[Bug tree-optimization/84383] New: [8 Regression] ICE in maybe_diag_stxncpy_trunc

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84383

            Bug ID: 84383
           Summary: [8 Regression] ICE in maybe_diag_stxncpy_trunc
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: ice-on-valid-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: jakub at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

int a;
struct S { char *s; };
void bar (struct S *);

void
foo (char *b)
{
  struct S c[4];
  bar (c);
  __builtin_strncpy (c[a].s, b, 32);
  c[a].s[31] = '\0';
}

ICEs at -O2 or fails under valgrind, like:
==32746== Conditional jump or move depends on uninitialised value(s)
==32746==    at 0x1234997: maybe_diag_stxncpy_trunc(gimple_stmt_iterator,
tree_node*, tree_node*) (tree-ssa-strlen.c:1881)
==32746==    by 0x1235595: handle_builtin_stxncpy(built_in_function,
gimple_stmt_iterator*) (tree-ssa-strlen.c:2079)
==32746==    by 0x1238BEE:
strlen_check_and_optimize_stmt(gimple_stmt_iterator*, bool*)
(tree-ssa-strlen.c:3142)
==32746==    by 0x123A225:
strlen_dom_walker::before_dom_children(basic_block_def*)
(tree-ssa-strlen.c:3475)

depending on what uninitialized value dstoff has.