Re: Complex DWARF expressions

Jan Kratochvil
2014-09-22T06:17:10Z

On Mon, 22 Sep 2014 07:59:20 +0200, Yao Qi wrote:
> Eli Zaretskii <eliz@gnu.org> writes:
>
> > Range 0x100aaf7-0x100ad4c: a complex DWARF expression:
> > 0: DW_OP_GNU_entry_value
> > 2: DW_OP_reg2 [$edx]
> > 3: DW_OP_stack_value
> >
> > "A variable in $edx" I understand, but what about the "complex DWARF
> > expression" parts? Is there any way a mere mortal such as myself can
> > decipher this to the effect of understanding in which register or at
> > what address can I look up the value, assuming that I know at which PC
> > address the program stopped?
> >
> > (Yes, I've looked at the DWARF Standard, but couldn't understand from
> > the description of these location descriptors how to convert them to
> > either a register or a memory address.)
>
> DW_OP_GNU_entry_value isn't in DWARF Standard and it is documented here
> http://www.dwarfstd.org/ShowIssue.php?issue=100909.1
>
> If I understand the doc above correctly, the entry above means if PC is
> within range 0x100aaf7-0x100ad4c, the value of new_width is the value of
> $edx at the moment entering this function. IOW, to get value of
> new_width, needs to unwind frame and read $edx.

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)

Jan