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staticlib with lto=fat now includes compiler_builtins as llvm bitcode after rustup update #146145

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staticlib with lto=fat now includes compiler_builtins as llvm bitcode after rustup update#146145
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rcvalle
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A-LTOArea: Link-time optimization (LTO)A-linkageArea: linking into static, shared libraries and binariesC-bugCategory: This is a bug.P-criticalCritical priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.

Description

@behdad
behdad
opened on Sep 3, 2025

In HarfBuzz we use cargo to build a staticlib, then link it to harfbuzz using meson. We use lto="fat" and that's essential to the performance of the generated rust code binary.

This used to work with gcc as the final linker with rustc 1.91.0-nightly (898aff7 2025-08-14), but after updating to rustc 1.91.0-nightly (51ff895 2025-09-02), now the generated libharfbuzz_rust.a includes: a harfbuzz_rust .o file that is an ELF binary, but also a compiler_builtins .o that is in LLVM IR bitcode. This makes the gcc linker fail linking with unknown file format error. Is this intentional?

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (51ff89506 2025-09-02)
binary: rustc
commit-hash: 51ff895062ba60a7cba53f57af928c3fb7b0f2f4
commit-date: 2025-09-02
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Activity

  1. behdad
    added
    C-bugCategory: This is a bug.
    on Sep 3, 2025
  2. rustbot
    added
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Sep 3, 2025

  3. Noratrieb commented on Sep 3, 2025

    @Noratrieb
    Noratrieb
    on Sep 3, 2025
    Member

    likely regressed in #145368, there's already been another issue about that PR #146109 @rcvalle @dianqk

  4. Noratrieb
    added
    A-linkageArea: linking into static, shared libraries and binaries
    regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    A-LTOArea: Link-time optimization (LTO)
    and removed
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Sep 3, 2025
  5. rustbot
    added
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on Sep 3, 2025

  6. lqd commented on Sep 3, 2025

    @lqd
    lqd
    on Sep 3, 2025
    Member

    If so, this should be fixed soon by the revert #146133

  7. apiraino commented on Sep 3, 2025

    @apiraino
    apiraino
    on Sep 3, 2025
    Contributor

    Assigning priority (discussion on Zulip).

    @rustbot label -I-prioritize +P-critical

  8. rustbot
    added
    P-criticalCritical priority
    and removed
    I-prioritizeIssue needs a team member to assess the impact. Will be replaced by P-{low,medium,high,critical}
    on Sep 3, 2025

  9. behdad commented on Sep 3, 2025

    @behdad
    behdad
    on Sep 3, 2025
    Author

    Note that even trying to do the final link with lld fails, with:

    ld.lld: error: src/rust/libharfbuzz_rust.a(compiler_builtins-e82ecba48a5a77b2.compiler_builtins.c83520de7ece3beb-cgu.00.rcgu.o): Unknown attribute kind (102) (Producer: 'LLVM21.1.0-rust-1.91.0-nightly' Reader: 'LLVM 19.1.7')

  10. bjorn3 commented on Sep 3, 2025

    @bjorn3
    bjorn3
    on Sep 3, 2025
    Member

    Is that the system lld or the rust-lld shipped with rustc? The system lld likely uses a too old LLVM version.

  11. 4 remaining items

  12. behdad commented on Sep 3, 2025

    @behdad
    behdad
    on Sep 3, 2025
    Author

    Does using lld together with a nightly rustc from before this regression also regress performance? Aka is the performance regression from using lld or from compiler-builtins participating in LTO.

    My bad. I was measuring with different codegen-units. Setting codegen-units=1 I see the same performance with all scenarios.

  13. rcvalle commented on Sep 3, 2025

    @rcvalle
    rcvalle
    on Sep 3, 2025
    Member

    @rustbot claim

  14. rustbot
    assigned
    rcvalle
    on Sep 3, 2025
  15. bors
    added a commit that references this issue on Sep 3, 2025

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    a1208bf
  16. github-actions
    added 2 commits that reference this issue on Sep 4, 2025

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    c6ec0de

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    94fc405

  17. rcvalle commented on Sep 4, 2025

    @rcvalle
    rcvalle
    on Sep 4, 2025
    Member

    Fixed in #146133.

  18. rcvalle
    closed this as completedon Sep 4, 2025
  19. rust-cloud-vms
    added a commit that references this issue on Oct 12, 2025

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    c1022eb
  20. makai410
    added 3 commits that reference this issue on Nov 8, 2025

    Auto merge of rust-lang#146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    3184d92

    Auto merge of rust-lang#146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    521a970

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    b52683c
  21. Kobzol
    added a commit that references this issue on Dec 21, 2025

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    b8a2607
  22. Kobzol
    added a commit that references this issue on Dec 29, 2025

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    d6ec1e2
  23. makai410
    added a commit that references this issue on Mar 27, 2026

    Auto merge of #146133 - rcvalle:rust-cfi-fix-145981, r=bjorn3

    98c17f2
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A-LTOArea: Link-time optimization (LTO)A-linkageArea: linking into static, shared libraries and binariesC-bugCategory: This is a bug.P-criticalCritical priorityT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.regression-from-stable-to-nightlyPerformance or correctness regression from stable to nightly.

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