Re: [PATCH] Improve debug info for various cases where we drop location info on the floor (PR debug/55608)

[Cary Coutant <ccoutant@google.com>]

> This patch adds DW_AT_const_value or DW_AT_location for unused static vars
> (thus, not really modified and their DECL_INITIAL can be used for
> location/constant value info), and optimizes various cases using
> DW_OP_GNU_implicit_pointer (e.g. DW_OP_addr <symbol> DW_OP_stack_value
> where symbol is either an optimized away static var (but with DW_AT_location
> or DW_AT_const_value on it's DIE), which would be otherwise dropped
> to avoid referencing non-existent symbol, can be replaced with
> DW_OP_GNU_implicit_pointer to that DIE.  Or, if symbol above is a constant string
> literal, we can create DW_TAG_dwarf_procedure with DW_AT_location
> DW_OP_implicit_value (and, surprisingly, GDB handles that out of the box).
>
> In cc1plus .debug_loc grew with this by about 3% and .debug_info grew by
> 0.25%, in libstdc++ the changes were in the noise.

Based on my reading of PR 55608, this isn't really a regression, and
it's definitely not a regression from 4.7 to 4.8. While the patch
looks OK to me, the size increase in .debug_loc and the scope of the
change suggest that this would be more appropriate for when Stage 1
reopens.

-cary