Fix incremental compilation for fat LTO - #163317

bjorn3 · 2026-09-25T10:18:19Z

Previously we would copy post-LTO artifacts into the incr comp cache for fat LTO despite them never getting used. Also the pre-LTO bitcode didn't get tracked and thus determine_cgu_reuse believed it had to regenerate bitcode for all CGUs even when all CGUs would be green.

Also move incr comp session dirs out of CodegenContext. CodegenContext will in the future be serialized and deserialized into a different rustc instance when -Zno-link/-Zlink-only is used. A separate incr comp session has to be used for both rustc invocations.

Part of rust-lang/compiler-team#908

rustbot · 2026-09-25T10:18:22Z

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

rustbot · 2026-09-25T10:18:25Z

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bjorn3 · 2026-09-25T12:12:09Z

            if !self.check_config(cfg) {
                debug!("check_attr: config does not match, ignoring attr");
-                return;
+                continue;


This caused us to skip all assertions after the first one with a cfg that doesn't match. Surprisingly nothing regressed by the lack of test coverage since the introduction of this bug.

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mejrs

Not sure about this otherwise.

r? @nnethercote perhaps?

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nnethercote

r=me with the nits addressed.

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nnethercote · 2026-10-02T03:03:32Z

+                };
+
+                // FIXME include pre-LTO bitcode in workproduct tracking
+                // FIXME add separate incr comp session for post-LTO outputs to use during link step


How important are these FIXMEs? Are you planning to fix them?

I'm working on a follow up PR to do this.

bjorn3 · 2026-10-02T09:01:09Z

@bors r=nnethercote

rust-bors · 2026-10-02T09:01:12Z

📌 Commit ebdd423 has been tentatively approved by nnethercote

It will be put into the queue for this repository once PR CI succeeds.

Fix incremental compilation for fat LTO Previously we would copy post-LTO artifacts into the incr comp cache for fat LTO despite them never getting used. Also the pre-LTO bitcode didn't get tracked and thus `determine_cgu_reuse` believed it had to regenerate bitcode for all CGUs even when all CGUs would be green. Also move incr comp session dirs out of `CodegenContext`. `CodegenContext` will in the future be serialized and deserialized into a different rustc instance when `-Zno-link`/`-Zlink-only` is used. A separate incr comp session has to be used for both rustc invocations. Part of rust-lang/compiler-team#908

…uwer Rollup of 13 pull requests Successful merges: - #163317 (Fix incremental compilation for fat LTO) - #163582 (Reapply "bootstrap: Enable rustdoc mergeable CCI for std and internal docs") - #151793 (Add mul_add_relaxed methods for floating-point types) - #162782 (Fix rustdoc ICE caused by mishandling of ambiguity errors) - #163010 (Miri can do dirfd now) - #163535 (Improve `DocStrings` perf) - #163576 (Fix `TypeOutlives` fast-path) - #163587 (Several small span improvements) - #163612 (fix `ValidateBoundVars`) - #163632 (bump rustc-build-sysroot) - #163635 (Revert note about signum of NaN) - #163644 (Add mailmap entry) - #163651 (Remove variants from `feature-gate-autodiff-use` test)

JonathanBrouwer · 2026-10-02T14:07:42Z

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #163650 (comment)

@bors try jobs=test-x86_64-mingw-1

rust-bors · 2026-10-02T14:07:51Z

This pull request was unapproved.

This PR was contained in a rollup (#163656), which was unapproved.

View changes since this unapproval

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

rust-bors · 2026-10-02T16:46:40Z

💔 Test for e4021fc failed: CI. Failed job:

try - test-x86_64-mingw-1 (web logs, enhanced plaintext logs)

CodegenContext will in the future be serialized and deserialized into a different rustc instance when -Zno-link/-Zlink-only is used. A separate incr comp session has to be used for both rustc invocations.

Previously we would copy post LTO artifacts into the incr comp cache for fat LTO despite them never getting used. Also the pre LTO bitcode didn't get tracked and thus determine_cgu_reuse believed it had to regenerate bitcode for all CGUs even when all CGUs would be green.

Co-authored-by: Bruno Kolenbrander <59372212+mejrs@users.noreply.github.com>

rustbot · 2026-10-04T14:36:33Z

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.

This reverts commit db2812a.

bjorn3 · 2026-10-04T14:37:12Z

Doesn't reproduce on Linux, so let's try bisecting it using CI.

@bors try jobs=test-x86_64-mingw-1

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

bjorn3 · 2026-10-04T15:42:23Z

@bors try jobs=test-x86_64-mingw-1

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

rust-bors · 2026-10-04T17:02:15Z

💔 Test for 22eb763 failed: CI. Failed job:

try - test-x86_64-mingw-1 (web logs, enhanced plaintext logs)

…testing" This reverts commit 728beca.

bjorn3 · 2026-10-04T17:25:00Z

@bors try jobs=test-x86_64-mingw-1

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

rust-bors · 2026-10-04T20:23:38Z

☀️ Try build successful (CI)
Build commit: a640230 (a6402304796db36e1ddf97b58ecdaa1676982c31)
Base parent: a639ea0 (a639ea0890c0977b1c45596fd6b5f9a70a1d67da)

rustbot assigned mejrs Sep 25, 2026

bjorn3 mentioned this pull request Sep 25, 2026

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

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bjorn3 force-pushed the lto_refactors22 branch from bc690ff to 980f03b Compare September 25, 2026 12:10

bjorn3 commented Sep 25, 2026

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bjorn3 changed the title ~~Fix incr comp for fat LTO~~ Fix incremental compilation for fat LTO Sep 27, 2026

mejrs reviewed Sep 30, 2026

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Comment thread compiler/rustc_codegen_llvm/src/back/llvm_backend.rs

rustbot assigned nnethercote and unassigned mejrs Sep 30, 2026

nnethercote approved these changes Oct 2, 2026

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JonathanBrouwer mentioned this pull request Oct 2, 2026

Rollup of 8 pull requests #163650

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JonathanBrouwer mentioned this pull request Oct 2, 2026

Rollup of 13 pull requests #163656

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rust-bors Bot pushed a commit that referenced this pull request Oct 2, 2026

Auto merge of #163317 - bjorn3:lto_refactors22, r=<try>

e4021fc

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

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bjorn3 and others added 7 commits October 4, 2026 14:36

Remove unused method

39a66e7

Move some LLVM specific options from Linker::link to cg_llvm

dc513a5

Move incr comp session dirs out of CodegenContext

414ab79

CodegenContext will in the future be serialized and deserialized into a different rustc instance when -Zno-link/-Zlink-only is used. A separate incr comp session has to be used for both rustc invocations.

Add test for incr comp fat LTO and fix assert_module_sources testing

728beca

Apply suggestion

7e5cd09

Co-authored-by: Bruno Kolenbrander <59372212+mejrs@users.noreply.github.com>

Fix typo

e672a1c

bjorn3 force-pushed the lto_refactors22 branch from ebdd423 to e672a1c Compare October 4, 2026 14:36

Revert "Improve work product tracking for fat LTO"

1244c1b

This reverts commit db2812a.

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rust-bors Bot pushed a commit that referenced this pull request Oct 4, 2026

Auto merge of #163317 - bjorn3:lto_refactors22, r=<try>

d20724a

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

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rust-bors Bot pushed a commit that referenced this pull request Oct 4, 2026

Auto merge of #163317 - bjorn3:lto_refactors22, r=<try>

22eb763

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

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Revert "Add test for incr comp fat LTO and fix assert_module_sources …

993bdd7

…testing" This reverts commit 728beca.

bjorn3 force-pushed the lto_refactors22 branch from 6dce75b to 993bdd7 Compare October 4, 2026 17:24

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rust-bors Bot pushed a commit that referenced this pull request Oct 4, 2026

Auto merge of #163317 - bjorn3:lto_refactors22, r=<try>

a640230

Fix incremental compilation for fat LTO try-job: test-x86_64-mingw-1

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