[llvm] r296253 - [DebugInfo] Skip implicit_const attributes when dumping .debug_info. NFC.

[David Blaikie via llvm-commits]

On Mon, Feb 27, 2017 at 10:30 AM Victor Leschuk <vleschuk at accesssoftek.com>
wrote:

> I think flag_present and implicit_const attributes shouldn't be treated as
> similar here: flag_present attributes physically do have a record in
> .debug_info section,
>

I think one of us is confused, because my understanding is that
flag_present attributes do not have any bit/bytes in the debug_info
representation.

To take the same example as before and look at the assembly:

this is the debug_info assembly for that subprogram:

        .byte   2                       # Abbrev [2] 0x2a:0x19
DW_TAG_subprogram
        .quad   .Lfunc_begin0           # DW_AT_low_pc
        .long   .Lfunc_end0-.Lfunc_begin0 # DW_AT_high_pc
        .byte   1                       # DW_AT_frame_base
        .byte   86
        .long   .Linfo_string3          # DW_AT_linkage_name
        .long   .Linfo_string4          # DW_AT_name
        .byte   1                       # DW_AT_decl_file
        .byte   1                       # DW_AT_decl_line
                                        # DW_AT_external
        .byte   0                       # End Of Children Mark

Note that no bytes were produced for the DW_AT_external (using
DW_FORM_flag_present in the abbrev) attribute.

>From the DWARF4 spec:

"A flag is represented explicitly as a single byte of data (DW_FORM_flag)
or implicitly (DW_FORM_flag_present). In the first case, if the flag has
value zero, it indicates the absence of the attribute; if the flag has a
non-zero value, it indicates the presence of the attribute. In the second
case, the attribute is implicitly indicated as present, and no value is
encoded in the debugging information entry itself."

> while implicit_const attributes are not physically mentioned there.
> Printing implicit_const values when dumping .debug_info can be more
> convenient for reader but it will actually violate the definition of dump.
> When dumping section contents we expect to see what is actually there,
> don't we?
>
> On 02/27/2017 09:04 PM, David Blaikie wrote:
>
>
>
> On Mon, Feb 27, 2017 at 10:00 AM Victor Leschuk <vleschuk at accesssoftek.com>
> wrote:
>
>
>
> On 02/27/2017 08:04 PM, David Blaikie wrote:
>
> Skipping these values doesn't seem to be correct - in the case of
> flag_present, I think the value is still rendered in the dump, is it not?
>
> Do you mean that the value is somehow mentioned in .debug_info section? It
> is not.
>
>
> Looks to me like it is:
>
> dumping debug info for this code: void f1() { }
>
> produces this:
>
>   .debug_abbrev contents:
>   ...
>   [2] DW_TAG_subprogram   DW_CHILDREN_no
>           ...
>           DW_AT_external  DW_FORM_flag_present
>
>   .debug_info contents:
>   ...
>   0x0000002a:   DW_TAG_subprogram [2]
>                   ...
>                   DW_AT_external [DW_FORM_flag_present] (true)
>
> I would expect similar behavior for const_value - that the attribute and
> its constant value is printed in the debug_info dump.
>
>
> I would expect the same for implicit_const values. (also, this needs test
> coverage either way)
>
> Yes, you are correct. I will create tests when we decide what's right
> behavior here.
>
> I think skipping is correct here: we are dumping .debug_info data based on
> attributes stored in AbbrevDecl, this code was based on assumption that
> every attribute has representation in .debug_info section, that was before
> implicit_const form was introduced.
>
>
> On Sat, Feb 25, 2017 at 5:27 AM Victor Leschuk via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
> Author: vleschuk
> Date: Sat Feb 25 07:15:57 2017
> New Revision: 296253
>
> URL: http://llvm.org/viewvc/llvm-project?rev=296253&view=rev
> Log:
> [DebugInfo] Skip implicit_const attributes when dumping .debug_info. NFC.
>
> When dumping .debug_info section we loop through all attributes mentioned
> in
> .debug_abbrev section and dump values using DWARFFormValue::extractValue().
> We need to skip implicit_const attributes here as their values are not
> really located in .debug_info but directly in .debug_abbrev. This patch
> fixes
> triggered assert() in DWARFFormValue::extractValue() caused by trying to
> access implicit_const values from .debug_info.
>
> Modified:
>     llvm/trunk/lib/DebugInfo/DWARF/DWARFDie.cpp
>
> Modified: llvm/trunk/lib/DebugInfo/DWARF/DWARFDie.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/DebugInfo/DWARF/DWARFDie.cpp?rev=296253&r1=296252&r2=296253&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/DebugInfo/DWARF/DWARFDie.cpp (original)
> +++ llvm/trunk/lib/DebugInfo/DWARF/DWARFDie.cpp Sat Feb 25 07:15:57 2017
> @@ -329,6 +329,12 @@ void DWARFDie::dump(raw_ostream &OS, uns
>
>          // Dump all data in the DIE for the attributes.
>          for (const auto &AttrSpec : AbbrevDecl->attributes()) {
> +          if (AttrSpec.Form == DW_FORM_implicit_const) {
> +            // We are dumping .debug_info section ,
> +            // implicit_const attribute values are not really stored here,
> +            // but in .debug_abbrev section. So we just skip such attrs.
> +            continue;
> +          }
>            dumpAttribute(OS, *this, &offset, AttrSpec.Attr, AttrSpec.Form,
>                          Indent);
>          }
>
>
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>
> --
> Best Regards,
> Victor
>
>
> --
> Best Regards,
> Victor
>
>
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