5, 8.4, 9.3, 10.3, 11.1) and the
results are identical to the git version.