feat: cross language LTO #3162
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feat: cross language LTO #3162
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Will take a look at the CI failures. FWIW I tested this locally with clang-18 and Rust 1.83 and it indeed works. When LTO is enabled for both Rust and C I see in-lining occurring across the language boundary |
havasd
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Jan 8, 2025
| args = [] | ||
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| # proc-macros do not benefit from LTO, and cannot be dynamically linked with LTO. | ||
| if mode in ["thin", "fat", "off"] and not is_exec_configuration(ctx) and crate_info.type != "proc-macro": |
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did you revert this intentionally?
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Yeah I was experimenting, but I switched this back to how it was before!
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If you're using Clang for your C toolchain, it's possible to do cross language optimizations between Rust and C, see Rust's Linker-plugin-based LTO docs for more info.
This PR adds a new
@rules_rust//rust/settings:experimental_cross_language_lto(off by default) which when building a Rust binary will set-Clinker-plugin-lto. If LTO is enabled for both Rust and C will then delay optimizations to the linker, resulting in cross language optimizations.This feature is highly experimental. To have it work correctly you need to be using a Clang toolchain with
lldas the linker that is recent enough to understand the bitcode emitted byrustc. There is much more we can do here to make the feature more stable, but this seemed like a good starting point?