[Dwarf-Discuss] DWARF: Hierarchies of abstract and concrete DIE instance trees

Simon Marchi simon.marchi at ericsson.com
Fri Sep 15 05:02:07 PDT 2017

Hi all,

First, a bit of context.  I'm currently investigating why GDB doesn't show
optimized out variables in "info locals".  The fix appears simple [1], but one
test case starts failing.  The symptom is that the local variable of func1, which
is inlined in main, appears twice (at least when compiled with GCC 5.4 and 7.1):

(gdb) bt
#0  bar () at /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.opt/inline-markers.c:27
#1  0x00000000004005b0 in func1 (arg1=0) at /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.opt/inline-locals.c:38
#2  main () at /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.opt/inline-locals.c:55
(gdb) frame
#1  0x00000000004005b0 in func1 (arg1=0) at /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.opt/inline-locals.c:38
(gdb) info locals
array = {... the right value ... }
array = <optimized out>

Note that in this case, array is not actually optimized out.  What I'm seeing
when debugging GDB is that it creates two "array" symbols while parsing the
DWARF debug info.  The interesting parts of the DWARF are:

- the abstract instance tree (thing that gets inlined)

 <1><29>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2a>   DW_AT_external    : 1
    <2a>   DW_AT_name        : (indirect string, offset: 0x70): func1
    <2e>   DW_AT_decl_file   : 1
    <2f>   DW_AT_decl_line   : 32
    <30>   DW_AT_prototyped  : 1
    <30>   DW_AT_type        : <0x50>
    <34>   DW_AT_inline      : 3        (declared as inline and inlined)
    <35>   DW_AT_sibling     : <0x50>
 <2><44>: Abbrev Number: 4 (DW_TAG_variable)
    <45>   DW_AT_name        : (indirect string, offset: 0x50): array
    <49>   DW_AT_decl_file   : 1
    <4a>   DW_AT_decl_line   : 34
    <4b>   DW_AT_type        : <0x57>

- the concrete instance tree (the place where it gets inlined)

 <2><9e>: Abbrev Number: 11 (DW_TAG_inlined_subroutine)
    <9f>   DW_AT_abstract_origin: <0x29>
    <a3>   DW_AT_low_pc      : 0x400585
    <ab>   DW_AT_high_pc     : 0x4c
    <b3>   DW_AT_call_file   : 1
    <b4>   DW_AT_call_line   : 55
 <3><be>: Abbrev Number: 13 (DW_TAG_lexical_block)
    <bf>   DW_AT_low_pc      : 0x400585
    <c7>   DW_AT_high_pc     : 0x4c
 <4><cf>: Abbrev Number: 14 (DW_TAG_variable)
    <d0>   DW_AT_abstract_origin: <0x44>
    <d4>   DW_AT_location    : 3 byte block: 91 e0 7d   (DW_OP_fbreg: -288)

The interesting thing here is that the hierarchies of the abstract and concrete
DIE trees are not exactly the same.  In the abstract tree the variable is a
direct child of the subprogram.  In the concrete tree, a lexical block is
inserted between the inlined subroutine and the variable.  For the visuals:

                                            child of
Abstract tree:  subprogram <------------------------------------- variable
                    A                                                A
                    |                                                |
        instance of |                                    instance of |
                    |                                                |
                    |         child of                  child of     |
Concrete tree:  inlined sub <---------- lexical block <---------- variable

When GDB parses that DWARF, it first creates a symbol when visiting the
concrete instance of the variable.  It then tries to inherit everything from
the inlined subroutine's abstract origin (including children) that wasn't
already explicitly referenced by its children.  For some reason, GDB doesn't
track properly that the abstract variable has already referenced, and creates a
second symbol.  That symbol coming from the abstract variable DIE doesn't have
a location, that's it appears as "optimized out".

My question is: should the hierarchies of the abstract and concrete trees match
exactly (IOW, should the concrete child's abstract origin's parent always be
the same as its parent's abstract origin).  And therefore, is this example
"legal" DWARF and GDB bug, or invalid DWARF and a GCC bug (or neither and I'm
completely lost).

With Clang, the lexical block is not there, and everything works as expected.



[1] https://github.com/simark/binutils-gdb/commit/3d16834e2d886d2dd57f93d27b39a4099ffc98fc.patch

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