Merge branch 'heftig/pack-relative-relocs'

rfcs/0023-pack-relative-relocs.rst

+===============================
+Add -Wl,-z,pack-relative-relocs
+===============================
+
+- Date proposed: 2023-09-03
+- RFC MR: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/0023
+
+Summary
+-------
+
+Add ``-Wl,-z,pack-relative-relocs`` to our LDFLAGS in order to reduce the size
+of relocations.
+
+Motivation
+----------
+
+``-z pack-relative-relocs`` moves relative relocations from the ``.rela.dyn``
+section into a new ``.relr.dyn`` section with a significantly more compact
+encoding, supported since glibc 2.36, GNU Binutils 2.38 and LLVM 15.
+
+This can reduce the size of libraries a lot, e.g. the installed size of
+``libphonenumber`` dropped from about 17 MB to 7 MB.
+
+``glibc`` already enabled this for all its shared objects. This can be confirmed
+by using ``readelf -d`` on an object and looking for the ``RELR`` tag.
+
+For more info, see `FS#72433`_, the related blog post from `MaskRay on RELR`_
+and the blog post from `Glandium on RELR`_ for Firefox.
+
+.. _`FS#72433`: https://bugs.archlinux.org/task/72433
+.. _`MaskRay on RELR`: https://maskray.me/blog/2021-10-31-relative-relocations-and-relr
+.. _`Glandium on RELR`: https://glandium.org/blog/?p=4297
+
+Specification
+-------------
+
+Change our distributed LDFLAGS to the following:
+
+::
+
+ LDFLAGS="-Wl,-O1 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now \
+          -Wl,-z,pack-relative-relocs"
+
+We now also use one ``-Wl`` for each logical flag so that we can easily split at
+whitespace. This does not change the meaning of the flags.
+
+Drawbacks
+---------
+
+This will break compatibility with tools that process relocations and that have
+not been updated. For example, ``eu-readelf -r`` does not decode the
+``.relr.dyn`` section and ``eu-elflint`` reports an error.
+
+We may encounter other regressions from this, but it seems unlikely that they
+are major. Since glibc enabled this internally we should have encountered such
+already.
+
+Unresolved Questions
+--------------------
+
+None.
+
+Alternatives Considered
+-----------------------
+
+None.