https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106783
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
```
/* To simplify debugging, compare IPA and local solutions. */
if (past_flags_known && summary)
{
size_t len = summary->arg_flags.length ();
if (past_flags.length () > len)
len = past_flags.length ();
for (size_t i = 0; i < len; i++)
{
int old_flags = i < past_flags.length () ? past_flags[i] : 0;
int new_flags = i < summary->arg_flags.length ()
? summary->arg_flags[i] : 0;
old_flags = remove_useless_eaf_flags
(old_flags, flags_from_decl_or_type (current_function_decl),
VOID_TYPE_P (TREE_TYPE (TREE_TYPE (current_function_decl))));
if (old_flags != new_flags)
{
if ((old_flags & ~new_flags) == 0
|| (new_flags & EAF_UNUSED))
fprintf (dump_file, " Flags for param %i improved:",
(int)i);
else
gcc_unreachable ();
dump_eaf_flags (dump_file, old_flags, false);
fprintf (dump_file, " -> ");
dump_eaf_flags (dump_file, new_flags, true);
}
}
```
(gdb) p old_flags
$1 = 960
(gdb) p new_flags
$2 = 828
(gdb) p dump_eaf_flags (dump_file, old_flags, true)
not_returned_directly not_returned_indirectly no_direct_read no_indirect_read
$4 = void
(gdb) p dump_eaf_flags (dump_file, new_flags, true)
no_direct_clobber no_indirect_clobber no_direct_escape no_indirect_escape
no_direct_read no_indirect_read
So what is happening is the `n/=0;` after modref1 is being turned into a
__builtin_trap and so basically modref1 and modref2 now disagree but only
because of the now it is unconditional trap.
Another testcase (fails at `-O1 -fno-tree-fre`):
```
int n;
__attribute__ ((noinline,noclone,returns_twice)) static int
bar (int)
{
n = 1;
if (n)
__builtin_unreachable();
return n;
}
int
foo (int x)
{
return bar (x);
}
```
Same as above, modref1 disagrees with modref2 but only because we go from a
conditional __builtin_unreachable to an unconditional one (this is why
-fno-tree-fre is needed to get the disagreement happening).
