Remove -Zemit-thin-lto flag by bjorn3 · Pull Request #152527 · rust-lang/rust

bjorn3 · 2026-02-12T11:07:38Z

As far as I can tell it was introduced in #98162 to allow fat LTO with -Clinker-plugin-lto. In #136840 a change was made to automatically disable ThinLTO summary generation when -Clinker-plugin-lto -Clto=fat is used, so we can safely remove it.

Fixes #152490

rustbot · 2026-02-12T11:07:43Z

r? @cuviper

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bjorn3 · 2026-02-12T11:08:25Z

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp


  ModulePassManager MPM;
-  bool NeedThinLTOBufferPasses = EmitThinLTO;
+  bool NeedThinLTOBufferPasses = true;


This used to be true when fat LTO is not used, so I ensured all fat LTO phases do NeedThinLTOBufferPasses = false below.

bjorn3 · 2026-02-12T11:08:54Z

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

-            MPM.addPass(BitcodeWriterPass(ThinLTODataOS));
-          }
+          MPM.addPass(ThinLTOBitcodeWriterPass(
+              ThinLTODataOS, EmitThinLTOSummary ? &ThinLinkDataOS : nullptr));


PreLinkNoLTO is never used when fat LTO is enabled.

bjorn3 · 2026-02-12T11:09:30Z

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

-    if (EmitThinLTO) {
+    // thin lto summaries prevent fat lto, so do not emit them if fat
+    // lto is requested. See PR #136840 for background information.
+    if (OptStage != LLVMRustOptStage::PreLinkFatLTO) {


This doesn't need to check LLVMRustOptStage::FatLTO as ThinLTOBufferRef is NULL in that case.

Then, how is this block "For ... LTO" at all?
(per the existing comment, assuming unqualified means full)

LTO runs this function twice. Once on each individual module (PreLinkFatLTO or PreLinkThinLTO). In this case ThinBufferRef is a valid pointer to write the bitcode to after optimizing. And a second time after doing module linking (FatLTO) or cross-module function importing (ThinLTO). In this case ThinBufferRef is NULL as we will emit object file(s) rather thaj bitcode.

I see, then perhaps the "For" comment can mention that this is only expected in the prelink phase.

cuviper · 2026-02-20T00:27:45Z

LGTM

@bors r+

rust-bors · 2026-02-20T00:27:48Z

📌 Commit 6dab8fa has been approved by cuviper

It is now in the queue for this repository.

…uviper Remove -Zemit-thin-lto flag As far as I can tell it was introduced in rust-lang#98162 to allow fat LTO with `-Clinker-plugin-lto`. In rust-lang#136840 a change was made to automatically disable ThinLTO summary generation when `-Clinker-plugin-lto -Clto=fat` is used, so we can safely remove it. Fixes rust-lang#152490

Rollup of 8 pull requests Successful merges: - #152057 (bootstrap: respect POSIX jobserver) - #152527 (Remove -Zemit-thin-lto flag) - #152818 (DOC: do not link to "nightly" in Iterator::by_ref() docstring) - #152840 (Add bootstrap snapshot tests for {`install`, `install src`}) - #152846 (Clarify some variable names in the query proc-macro) - #152858 (Fix typo in doc for core::mem::type_info::Struct) - #152861 (resolve: do not suggest `_` for unresolved imports) - #152873 (std::ops::ControlFlow - use "a" before `Result`)

bjorn3 · 2026-02-20T11:05:46Z

@bors r-

Failed in #152886 (comment)

rust-bors · 2026-02-20T11:05:53Z

Commit 6dab8fa has been unapproved.

This PR was contained in a rollup (#152886), which was also unapproved.

bjorn3 · 2026-02-20T11:07:32Z

@bors try jobs=aarch64-gnu-debug

Remove -Zemit-thin-lto flag try-job: aarch64-gnu-debug

As far as I can tell it was introduced to allow fat LTO with -Clinker-plugin-lto. Later a change was made to automatically disable ThinLTO summary generation when -Clinker-plugin-lto -Clto=fat is used, so we can safely remove it.

rust-bors · 2026-02-20T12:24:21Z

☀️ Try build successful (CI)
Build commit: 92e1558 (92e15585e3e15dbe0a30d3f961d9033e05445be9, parent: 59fd4ef94daa991e6797b5aa6127e824f3067def)

rustbot · 2026-02-20T12:24:35Z

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.

bjorn3 · 2026-02-20T12:24:51Z

@bors r=cuviper

Only change since review is an adjusted test.

rust-bors · 2026-02-20T12:24:54Z

📌 Commit c51cd0e has been approved by cuviper

It is now in the queue for this repository.

rustbot assigned cuviper Feb 12, 2026

bjorn3 commented Feb 12, 2026

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bjorn3 force-pushed the remove_z_emit_thin_lto branch from df6f555 to 3a1c7d4 Compare February 13, 2026 12:40

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This was referenced Feb 15, 2026

Move LTO from the codegen coordinator thread to link_binary rust-lang/compiler-team#908

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Simplify ThinLTO handling #152670

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bjorn3 force-pushed the remove_z_emit_thin_lto branch from 3a1c7d4 to e93ee27 Compare February 15, 2026 20:08

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bjorn3 force-pushed the remove_z_emit_thin_lto branch from 7bb4fe1 to 6dab8fa Compare February 19, 2026 10:42

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jhpratt mentioned this pull request Feb 20, 2026

Rollup of 8 pull requests #152886

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rust-bors bot pushed a commit that referenced this pull request Feb 20, 2026

Auto merge of #152527 - bjorn3:remove_z_emit_thin_lto, r=<try>

92e1558

Remove -Zemit-thin-lto flag try-job: aarch64-gnu-debug

bjorn3 added 2 commits February 20, 2026 12:19

Remove -Zemit-thin-lto flag

6366a69

As far as I can tell it was introduced to allow fat LTO with -Clinker-plugin-lto. Later a change was made to automatically disable ThinLTO summary generation when -Clinker-plugin-lto -Clto=fat is used, so we can safely remove it.

Deduplicate some code in LLVMRustOptimize

c51cd0e

bjorn3 force-pushed the remove_z_emit_thin_lto branch from 953103a to c51cd0e Compare February 20, 2026 12:24

rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 20, 2026

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bjorn3 commented Feb 12, 2026 •

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