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Merged
rust-bors merged 2 commits into from
Feb 21, 2026
Merged

Remove -Zemit-thin-lto flag #152527

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6366a69
Remove -Zemit-thin-lto flag
bjorn3 Feb 12, 2026
c51cd0e
Deduplicate some code in LLVMRustOptimize
bjorn3 Feb 12, 2026
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3 changes: 1 addition & 2 deletions compiler/rustc_codegen_llvm/src/back/write.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -786,7 +786,6 @@ pub(crate) unsafe fn llvm_optimize(
config.verify_llvm_ir,
config.lint_llvm_ir,
thin_lto_buffer,
config.emit_thin_lto,
config.emit_thin_lto_summary,
merge_functions,
unroll_loops,
Expand Down Expand Up @@ -1033,7 +1032,7 @@ pub(crate) fn codegen(
"LLVM_module_codegen_make_bitcode",
&*module.name,
);
ThinBuffer::new(llmod, config.emit_thin_lto)
ThinBuffer::new(llmod, cgcx.lto != Lto::Fat)
};
let data = thin.data();
let _timer = prof
Expand Down
1 change: 0 additions & 1 deletion compiler/rustc_codegen_llvm/src/llvm/ffi.rs
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Expand Up @@ -2376,7 +2376,6 @@ unsafe extern "C" {
VerifyIR: bool,
LintIR: bool,
ThinLTOBuffer: Option<&mut *mut ThinLTOBuffer>,
EmitThinLTO: bool,
EmitThinLTOSummary: bool,
MergeFunctions: bool,
UnrollLoops: bool,
Expand Down
4 changes: 0 additions & 4 deletions compiler/rustc_codegen_ssa/src/back/write.rs
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Expand Up @@ -101,7 +101,6 @@ pub struct ModuleConfig {
pub emit_ir: bool,
pub emit_asm: bool,
pub emit_obj: EmitObj,
pub emit_thin_lto: bool,
pub emit_thin_lto_summary: bool,

// Miscellaneous flags. These are mostly copied from command-line
Expand Down Expand Up @@ -212,9 +211,6 @@ impl ModuleConfig {
false
),
emit_obj,
// thin lto summaries prevent fat lto, so do not emit them if fat
// lto is requested. See PR #136840 for background information.
emit_thin_lto: sess.opts.unstable_opts.emit_thin_lto && sess.lto() != Lto::Fat,
emit_thin_lto_summary: if_regular!(
sess.opts.output_types.contains_key(&OutputType::ThinLinkBitcode),
false
Expand Down
1 change: 0 additions & 1 deletion compiler/rustc_interface/src/tests.rs
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Expand Up @@ -796,7 +796,6 @@ fn test_unstable_options_tracking_hash() {
tracked!(dwarf_version, Some(5));
tracked!(embed_metadata, false);
tracked!(embed_source, true);
tracked!(emit_thin_lto, false);
tracked!(emscripten_wasm_eh, false);
tracked!(export_executable_symbols, true);
tracked!(fewer_names, Some(true));
Expand Down
34 changes: 14 additions & 20 deletions compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
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Expand Up @@ -566,7 +566,7 @@ extern "C" LLVMRustResult LLVMRustOptimize(
LLVMModuleRef ModuleRef, LLVMTargetMachineRef TMRef,
LLVMRustPassBuilderOptLevel OptLevelRust, LLVMRustOptStage OptStage,
bool IsLinkerPluginLTO, bool NoPrepopulatePasses, bool VerifyIR,
bool LintIR, LLVMRustThinLTOBuffer **ThinLTOBufferRef, bool EmitThinLTO,
bool LintIR, LLVMRustThinLTOBuffer **ThinLTOBufferRef,
bool EmitThinLTOSummary, bool MergeFunctions, bool UnrollLoops,
bool SLPVectorize, bool LoopVectorize, bool DisableSimplifyLibCalls,
bool EmitLifetimeMarkers, registerEnzymeAndPassPipelineFn EnzymePtr,
Expand Down Expand Up @@ -808,44 +808,35 @@ extern "C" LLVMRustResult LLVMRustOptimize(
}

ModulePassManager MPM;
bool NeedThinLTOBufferPasses = EmitThinLTO;
bool NeedThinLTOBufferPasses = true;
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@bjorn3 bjorn3 Feb 12, 2026

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

auto ThinLTOBuffer = std::make_unique<LLVMRustThinLTOBuffer>();
raw_string_ostream ThinLTODataOS(ThinLTOBuffer->data);
raw_string_ostream ThinLinkDataOS(ThinLTOBuffer->thin_link_data);
bool IsLTO = OptStage == LLVMRustOptStage::ThinLTO ||
OptStage == LLVMRustOptStage::FatLTO;
if (!NoPrepopulatePasses) {
for (const auto &C : PipelineStartEPCallbacks)
PB.registerPipelineStartEPCallback(C);
for (const auto &C : OptimizerLastEPCallbacks)
PB.registerOptimizerLastEPCallback(C);

// The pre-link pipelines don't support O0 and require using
// buildO0DefaultPipeline() instead. At the same time, the LTO pipelines do
// support O0 and using them is required.
if (OptLevel == OptimizationLevel::O0 && !IsLTO) {
for (const auto &C : PipelineStartEPCallbacks)
PB.registerPipelineStartEPCallback(C);
for (const auto &C : OptimizerLastEPCallbacks)
PB.registerOptimizerLastEPCallback(C);

// We manually schedule ThinLTOBufferPasses below, so don't pass the value
// to enable it here.
MPM = PB.buildO0DefaultPipeline(OptLevel);
} else {
for (const auto &C : PipelineStartEPCallbacks)
PB.registerPipelineStartEPCallback(C);
for (const auto &C : OptimizerLastEPCallbacks)
PB.registerOptimizerLastEPCallback(C);

switch (OptStage) {
case LLVMRustOptStage::PreLinkNoLTO:
if (ThinLTOBufferRef) {
// This is similar to LLVM's `buildFatLTODefaultPipeline`, where the
// bitcode for embedding is obtained after performing
// `ThinLTOPreLinkDefaultPipeline`.
MPM.addPass(PB.buildThinLTOPreLinkDefaultPipeline(OptLevel));
if (EmitThinLTO) {
MPM.addPass(ThinLTOBitcodeWriterPass(
ThinLTODataOS, EmitThinLTOSummary ? &ThinLinkDataOS : nullptr));
} else {
MPM.addPass(BitcodeWriterPass(ThinLTODataOS));
}
MPM.addPass(ThinLTOBitcodeWriterPass(
ThinLTODataOS, EmitThinLTOSummary ? &ThinLinkDataOS : nullptr));
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@bjorn3 bjorn3 Feb 12, 2026

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PreLinkNoLTO is never used when fat LTO is enabled.

*ThinLTOBufferRef = ThinLTOBuffer.release();
MPM.addPass(PB.buildModuleOptimizationPipeline(
OptLevel, ThinOrFullLTOPhase::None));
Expand All @@ -870,6 +861,7 @@ extern "C" LLVMRustResult LLVMRustOptimize(
break;
case LLVMRustOptStage::FatLTO:
MPM = PB.buildLTODefaultPipeline(OptLevel, nullptr);
NeedThinLTOBufferPasses = false;
break;
}
}
Expand All @@ -895,9 +887,11 @@ extern "C" LLVMRustResult LLVMRustOptimize(
MPM.addPass(CanonicalizeAliasesPass());
MPM.addPass(NameAnonGlobalPass());
}
// For `-Copt-level=0`, ThinLTO, or LTO.
// For `-Copt-level=0`, and the pre-link fat/thin LTO stages.
if (ThinLTOBufferRef && *ThinLTOBufferRef == nullptr) {
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) {
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@bjorn3 bjorn3 Feb 12, 2026

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

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@cuviper cuviper Feb 16, 2026

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

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@bjorn3 bjorn3 Feb 16, 2026

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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.

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@cuviper cuviper Feb 17, 2026

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

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@bjorn3 bjorn3 Feb 17, 2026

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Done

MPM.addPass(ThinLTOBitcodeWriterPass(
ThinLTODataOS, EmitThinLTOSummary ? &ThinLinkDataOS : nullptr));
} else {
Expand Down
2 changes: 0 additions & 2 deletions compiler/rustc_session/src/options.rs
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Expand Up @@ -2335,8 +2335,6 @@ options! {
"embed source text in DWARF debug sections (default: no)"),
emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
"emit a section containing stack size metadata (default: no)"),
emit_thin_lto: bool = (true, parse_bool, [TRACKED],
"emit the bc module with thin LTO info (default: yes)"),
emscripten_wasm_eh: bool = (true, parse_bool, [TRACKED],
"Use WebAssembly error handling for wasm32-unknown-emscripten"),
enforce_type_length_limit: bool = (false, parse_bool, [TRACKED],
Expand Down
2 changes: 1 addition & 1 deletion tests/run-make/issue-84395-lto-embed-bitcode/rmake.rs
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Expand Up @@ -18,7 +18,7 @@ fn main() {
.arg("-Clinker-plugin-lto")
.arg(format!("-Clinker={}", env_var("CLANG")))
.arg("-Clink-arg=-Wl,--plugin-opt=-lto-embed-bitcode=optimized")
.arg("-Zemit-thin-lto=no")
.arg("-Clto=fat")
.run();

llvm_objcopy().dump_section(".llvmbc", "test.bc").arg("test").run();
Expand Down
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