Enable LTO by default

rfcs/0004-lto-by-default.rst

+==============
+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 particulary beneficial in large C++
+programs.
+
+Some useful reading:
+ * http://www.ucw.cz/~hubicka/slides/opensuse2018-e.pdf
+ * https://hubicka.blogspot.com/2019/05/gcc-9-link-time-and-inter-procedural.html
+
+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 has additions to strip the LTO bytecodes from
+any installed .o/.a files. [2]
+
+RedHat 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 addtion, 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.
+
+[4] https://sourceware.org/bugzilla/show_bug.cgi?id=23710
+
+Unresolved Questions
+--------------------
+
+None.
+