[DWARF] Fix signedness issue in DWARF functions (1)

[Eric Botcazou]

Hi,

the compiler can synthesize DWARF functions to describe the location and size
of components in discriminated record types with variant part in Ada, but a
limitation is that most quantities must have DWARF2_ADDR_SIZE or else be the
result of a zero-extension to DWARF2_ADDR_SIZE of a smaller quantity, as
documented in loc_list_from_tree_1:

  /* ??? Most of the time we do not take proper care for sign/zero
     extending the values properly.  Hopefully this won't be a real
     problem...  */

Now in Ada discriminants may be either signed or unsigned, so this limitation
is problematic.  Therefore the attached patch adds a strict_signedness field
to the loc_descr_context that is passed around in parts of the DWARF back-end
and changes loc_list_from_tree_1 to act upon it being set to true.  It also
contains a small optimization to avoid emitting useless comparisons with 0.

Tested on x86-64/Linux, both GCC and GDB, OK for the mainline?


2021-04-26  Eric Botcazou  <ebotcazou@adacore.com>

	* dwarf2out.c (scompare_loc_descriptor): Fix head comment.
	(is_handled_procedure_type): Likewise.
	(struct loc_descr_context): Add strict_signedness field.
	(resolve_args_picking_1): Deal with DW_OP_[GNU_]deref_type,
	DW_OP_[GNU_]convert and DW_OP_[GNU_]reinterpret.
	(resolve_args_picking): Minor tweak.
	(function_to_dwarf_procedure): Initialize strict_signedness field.
	(type_byte_size): Likewise.
	(field_byte_offset): Likewise.
	(gen_descr_array_type_die): Likewise.
	(gen_variant_part): Likewise.
	(loc_list_from_tree_1) <CALL_EXPR>: Tidy up and set strict_signedness
	to true when a context is present before evaluating the arguments.
	<COND_EXPR>: Do not generate a useless comparison with zero.
	When dereferencing an address, if strict_signedness is true and the
	type is small and signed, use DW_OP_deref_type to do the dereference
	and then DW_OP_convert to convert back to the generic type.

-- 
Eric Botcazou
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