rustc_llvm: Emit module summaries when using -Clto=fat by erickt · Pull Request #159029 · rust-lang/rust

erickt · 2026-07-09T17:55:43Z

Currently, module summaries are only emitted with thin lto. If we would link full/fat lto'd rust code against lto'd c++ code built with CFI (or WPD), those passes would fail during the link step because the participating rust modules are missing module summaries. Rust code does not know at compile-time if it would be participating in some special link which may require module summaries, so this PR ensures module summaries are unconditionally emitted for full/fat lto, just like with thin lto.

The WriteBitcodeToFile function just invokes the normal BitcodeWriterPass under the hood, but doesn't provide a way to set the argument for emitting module summaries. So this patch just adds the pass directly and sets that argument.

This is a rebase of @PiJoules's #158099, which also should fix up the tests with the gcc tools.

rustbot · 2026-07-09T17:55:48Z

r? @cuviper

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PiJoules

Not a rust approver but LGTM from me with one nit

View changes since this review

PiJoules · 2026-07-09T18:01:23Z

cc @ilovepi

rustbot · 2026-07-10T22:33:35Z

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.

erickt · 2026-07-18T12:08:15Z

@rustbot ready

erickt · 2026-07-21T18:12:17Z

@rustbot ready

erickt · 2026-07-22T13:37:38Z

@bjorn3 - I think I did a force push and lost a comment from you. Does the latest version still have the concern you mentioned?

cuviper · 2026-07-23T19:23:11Z

I assume they deleted their own comment, but do speak up if that's still relevant.

Otherwise, it looks good to me!

@bors r+

rust-bors · 2026-07-23T19:23:14Z

📌 Commit fe44964 has been approved by cuviper

It is now in the queue for this repository.

bjorn3 · 2026-07-23T19:28:35Z

I don't fully remember what my comment was, but I did indeed delete it when I realized I was thinking wrong. I don't have any objections to this PR.

@PiJoules

…iper rustc_llvm: Emit module summaries when using -Clto=fat Currently, module summaries are only emitted with thin lto. If we would link full/fat lto'd rust code against lto'd c++ code built with CFI (or WPD), those passes would fail during the link step because the participating rust modules are missing module summaries. Rust code does not know at compile-time if it would be participating in some special link which may require module summaries, so this PR ensures module summaries are unconditionally emitted for full/fat lto, just like with thin lto. The WriteBitcodeToFile function just invokes the normal BitcodeWriterPass under the hood, but doesn't provide a way to set the argument for emitting module summaries. So this patch just adds the pass directly and sets that argument. This is a rebase of @PiJoules's rust-lang#158099, which also should fix up the tests with the gcc tools.

@PiJoules

…iper rustc_llvm: Emit module summaries when using -Clto=fat Currently, module summaries are only emitted with thin lto. If we would link full/fat lto'd rust code against lto'd c++ code built with CFI (or WPD), those passes would fail during the link step because the participating rust modules are missing module summaries. Rust code does not know at compile-time if it would be participating in some special link which may require module summaries, so this PR ensures module summaries are unconditionally emitted for full/fat lto, just like with thin lto. The WriteBitcodeToFile function just invokes the normal BitcodeWriterPass under the hood, but doesn't provide a way to set the argument for emitting module summaries. So this patch just adds the pass directly and sets that argument. This is a rebase of @PiJoules's rust-lang#158099, which also should fix up the tests with the gcc tools.

jhpratt · 2026-07-24T01:30:24Z

@bors r- #159803 (comment)

---- [run-make] tests/run-make/fat-lto-module-summary stdout ----

error: rmake recipe failed to complete
status: exit status: 1
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/fat-lto-module-summary/rmake_out" && env -u RUSTFLAGS -u __RUSTC_DEBUG_ASSERTIONS_ENABLED -u __STD_DEBUG_ASSERTIONS_ENABLED AR="arm-none-eabi-ar" BUILD_ROOT="/checkout/obj/build/x86_64-unknown-linux-gnu" CC="arm-none-eabi-gcc" CC_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -mthumb -march=armv6s-m -gz" CXX="arm-none-eabi-g++" CXX_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -mthumb -march=armv6s-m -gz" HOST_RUSTC_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/build/run_make_support/ca584b3561373ef1/out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" LD_LIB_PATH_ENVVAR="LD_LIBRARY_PATH" LLVM_BIN_DIR="/checkout/obj/build/x86_64-unknown-linux-gnu/llvm/bin" LLVM_COMPONENTS="aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils abi aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgputargetmca amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cas cfguard cgdata codegen codegentypes core coroutines coverage csky cskyasmparser cskycodegen cskydesc cskydisassembler cskyinfo debuginfobtf debuginfocodeview debuginfodwarf debuginfodwarflowlevel debuginfogsym debuginfologicalview debuginfomsf debuginfopdb demangle dlltooldriver dtlto dwarfcfichecker dwarflinker dwarflinkerclassic dwarflinkerparallel dwp engine executionengine extensions filecheck frontendatomic frontenddirective frontenddriver frontendhlsl frontendoffloading frontendopenacc frontendopenmp fuzzercli fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo hipstdpar instcombine instrumentation interfacestub interpreter ipo irprinter irreader jitlink libdriver lineeditor linker loongarch loongarchasmparser loongarchcodegen loongarchdesc loongarchdisassembler loongarchinfo lto m68k m68kasmparser m68kcodegen m68kdesc m68kdisassembler m68kinfo mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts objcopy object objectyaml option orcdebugging orcjit orcshared orctargetprocess passes plugins powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvtargetmca runtimedyld sandboxir scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support supportlsp symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target targetparser telemetry textapi textapibinaryreader transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo webassemblyutils windowsdriver windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86targetmca xray xtensa xtensaasmparser xtensacodegen xtensadesc xtensadisassembler xtensainfo" LLVM_FILECHECK="/checkout/obj/build/x86_64-unknown-linux-gnu/llvm/build/bin/FileCheck" NODE="/usr/bin/node" PYTHON="/usr/bin/python3" RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUSTC_LINKER="arm-none-eabi-gcc" RUSTDOC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" SOURCE_ROOT="/checkout" TARGET="thumbv6m-none-eabi" TARGET_EXE_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/thumbv6m-none-eabi/lib" __BOOTSTRAP_JOBS="4" __RMAKE_VERBOSE_SUBPROCESS_OUTPUT="1" __STD_REMAP_DEBUGINFO_ENABLED="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/fat-lto-module-summary/rmake"
stdout: none
--- stderr -------------------------------
command failed at line 4
LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/fat-lto-module-summary/rmake_out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib:/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/build/run_make_support/ca584b3561373ef1/out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/fat-lto-module-summary/rmake_out" "foo.rs" "--crate-type" "lib" "-Clto=fat" "--emit=llvm-bc" "--target=thumbv6m-none-eabi"
output status: `exit status: 1`
=== STDOUT ===



=== STDERR ===
error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv6m-none-eabi` target may not support the standard library
  = note: `std` is required by `<unknown>` because it does not declare `#![no_std]`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

error: cannot resolve a prelude import

error: aborting due to 2 previous errors

rust-bors · 2026-07-24T01:30:30Z

This pull request was unapproved.

This PR was contained in a rollup (#159803), which was unapproved.

View changes since this unapproval

Currently, module summaries are only emitted with thin lto. If we would link full/fat lto'd rust code against lto'd c++ code built with CFI (or WPD), those passes would fail during the link step because the participating rust modules are missing module summaries. Rust code does not know at compile-time if it would be participating in some special link which may require module summaries, so this PR ensures module summaries are unconditionally emitted for full/fat lto, just like with thin lto. The WriteBitcodeToFile function just invokes the normal BitcodeWriterPass under the hood, but doesn't provide a way to set the argument for emitting module summaries. So this patch just adds the pass directly and sets that argument. Finally, now that we're generating module summaries for `-Clto=fat` without using ThinLTO, we need to update `rustc-metadata` to keep around a temporary directory that contains the `.rmeta` for a little longer before we deserialize it.

erickt · 2026-07-27T19:22:01Z

Shucks, forgot to mark that test as #[no_std]. I think this latest version should fix it.

@bors try jobs=dist-various-1

@bors ready

rustc_llvm: Emit module summaries when using -Clto=fat try-job: dist-various-1

rust-bors · 2026-07-27T19:22:08Z

Unknown command "ready". Run @bors help or go to https://bors.rust-lang.org/help to see available commands.

rust-bors · 2026-07-27T20:53:47Z

☀️ Try build successful (CI)
Build commit: c08502d (c08502d6caf3719ef2957616d2a2b92a35100685)
Base parent: 4fefe36 (4fefe3656700e5528edddc2336667818917dfc70)

erickt · 2026-07-28T00:03:23Z

@rustbot ready

cuviper · 2026-08-05T17:24:02Z

@bors r+ rollup=iffy

rust-bors · 2026-08-05T17:24:05Z

📌 Commit a4bc26a has been approved by cuviper

It is now in the queue for this repository.

…uwer Rollup of 12 pull requests Successful merges: - #159014 ([rustdoc] Do not take `doc(cfg())` into account when filtering doctests) - #159029 (rustc_llvm: Emit module summaries when using -Clto=fat) - #160574 (Update rustc-perf submodule) - #154585 (treat no_mangle_generic_items as hard error instead of lint warning) - #159473 (Fix, simplify, and document doc meta finalize mode) - #157489 (Add CoerceShared field-wise reborrow WF checks) - #160532 (Add Enzyme bugfix to support rust+llvm23) - #160545 (Reorder the methods in `#[rustc_must_implement_one_of]`) - #160558 (Revert "codegen_ssa: no dbginfo for scalable vec local w/ `-O0`") - #160566 (Re-enable bool indexing assembly test for LLVM 23) - #160569 (Remove `OnDuplicate::Custom`) - #160576 ([rustdoc] Create output file after we checked that the standalone markdown file is valid)

@PiJoules

Rollup merge of #159029 - erickt:lto-module-summaries, r=cuviper rustc_llvm: Emit module summaries when using -Clto=fat Currently, module summaries are only emitted with thin lto. If we would link full/fat lto'd rust code against lto'd c++ code built with CFI (or WPD), those passes would fail during the link step because the participating rust modules are missing module summaries. Rust code does not know at compile-time if it would be participating in some special link which may require module summaries, so this PR ensures module summaries are unconditionally emitted for full/fat lto, just like with thin lto. The WriteBitcodeToFile function just invokes the normal BitcodeWriterPass under the hood, but doesn't provide a way to set the argument for emitting module summaries. So this patch just adds the pass directly and sets that argument. This is a rebase of @PiJoules's #158099, which also should fix up the tests with the gcc tools.

…uwer Rollup of 12 pull requests Successful merges: - rust-lang/rust#159014 ([rustdoc] Do not take `doc(cfg())` into account when filtering doctests) - rust-lang/rust#159029 (rustc_llvm: Emit module summaries when using -Clto=fat) - rust-lang/rust#160574 (Update rustc-perf submodule) - rust-lang/rust#154585 (treat no_mangle_generic_items as hard error instead of lint warning) - rust-lang/rust#159473 (Fix, simplify, and document doc meta finalize mode) - rust-lang/rust#157489 (Add CoerceShared field-wise reborrow WF checks) - rust-lang/rust#160532 (Add Enzyme bugfix to support rust+llvm23) - rust-lang/rust#160545 (Reorder the methods in `#[rustc_must_implement_one_of]`) - rust-lang/rust#160558 (Revert "codegen_ssa: no dbginfo for scalable vec local w/ `-O0`") - rust-lang/rust#160566 (Re-enable bool indexing assembly test for LLVM 23) - rust-lang/rust#160569 (Remove `OnDuplicate::Custom`) - rust-lang/rust#160576 ([rustdoc] Create output file after we checked that the standalone markdown file is valid)

rustbot assigned cuviper Jul 9, 2026

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rustc_llvm: Emit module summaries when using -Clto=fat #158099

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PiJoules mentioned this pull request Jul 20, 2026

Ensure full LTO'd bitcode goes through the normal LTO pipeline instead of the thinlto pipeline #159624

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Rollup of 23 pull requests #159793

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Rollup of 11 pull requests #159803

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rust-bors Bot pushed a commit that referenced this pull request Jul 27, 2026

Auto merge of #159029 - erickt:lto-module-summaries, r=<try>

c08502d

rustc_llvm: Emit module summaries when using -Clto=fat try-job: dist-various-1

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Rollup of 12 pull requests #160586

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rust-bors Bot merged commit 6a69014 into rust-lang:main Aug 6, 2026
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