[Bug debug/93941] New: gcc generates wrong debug information at -O2

[qrzhang at gatech dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93941

            Bug ID: 93941
           Summary: gcc generates wrong debug information at -O2
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qrzhang at gatech dot edu
  Target Milestone: ---

It appears to be a recent regression.
My bisect points to g:82e8e335f917b9ce40801838c  (needs double check).



$ gcc-trunk -v
gcc version 10.0.1 20200225 (experimental) [master revision
2473c81cb2d:31b6c240eeb:81c833b311b16cfd87a947374d5ffbbd48facd03] (GCC)


## correct output
$ gcc abc.c ; ./a.out
10


## wrong value (i=1) at -O2
$ gcc-trunk -g -O2 abc.c
$ gdb -x cmds -batch a.out
Breakpoint 1 at 0x400451: file abc.c, line 15.

Breakpoint 1, main () at abc.c:15
15        printf("%d\n", i);
$1 = 1
Kill the program being debugged? (y or n) [answered Y; input not from terminal]
[Inferior 1 (process 2788) killed]



$ cat abc.c
volatile int a;
char b[10][10][1];
int c;
int main() {
  int i, d;
  i = 0;
  for (; i < 10; i++) {
    c = 0;
    for (; c < 10; c++) {
      d = 0;
      for (; d < 1; d++)
        a = b[i][c][d];
    }
  }
  printf("%d\n", i);
}

$ cat cmds
b abc.c:15
r
p i
kill
q