0023 Add -Wl,-z,pack-relative-relocs

Add -Wl,-z,pack-relative-relocs #

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.

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.

November 6, 2024