Re: Complex DWARF expressions

Eli Zaretskii
2014-09-22T18:21:27Z

> Date: Mon, 22 Sep 2014 08:16:54 +0200
> From: Jan Kratochvil <jan.kratochvil@redhat.com>
> Cc: Eli Zaretskii <eliz@gnu.org>, gdb@sourceware.org
>
> But $edx at the caller would be usually callee-clobbered so one would not be
> able to read the value. This is why the caller's call instruction is described
> by:
>
> <8><1663ca>: Abbrev Number: 24 (DW_TAG_GNU_call_site)
> <1663cb> DW_AT_low_pc : 0x814d44f
> <1663cf> DW_AT_abstract_origin: <0x15e7bc>
> <9><1663d8>: Abbrev Number: 3 (DW_TAG_GNU_call_site_parameter)
> <1663d9> DW_AT_location : 1 byte block: 52 (DW_OP_reg2 (edx))
> <1663db> DW_AT_GNU_call_site_value: 1 byte block: 30 (DW_OP_lit0)
>
> So one finds matching DW_TAG_GNU_call_site and then one finds
> DW_TAG_GNU_call_site_parameter with matching DW_AT_location there.
>
> These rules have to be applied recursively, as in many cases there is for
> example:
>
> <6><1669c2>: Abbrev Number: 3 (DW_TAG_GNU_call_site_parameter)
> <1669c3> DW_AT_location : 1 byte block: 51 (DW_OP_reg1 (ecx))
> <1669c5> DW_AT_GNU_call_site_value: 7 byte block: f3 1 51 a ff ff 1a (DW_OP_GNU_entry_value: (DW_OP_reg1 (ecx)); DW_OP_const2u: 65535; DW_OP_and)

Why can't GDB apply all this, and show a value for a given PC? We
don't really expect anyone but a few DWARF specialists to understand
all that you said above (I, for example, didn't understand any of it),
right?