[llvm-dev] Increasing address pool reuse/reducing .o file size in DWARFv5

[Eric Christopher via llvm-dev]

For the record I think the options being committed so we can get a better
idea on how it looks would be fine. We should definitely figure out what
seems to work best and leave it there, but in the meantime I think your
plan sounds good.

-eric

On Mon, Dec 30, 2019 at 12:08 PM David Blaikie <dblaikie at gmail.com> wrote:

> tl;dr: in DWARFv5, using DW_AT_ranges even when the range is contiguous
> reduces linked, uncompressed debug_addr size for optimized builds by 93%
> and reduces total .o file size (with compression and split) by 15%. It does
> grow .dwo file size a bit - DWARFv5, no compression, not split shows the
> net effect if all bytes are equal: -O3 clang binary grows by 0.4%, -O0
> clang binary shrinks by 0.1%
> Should we enable this strategy by default for DWARFv5, for DWARFv5+Split
> DWARF, or not by default at all/only under a flag?
>
>
>
> So, I've brought this up a few times before - that DWARFv5 does a pretty
> good job of reducing relocations (& reducing .o file size with Split DWARF)
> by allowing many uses of addresses to include some kind of address+offset
> (debug_rnglists and loclists allowing "base_address" then offset_pairs (an
> improvement over similar functionality in DWARFv4 because the offset pairs
> can be uleb encoded - so they can be quite compact))
>
> But one place that DWARFv5 misses to reduce relocations further is direct
> addresses from debug_info, such as DW_AT_low_pc.
>
> For a while I've wondered if we could use an extension form for
> addr+offset, and I prototyped this without an extension attribute, but
> instead using exprloc. This has slightly higher overhead to express the...
> expression. (it's 9 bytes in total, could be as few as 5 with a custom form)
>
> But I had another idea that's more instantly deployable: Why not use
> DW_AT_ranges even when the range is contiguous? That way the low_pc that
> previously couldn't use an existing address pool entry + offset, could use
> the rnglist support for base address.
>
> The only unnecessary address pool entries that remain that I've found are
> DW_AT_low_pc for DW_TAG_labels - but there's only a handful of those in
> most code. So the "ranges everywhere" strategy gets the addresses for
> optimized clang down from 4758 (v4 address pool used 9923 addresses... ) to
> 342, with about ~4 "extra" addresses for DW_TAG_labels.
>
> This could also be a bit less costly if DWARFv5 rnglists didn't use a
> separate offset table (instead encoding the offsets directly in debug_info,
> rather than using indexes)
>
> I have patches for both the addr+offset exprloc and for the ranges-always,
> both with -mllvm flags - do people think they're both worth committing for
> experimentation? Neither? Default on in some cases (like Split DWARF)?
>
> Thanks,
> - Dave
>
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