LTO by Default
- Date proposed: 2021-02-04
- RFC MR: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/0004
Summary
Enable link time optimization (LTO) of packages by default by adding the -flto flag. This provides smaller, faster executables/DSOs, and improves GCC diagnostics.
Motivation
Arch packages are super slow, and they need to be super fast! One way to improve optimisation is through LTO.
LTO gives GCC the capability of dumping its internal representation (GIMPLE) to disk, so that all the different compilation units that make up a single executable can be optimized as a single module. This expands the scope of inter-procedural optimizations to encompass the whole program (or, rather, everything that is visible at link time). Clang does similar LTO, but dumps its internal representation as LLVM byte-code.
When SUSE implemented LTO, they found the overall size of installed binaries decreased by 5%. The benefits are particularly beneficial in large C++ programs.
Alternatives Considered
Remaining with super slow and unoptimised binaries.
Specification
Your friendly pacman/makepkg developers have provided a new lto option [1] in makepkg.conf (to be released). This will be enabled by default and add -flto to CFLAGS and CXXFLAGS. PKGBUILDs that need to disable LTO for some reason, can add
options=('!lto')
Additionally, makepkg will have additions to strip the LTO bytecodes from any installed .o/.a files. [2]
Red Hat found some issues using -flto with old configure scripts [3]. These packages can be detected by running configure with and without LTO enabled and comparing the generated config.h files. We can provide a simple sed script as a makepkg-template in devtools to fix these packages, and do a scan of the repos to automatically detect packages that need the fix added to their PKGBUILD.
[1] https://lists.archlinux.org/pipermail/pacman-dev/2021-March/024908.html [2] https://lists.archlinux.org/pipermail/pacman-dev/2021-March/024911.html [3] https://bugzilla.redhat.com/show_bug.cgi?id=1789149
Drawbacks
There are still potential bugs in LTO that cause binaries to blow up in size. These are mostly already fixed by other distributions.
Some packages will fail to build with LTO. This results in one line being added to their PKGBUILD.
There is some increased overhead when linking.
LTO can make debugging more difficult, with inlining of function occurring across build units. In addition, the use of LTO can cause the memory usage of GBD to markedly increase [4]. This can be worked around by debugging in a non-LTO build if needed (and assuming that LTO is not the cause of the issue.
Unresolved Questions
None.