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Fix linker-plugin-lto: use -flto=thin #1594

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Merged
merged 3 commits into from
Oct 24, 2025
+8 −5
Merged

Fix linker-plugin-lto: use -flto=thin #1594

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34b804c
Fix linker-plugin-lto: use `-flto=thin`
NobodyXu Oct 13, 2025
7fac057
Clarify purpose of unused 'lto' variable in flags.rs
NobodyXu Oct 17, 2025
27698d1
Update comments on thin LTO for linker plugin
NobodyXu Oct 17, 2025
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13 changes: 8 additions & 5 deletions src/flags.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -140,6 +140,8 @@ impl<'this> RustcCodegenFlags<'this> {
// https://doc.rust-lang.org/rustc/codegen-options/index.html#control-flow-guard
"-Ccontrol-flow-guard" => self.control_flow_guard = value.or(Some("true")),
// https://doc.rust-lang.org/rustc/codegen-options/index.html#lto
//
// This variable is currently unused, we just keep it in case we need it in future
"-Clto" => self.lto = value.or(Some("true")),
// https://doc.rust-lang.org/rustc/linker-plugin-lto.html
"-Clinker-plugin-lto" => self.linker_plugin_lto = Some(true),
Expand Down Expand Up @@ -322,11 +324,12 @@ impl<'this> RustcCodegenFlags<'this> {
// https://doc.rust-lang.org/rustc/linker-plugin-lto.html
if self.linker_plugin_lto.unwrap_or(false) {
// https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-flto
let cc_val = match self.lto {
Some("thin") => "thin",
_ => "full",
};
push_if_supported(format!("-flto={cc_val}").into());
// In order to use linker-plugin-lto to achieve cross-lang lto, cc has to use thin LTO
// to compile the c/c++ libraries because llvm linker plugin/lld uses thin LTO by default.
// And for thin LTO in linker plugin to work, the archive also has to be compiled using thin LTO,
// since thin LTO generates extra information that fat LTO does not generate that
// is required for thin LTO process.
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Six "thin LTO" in two sentences. Well, not that I care that much, but it does sting the eye. It also feels that it would look worse in isolation, i.e. not in the diff context. I mean

- push_if_supported(format!("-flto={cc_val}").into());
+ // It has to be thin LTO because llvm linker plugin/lld uses thin LTO by default.

looks like meaningful "rebuttal," while

// https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-flto
// It has to be thin LTO because llvm linker plugin/lld uses thin LTO by default.

feels like "wow, where does it come from?"

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Thanks updated

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Five "thin LTO"-s, only one down... But what does second sentence add now? It's basically the first sentence... Again, not that I care that much :-) Just do you :-)

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hmm if it can be understood by another maintainer with no context before hand, then I think it's good

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FWIW, I think this might become problematic with rustc_codegen_gcc, as GCC has -flto but not -flto=thin (the way GCC does LTO is different). I believe rustc_codegen_gcc already has some support for cross-language LTO so it's not just theoretical. cc @antoyo

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@NobodyXu NobodyXu Oct 21, 2025 •
edited
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-flto is only enabled for clang, it's currently always disabled for gcc

push_if_supported("-flto=thin".into());
}
// https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mguard
if let Some(value) = self.control_flow_guard {
Expand Down
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