Remove -Zemit-thin-lto flag

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.

Author: bjorn3

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -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,
@@ -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

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2376,7 +2376,6 @@ unsafe extern "C" {
         VerifyIR: bool,
         LintIR: bool,
         ThinLTOBuffer: Option<&mut *mut ThinLTOBuffer>,
-        EmitThinLTO: bool,
         EmitThinLTOSummary: bool,
         MergeFunctions: bool,
         UnrollLoops: bool,

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -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
@@ -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

compiler/rustc_interface/src/tests.rs

@@ -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));

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -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,
@@ -808,7 +808,7 @@ extern "C" LLVMRustResult LLVMRustOptimize(
   }
 
   ModulePassManager MPM;
-  bool NeedThinLTOBufferPasses = EmitThinLTO;
+  bool NeedThinLTOBufferPasses = true;
   auto ThinLTOBuffer = std::make_unique<LLVMRustThinLTOBuffer>();
   raw_string_ostream ThinLTODataOS(ThinLTOBuffer->data);
   raw_string_ostream ThinLinkDataOS(ThinLTOBuffer->thin_link_data);
@@ -840,12 +840,8 @@ extern "C" LLVMRustResult LLVMRustOptimize(
           // 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));
           *ThinLTOBufferRef = ThinLTOBuffer.release();
           MPM.addPass(PB.buildModuleOptimizationPipeline(
               OptLevel, ThinOrFullLTOPhase::None));
@@ -870,6 +866,7 @@ extern "C" LLVMRustResult LLVMRustOptimize(
         break;
       case LLVMRustOptStage::FatLTO:
         MPM = PB.buildLTODefaultPipeline(OptLevel, nullptr);
+        NeedThinLTOBufferPasses = false;
         break;
       }
     }
@@ -895,9 +892,11 @@ 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) {
       MPM.addPass(ThinLTOBitcodeWriterPass(
           ThinLTODataOS, EmitThinLTOSummary ? &ThinLinkDataOS : nullptr));
     } else {

compiler/rustc_session/src/options.rs

@@ -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],

tests/run-make/issue-84395-lto-embed-bitcode/rmake.rs

@@ -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();