[Bug middle-end/97047] New: missing warning reading past the end of a constant string returned from a function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97047

            Bug ID: 97047
           Summary: missing warning reading past the end of a constant
                    string returned from a function
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The first call to memcpy below triggers a warning for reading past the end of
the string returned from f(), but the second call doesn't.

$ cat x.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout x.c
const char* f (void) { return "123"; }

char a[32];

void g (void)
{
  __builtin_memcpy (a, "123", sizeof a);   // warning (good)
}

void h (void)
{
  __builtin_memcpy (a, f (), sizeof a);    // missing warning (bug)
}

;; Function f (f, funcdef_no=0, decl_uid=1931, cgraph_uid=1, symbol_order=0)

f ()
{
  <bb 2> [local count: 1073741824]:
  return "123";

}


x.c: In function ‘g’:
x.c:7:3: warning: ‘__builtin_memcpy’ forming offset [4, 31] is out of the
bounds [0, 4] [-Warray-bounds]
    7 |   __builtin_memcpy (a, "123", sizeof a);   // warning (good)
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

;; Function g (g, funcdef_no=1, decl_uid=1935, cgraph_uid=2, symbol_order=2)

g ()
{
  <bb 2> [local count: 1073741824]:
  __builtin_memcpy (&a, "123", 32); [tail call]
  return;

}



;; Function h (h, funcdef_no=2, decl_uid=1938, cgraph_uid=3, symbol_order=3)

h ()
{
  <bb 2> [local count: 1073741824]:
  MEM <unsigned char[32]> [(char * {ref-all})&a] = MEM <unsigned char[32]>
[(char * {ref-all})"123"];
  return;

}