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Date: Mon, 22 Sep 2014 18:21:00 -0000
From: Eli Zaretskii <eliz@gnu.org>
Subject: Re: Complex DWARF expressions
In-reply-to: <20140922061654.GA15537@host2.jankratochvil.net>
To: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: yao@codesourcery.com, gdb@sourceware.org
Reply-to: Eli Zaretskii <eliz@gnu.org>
Message-id: <83a95ry0rp.fsf@gnu.org>
References: <83sijkydk4.fsf@gnu.org> <87a95sxkjr.fsf@codesourcery.com> <20140922061654.GA15537@host2.jankratochvil.net>
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> 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?