Re: F34 Change proposal: Debug Info Standardization (from DWZ to -fdebug-types-section) (System-Wide Change proposal)

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On Mon, 28 Sep 2020 16:51:02 +0200, Jan Kratochvil wrote:
> To make DWZ better consumable it needs to have the partial units separately
> parseable. That way they can be shared at IR level and not just at DWARF level
> That means:
>  * DW_TAG_partial_unit should have DW_AT_language.
>  * DW_TAG_partial_unit must contain only types (struct/class).
>    Currently they contain for example also static constant variables but when
>    you parse such independent DW_TAG_partial_unit into which dictionary you
>    will register such variable? That makes no sense.
> Currently DWZ has benefits only with DWZ common file. Without DWZ common files
> DWZ produces 1.6% files bigger than -fdebug-types-section. Therefore if the
> DWZ common files saving of 3.3% per Fedora distribution size is worth it then
> the existing DWZ tool should be dropped anyway as one can use normal
> -fdebug-types-section and one can just write a simple tool moving DW_UT_type
> units to the DWZ common file and converting DW_AT_signature declarations to
> DW_FORM_ref_sup4 and we are done.

I have measured this "a different common file" for DW_UT_type and it brings
18.14% size reduction of what DWZ does. With additional dropping of dead DIEs
+ dropping -fdebug-types-section type declaratins which achieves about 28%
this makes it together 46% of what DWZ reduces possible without those
overcomplicated constructs of DWZ.

Original comparison of plain -fdebug-types-section DWZ makes the Fedora
distribution 3.3% smaller.

-fdebug-types-section with some simple optimizations (just reusing existng
-fdebug-types-section code in consumers + DWZ common file opening in consumers
together with removal of dead DIEs) DWZ is still slightly smaller but already
only by 1.8% of the Fedora distribution size.

Sure my proposal does not expect that few percents matter. I have checked
further possibilities based on a the mail replies here which seem to insist on
any debuginfo size savings.


Jan
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