[Bug analyzer/103658] New: missing -Wanalyzer-use-of-uninitialized-value at -O1 and below for an array access

[msebor at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103658

            Bug ID: 103658
           Summary: missing -Wanalyzer-use-of-uninitialized-value at -O1
                    and below for an array access
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

I came across this while comparing the middle end -Wuninitialized with 
-Wanalyzer-use-of-uninitialized-value.  They both trigger at -O2.  At -O1, only
-Wuninitialized triggers, even though the IL looks the same between the two
levels (as far as I can tell).

$ cat z.c && gcc -O1 -S -Wall -fdump-tree-uninit=/dev/stdout -fanalyzer z.c
int f (int i, int j)
{
  int a[3];
  a[0] = 1;
  a[1] = 2;

  if (i < 1) i = 1;
  if (j < 1) j = 1;
  return a[i + j];
}

;; Function f (f, funcdef_no=0, decl_uid=1979, cgraph_uid=1, symbol_order=0)

z.c: In function ‘f’:
z.c:9:11: warning: ‘a’ is used uninitialized [-Wuninitialized]
    9 |   return a[i + j];
      |          ~^~~~~~~
z.c:3:7: note: ‘a’ declared here
    3 |   int a[3];
      |       ^
int f (int i, int j)
{
  int a[3];
  int _1;
  int _6;
  int _8;
  int _9;

  <bb 2> [local count: 1073741824]:
  _8 = MAX_EXPR <i_5(D), 1>;
  _6 = MAX_EXPR <j_7(D), 1>;
  _1 = _6 + _8;
  _9 = a[_1];
  a ={v} {CLOBBER};
  return _9;

}