debug: Fix __int128 handling in dwarf2out [PR99562]

debug: Fix __int128 handling in dwarf2out [PR99562]

The PR66728 changes broke __int128 handling.
It emits wide_int numbers in their minimum unsigned precision
rather than in their full precision.
The problem is then that e.g. the DW_OP_implicit_value path:
          int_mode = as_a <scalar_int_mode> (mode);
          loc_result = new_loc_descr (DW_OP_implicit_value,
                                      GET_MODE_SIZE (int_mode), 0);
          loc_result->dw_loc_oprnd2.val_class = dw_val_class_wide_int;
          loc_result->dw_loc_oprnd2.v.val_wide = ggc_alloc<wide_int> ();
          *loc_result->dw_loc_oprnd2.v.val_wide = rtx_mode_t (rtl, int_mode);
emits invalid DWARF.  In particular this patch fixes there multiple
occurences of:
        .byte   0x9e    # DW_OP_implicit_value
        .uleb128 0x10
        .quad   0xffffffffffffffff
+       .quad   0
        .quad   .LVL46  # Location list begin address (*.LLST40)
        .quad   .LFE14  # Location list end address (*.LLST40)
where we said the value has 16 byte size but then only emitted 8 byte value.
My understanding is that most of the places that use val_wide expect
the precision they chose (the one of the mode they want etc.), the only
exception is the add_const_value_attribute case where it deals with
VOIDmode CONST_WIDE_INTs, for that I agree when we don't have a mode
we need to fallback to minimum precision (not sure if maximum of
min_precision UNSIGNED and SIGNED wouldn't be better, then consumers
would know if it is signed or unsigned by looking at the MSB),
but that code already computes the precision, just decided to
create the wide_int with much larger precision (e.g. 512 bit
on x86_64).

2021-03-22  Jakub Jelinek  <jakub@redhat.com>

	PR debug/99562
	PR debug/66728
	* dwarf2out.c (get_full_len): Use get_precision rather than
	min_precision.
	(add_const_value_attribute): Make sure add_AT_wide argument has
	precision prec rather than some very wide one.