On 12/2/20 9:48 AM, Jakub Jelinek via Gcc-patches wrote:
> Hi!
>
> As discussed in https://sourceware.org/bugzilla/show_bug.cgi?id=26987 ,
> for very large bounds (which don't fit into HOST_WIDE_INT) GCC emits invalid
> DWARF.
> In DWARF2, DW_AT_{lower,upper}_bound were constant reference class.
> In DWARF3 they are block constant reference and the
> Static and Dynamic Properties of Types
> chapter says:
> "For a block, the value is interpreted as a DWARF expression; evaluation of
> the expression
> yields the value of the attribute."
> In DWARF4/5 they are constant exprloc reference class.
> Now, for add_AT_wide we use DW_FORM_data16 (valid in constant class)
> when -gdwarf-5, but otherwise just use DW_FORM_block1, which is not constant
> class, but block.
> For DWARF3 this means emitting clearly invalid DWARF, because the
> DW_FORM_block1 should contain a DWARF expression, not random bytes
> containing the constant directly.
> For DWARF2/DWARF4/5 it could be considered a GNU extension, but a very badly
> designed one when it means something different in DWARF3.
>
> The following patch uses add_AT_wide only if we know we'll be using
> DW_FORM_data16, and otherwise wastes 2 extra bytes and emits in there
> DW_OP_implicit_value <size> before the constant.
>
> Ok for trunk if it passes bootstrap/regtest?
>
> 2020-12-02 Jakub Jelinek <ja...@redhat.com>
>
> * dwarf2out.c (add_scalar_info): Only use add_AT_wide for 128-bit
> constants and only in dwarf-5 or later, where DW_FORM_data16 is
> available. Otherwise use DW_FORM_block*/DW_FORM_exprloc with
> DW_OP_implicit_value to describe the constant.
OK.
jeff
