llvm dev - Feb 2020 - Fwd: Implicit_const implementation related query.

Forwarding to Mailing list.
Hello Everyone,

I'm trying to implement DW_FORM_implicit_const as an experimental feature
to evaluate its gain in the overall size reduction in debug info.

In my initial implementation, DW_FORM_implicit_const is emitted only when a
*constant* attribute is same across multiple DIE's.

This is accomplished, in process of *uniquing* of Abbrevs i.e if an Abbrev
is duplicate that suggests it's shared by multiple DIE's, so we replaced
the constant attributes[DW_AT_decl_file] {value, form}  with
DW_FORM_implicit_const in corresponding Abbrev.

The problem I'm facing, is in deletion of  the attribute[DW_AT_decl_file]
from DIE's[debug_info] that are using this abbrev.  These DIE's are laid
out as DIEValueList, Is there a way to cleanly remove a node[Attribute]
from this list ??

Can anybody share comments/thoughts on the implementation side of this and
suggest  how to overcome the deletion of attributes from the DIEs in the
debug_info section.

Thank You,
Sourabh.
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Dynamically choosing form based on usage isn't something that's been
done
in LLVM's DWARF emission thus far and would probably be a significant
amount of work to implement. (I don't really recommend it)

But the way the existing similar support for DW_FORM_flag_present works is
that it's explicitly chosen when creating the attributes/DIEs - it might be
that by collecting some data you could show that most values for certain
attributes on certain DIEs share the same value and then explicitly
construct those using implicit_const always, rather than conditionally.
(eg: maybe the private/public part of member descriptions could always be
emitted with implicit_const)

This sort of hardcoded heuristic approach of course will not always yield
optimal results but might have some gains.

On Mon, Feb 10, 2020 at 11:24 PM Sourabh Singh Tomar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> Forwarding to Mailing list.
> Hello Everyone,
>
> I'm trying to implement DW_FORM_implicit_const as an experimental
feature
> to evaluate its gain in the overall size reduction in debug info.
>
> In my initial implementation, DW_FORM_implicit_const is emitted only when
> a *constant* attribute is same across multiple DIE's.
>
> This is accomplished, in process of *uniquing* of Abbrevs i.e if an Abbrev
> is duplicate that suggests it's shared by multiple DIE's, so we
replaced
> the constant attributes[DW_AT_decl_file] {value, form}  with
> DW_FORM_implicit_const in corresponding Abbrev.
>
> The problem I'm facing, is in deletion of  the
attribute[DW_AT_decl_file]
> from DIE's[debug_info] that are using this abbrev.  These DIE's are
laid
> out as DIEValueList, Is there a way to cleanly remove a node[Attribute]
> from this list ??
>
> Can anybody share comments/thoughts on the implementation side of this and
> suggest  how to overcome the deletion of attributes from the DIEs in the
> debug_info section.
>
> Thank You,
> Sourabh.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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