Project:Toolchain/LTO

Plan

• Add filter-lto to flag-o-matic.eclass
  • Trivial glob is a bit messy but also matches stuff like -fno-lto, hence this is needed (e.g. bug #838115)
  • Needs to handle non-obvious flags which require LTO (e.g. bug #853280)

• Review blockers of the LTO tracker (bug #618550) and fix appropriately

• Review list of failures from the gentooLTO overlay and fix appropriately

• Formulate a policy on USE=lto
  • Likely will mean "workarounds are needed in build system" and not "upstream support LTO for this package"
  • How can we express the latter though? Many of us would be happy using LTO where we know upstream support it
  • See https://marc.info/?l=gentoo-project&m=161957141921902&w=2

Signs a package is unsafe

• Any -Wodr warnings
• Any -Wlto-type-mismatch warnings
• Any -Wstrict-aliasing warnings
• Test failures with LTO but passes without
• Failure to build with e.g. multiple LTO partitions

Other distros

• Debian plans to default to LTO either for bookworm or bookworm+1.

• Fedora has defaulted to LTO since Fedora 33 (2021)
  • https://fedoraproject.org/wiki/LTOByDefault
  • https://fedoraproject.org/wiki/Changes/LTOBuildImprovements

• openSUSE defaulted to LTO in 2019
  • https://lists.opensuse.org/archives/list/factory@lists.opensuse.org/message/UT2YVWPZK2IZ5EUHMSHNCW3Q72CMPWCJ/