Created attachment 14984 [details]
bitint-39a.s

gdb 12.1 (haven't tried more recent) crashes with
../../gdb/dwarf2/attribute.h:90: internal-error: as_unsigned: Assertion `form_is_unsigned ()' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
when trying to debug following testcase:
int v;

__attribute__((noinline, noclone))
void bar (void)
{
  v++;
}

__attribute__((noinline, noclone))
signed _BitInt (19) t111_1mul (unsigned _BitInt (19) x, signed _BitInt (19) y)
{
  signed _BitInt (19) r;
  if (__builtin_mul_overflow (x, y, &r))
    bar ();
  return r;
}

__attribute__((noinline, noclone))
signed _BitInt (19) t111_2mul (signed _BitInt (19) y)
{
  unsigned _BitInt (19) x = (2);
  signed _BitInt (19) r;
  if (__builtin_mul_overflow (x, y, &r))
    bar ();
  return r;
}

__attribute__((noinline, noclone))
signed _BitInt (19) t111_3mul (signed _BitInt (19) y)
{
  signed _BitInt (19) r;
  if (__builtin_mul_overflow ((unsigned _BitInt (19)) (2), y, &r))
    bar ();
  return r;
}

__attribute__((noinline, noclone))
signed _BitInt (19) t111_4mul (unsigned _BitInt (19) x)
{
  signed _BitInt (19) y = (-262143wb - 1 / 2);
  signed _BitInt (19) r;
  if (__builtin_mul_overflow (x, y, &r))
    bar ();
  return r;
}

__attribute__((noinline, noclone))
signed _BitInt (19) t111_5mul (unsigned _BitInt (19) x)
{
  signed _BitInt (19) r;
  if (__builtin_mul_overflow
      (x, (signed _BitInt (19)) (-262143wb - 1 / 2), &r))
    bar ();
  return r;
}

__attribute__((noinline, noclone))
void t111mul (void)
{
  unsigned _BitInt (19) x = (2);
  signed _BitInt (19) y = (-262143wb - 1 / 2);
  signed _BitInt (19) r1, r2;
  v = 0;
  if (t111_1mul (x, y) != (signed _BitInt (19)) (-262143wb - 1)
      || t111_2mul (y) != (signed _BitInt (19)) (-262143wb - 1)
      || t111_3mul (y) != (signed _BitInt (19)) (-262143wb - 1)
      || t111_4mul (x) != (signed _BitInt (19)) (-262143wb - 1)
      || t111_5mul (x) != (signed _BitInt (19)) (-262143wb - 1))
    __builtin_abort ();
  if (__builtin_mul_overflow (x, y, &r1))
    bar ();
  if (r1 != (signed _BitInt (19)) (-262143wb - 1))
    __builtin_abort ();
  if (__builtin_mul_overflow
      ((unsigned _BitInt (19)) (2),
       (signed _BitInt (19)) (-262143wb - 1 / 2), &r2))
    bar ();
  if (r2 != (signed _BitInt (19)) (-262143wb - 1) || v != 7 * 0)
    __builtin_abort ();
}

int
main ()
{
  t111mul ();
  return 0;
}

with GCC trunk patched with https://gcc.gnu.org/bugzilla/attachment.cgi?id=55572&action=diff
compiled with -g -std=gnu2x.
The DW_TAG_base_type for _BitInt uses
        .byte   0x4     # uleb128 0x4; (abbrev code)
        .byte   0x24    # uleb128 0x24; (TAG: DW_TAG_base_type)
        .byte   0       # DW_children_no
        .byte   0xb     # uleb128 0xb; (DW_AT_byte_size)
        .byte   0x21    # uleb128 0x21; (DW_FORM_implicit_const)
        .byte   0x4     # sleb128 4
        .byte   0x3e    # uleb128 0x3e; (DW_AT_encoding)
        .byte   0xb     # uleb128 0xb; (DW_FORM_data1)
        .byte   0xd     # uleb128 0xd; (DW_AT_bit_size)
        .byte   0x21    # uleb128 0x21; (DW_FORM_implicit_const)
        .byte   0x13    # sleb128 19
        .byte   0x3     # uleb128 0x3; (DW_AT_name)
        .byte   0xe     # uleb128 0xe; (DW_FORM_strp)
        .byte   0
        .byte   0
but I don't see anything wrong on that, DW_FORM_implicit_const always uses sleb128
encoding of the constant and all that matters is whether that constant is signed or unsigned, not that it uses sleb128 encoding.
The crash is after
run
up