Add Windows ThinLTO arguments#747
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Use LLD COFF ThinLTO options and /start-lib groups for Windows targets. Use NUL for indexless Windows backend actions.
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Jun 13, 2026
| else: | ||
| # An empty input indicates not to perform cross-module optimization. | ||
| build_variables["thinlto_index"] = "/dev/null" | ||
| build_variables["thinlto_index"] = ( |
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@lilygorsheneva would you mind taking a peek at this one? This is all net-new behavior for windows that has never worked so hopefully it shouldn't be dangerous for google-internal usage.
All the other bits we can vendor into our toolchain module but this part is impossible to change externally, and while we can carry a patch, bzlmod rule mean every single consumer of our toolchain would also need to carry a patch, which is a huge pain.
Would be amazing to get this landed and a new rules_cc release cut, if possible! 🙏
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ok we found a pretty hacky way to workaround this but would still be good to move back to upstream!
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Windows rule-based toolchains currently reuse ELF LLD ThinLTO plugin options and
/dev/null, which are not valid for LLD's COFF driver or Windows backend actions. This change selects the LLD COFF ThinLTO options, uses/start-liband/end-libfor object-file groups, and usesNULfor indexless Windows backend actions.This allows Windows toolchains that use LLD and provide
supports_start_end_libto enable rules_cc distributed ThinLTO.