Send -fno-lto when linker plugin LTO is not requested to avoid having GCC do LTO when using rustc_codegen_gcc by antoyo · Pull Request #159924 · rust-lang/rust

antoyo · 2026-07-25T19:10:56Z

rustbot · 2026-07-25T19:11:00Z

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bjorn3 · 2026-07-25T19:57:33Z

Thinking about this again, this would cause a compilation failure if there is any C code compiled with -flto, right? Older versions of cc did that whenever rust LTO is enabled even though no LTO between C and Rust happens in that case. Only LTO between Rust crates and separately between C objects. And as it so happens rustc is currently stuck on said older version of the cc crate as disabling LTO for Jemalloc is a perf regression.

antoyo · 2026-07-25T20:14:44Z

Thinking about this again, this would cause a compilation failure if there is any C code compiled with -flto, right?

You mean objects files that are not fat compiled with GCC?
Is this a case this is currently supported?
That would not work either when using LLD which is the default, no?
I would assume the way to support this would be to explicitly use a linker plugin.
What are your thoughts on this?

bjorn3 · 2026-07-25T20:43:40Z

That would not work either when using LLD which is the default, no?

It should currently work without extra configuration when using Clang as C compiler and either Clang as linker driver or LLD as linker though.

antoyo · 2026-07-25T20:52:05Z

It should currently work without extra configuration when using Clang as C compiler and either Clang as linker driver or LLD as linker though.

So, we would need to only emit -fno-lto when gcc is used as the linker, right?
Is there a way to detect this?
If not, would using self.codegen_backend == "gcc" like we did in push_linker_plugin_lto_args be OK?

fee1-dead · 2026-08-01T17:18:29Z

r? bjorn3

@jieyouxu

…r=Kobzol Make `run-make` testsuite work with other codegen backend than LLVM Needed for rust-lang#159924. Currently, we always run `run-make` testsuite with the codegen backend rustc was compiled with. However, in CI it's compiled with LLVM, so when we want to test with GCC (with `--test-codegen-backend`), it compiles `rmake.rs` with the GCC backend, but when running the test, it doesn't use the GCC backend since it just calls `rustc`. So to get around that, I now pass the codegen backend through the environment and set it in the `rustc` function of `run_make_support`. To be noted that for now it's only for the `rustc` function, no other command uses it. Should I extend it right away for all commands (well, likely only `cargo`) or just `rustc` for now is enough? r? @jieyouxu

@jieyouxu

…r=Kobzol Make `run-make` testsuite work with other codegen backend than LLVM Needed for rust-lang#159924. Currently, we always run `run-make` testsuite with the codegen backend rustc was compiled with. However, in CI it's compiled with LLVM, so when we want to test with GCC (with `--test-codegen-backend`), it compiles `rmake.rs` with the GCC backend, but when running the test, it doesn't use the GCC backend since it just calls `rustc`. So to get around that, I now pass the codegen backend through the environment and set it in the `rustc` function of `run_make_support`. To be noted that for now it's only for the `rustc` function, no other command uses it. Should I extend it right away for all commands (well, likely only `cargo`) or just `rustc` for now is enough? r? @jieyouxu

@jieyouxu

…r=Kobzol Make `run-make` testsuite work with other codegen backend than LLVM Needed for rust-lang#159924. Currently, we always run `run-make` testsuite with the codegen backend rustc was compiled with. However, in CI it's compiled with LLVM, so when we want to test with GCC (with `--test-codegen-backend`), it compiles `rmake.rs` with the GCC backend, but when running the test, it doesn't use the GCC backend since it just calls `rustc`. So to get around that, I now pass the codegen backend through the environment and set it in the `rustc` function of `run_make_support`. To be noted that for now it's only for the `rustc` function, no other command uses it. Should I extend it right away for all commands (well, likely only `cargo`) or just `rustc` for now is enough? r? @jieyouxu

@jieyouxu

Rollup merge of #162482 - GuillaumeGomez:run-make-backends, r=Kobzol Make `run-make` testsuite work with other codegen backend than LLVM Needed for #159924. Currently, we always run `run-make` testsuite with the codegen backend rustc was compiled with. However, in CI it's compiled with LLVM, so when we want to test with GCC (with `--test-codegen-backend`), it compiles `rmake.rs` with the GCC backend, but when running the test, it doesn't use the GCC backend since it just calls `rustc`. So to get around that, I now pass the codegen backend through the environment and set it in the `rustc` function of `run_make_support`. To be noted that for now it's only for the `rustc` function, no other command uses it. Should I extend it right away for all commands (well, likely only `cargo`) or just `rustc` for now is enough? r? @jieyouxu

GuillaumeGomez · 2026-09-09T09:52:16Z

#162482 got merged, so after a rebase, this PR should be ready to go.

rustbot · 2026-09-09T15:13:40Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

… GCC do LTO when using rustc_codegen_gcc

antoyo · 2026-09-09T18:53:43Z

@@ -1,4 +1,5 @@
 //@ ignore-cross-compile (need to run fake linker)
+//@ ignore-backends: gcc


Not sure if there would be a way to make this test "generic" on whether the flag -fno-lto is present or not, but I guess it's not super important anyway.

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antoyo · 2026-09-09T18:55:59Z

The test now passes.
@bjorn3: This is ready for review.

rustbot assigned fee1-dead Jul 25, 2026

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 25, 2026

antoyo force-pushed the fix/fno-lto branch from a4242c7 to c1af4ca Compare July 25, 2026 19:17

antoyo force-pushed the fix/fno-lto branch from c1af4ca to c553de8 Compare July 25, 2026 20:41

rustbot assigned bjorn3 and unassigned fee1-dead Aug 1, 2026

antoyo force-pushed the fix/fno-lto branch from c553de8 to 4de67b9 Compare August 11, 2026 14:22

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GuillaumeGomez mentioned this pull request Sep 8, 2026

Make run-make testsuite work with other codegen backend than LLVM #162482

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antoyo force-pushed the fix/fno-lto branch from 4de67b9 to f44c5df Compare September 9, 2026 15:13

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Send -fno-lto when linker plugin LTO is not requested to avoid having…

6a9b179

… GCC do LTO when using rustc_codegen_gcc

antoyo force-pushed the fix/fno-lto branch from f44c5df to 6a9b179 Compare September 9, 2026 16:16

antoyo commented Sep 9, 2026

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Send -fno-lto when linker plugin LTO is not requested to avoid having GCC do LTO when using rustc_codegen_gcc - #159924

Send -fno-lto when linker plugin LTO is not requested to avoid having GCC do LTO when using rustc_codegen_gcc#159924
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		@@ -1,4 +1,5 @@
		//@ ignore-cross-compile (need to run fake linker)
		//@ ignore-backends: gcc

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