thinlto: only build summary file if needed

If we don't do this, some versions of LLVM (at least 17, experimentally)
will double-emit some error messages, which is how I noticed this. Given
that it seems to be costing some extra work, let's only request the
summary bitcode production if we'll actually bother writing it down,
otherwise skip it.

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -229,9 +229,9 @@ pub(crate) fn run_thin(
     thin_lto(cgcx, &dcx, modules, upstream_modules, cached_modules, &symbols_below_threshold)
 }
 
-pub(crate) fn prepare_thin(module: ModuleCodegen<ModuleLlvm>) -> (String, ThinBuffer) {
+pub(crate) fn prepare_thin(module: ModuleCodegen<ModuleLlvm>, emit_summary: bool) -> (String, ThinBuffer) {
     let name = module.name;
-    let buffer = ThinBuffer::new(module.module_llvm.llmod(), true);
+    let buffer = ThinBuffer::new(module.module_llvm.llmod(), true, emit_summary);
     (name, buffer)
 }
 
@@ -671,9 +671,9 @@ unsafe impl Send for ThinBuffer {}
 unsafe impl Sync for ThinBuffer {}
 
 impl ThinBuffer {
-    pub fn new(m: &llvm::Module, is_thin: bool) -> ThinBuffer {
+    pub fn new(m: &llvm::Module, is_thin: bool, emit_summary: bool) -> ThinBuffer {
         unsafe {
-            let buffer = llvm::LLVMRustThinLTOBufferCreate(m, is_thin);
+            let buffer = llvm::LLVMRustThinLTOBufferCreate(m, is_thin, emit_summary);
             ThinBuffer(buffer)
         }
     }

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -716,9 +716,8 @@ pub(crate) unsafe fn codegen(
             let _timer = cgcx
                 .prof
                 .generic_activity_with_arg("LLVM_module_codegen_make_bitcode", &*module.name);
-            let thin = ThinBuffer::new(llmod, config.emit_thin_lto);
+            let thin = ThinBuffer::new(llmod, config.emit_thin_lto, config.emit_thin_lto_index);
             let data = thin.data();
-            let index_data = thin.thin_link_data();
 
             if let Some(bitcode_filename) = bc_out.file_name() {
                 cgcx.prof.artifact_size(
@@ -728,7 +727,8 @@ pub(crate) unsafe fn codegen(
                 );
             }
 
-            if let Some(thin_link_bitcode_filename) = bc_index_out.file_name() {
+            if config.emit_thin_lto_index && let Some(thin_link_bitcode_filename) = bc_index_out.file_name() {
+                let index_data = thin.thin_link_data();
                 cgcx.prof.artifact_size(
                     "llvm_bitcode_summary",
                     thin_link_bitcode_filename.to_string_lossy(),

compiler/rustc_codegen_llvm/src/lib.rs

@@ -240,8 +240,8 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     ) -> Result<CompiledModule, FatalError> {
         back::write::codegen(cgcx, dcx, module, config)
     }
-    fn prepare_thin(module: ModuleCodegen<Self::Module>) -> (String, Self::ThinBuffer) {
-        back::lto::prepare_thin(module)
+    fn prepare_thin(module: ModuleCodegen<Self::Module>, emit_summary: bool) -> (String, Self::ThinBuffer) {
+        back::lto::prepare_thin(module, emit_summary)
     }
     fn serialize_module(module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer) {
         (module.name, back::lto::ModuleBuffer::new(module.module_llvm.llmod()))

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2350,7 +2350,7 @@ extern "C" {
     #[allow(improper_ctypes)]
     pub fn LLVMRustModuleInstructionStats(M: &Module, Str: &RustString);
 
-    pub fn LLVMRustThinLTOBufferCreate(M: &Module, is_thin: bool) -> &'static mut ThinLTOBuffer;
+    pub fn LLVMRustThinLTOBufferCreate(M: &Module, is_thin: bool, emit_summary: bool) -> &'static mut ThinLTOBuffer;
     pub fn LLVMRustThinLTOBufferFree(M: &'static mut ThinLTOBuffer);
     pub fn LLVMRustThinLTOBufferPtr(M: &ThinLTOBuffer) -> *const c_char;
     pub fn LLVMRustThinLTOBufferLen(M: &ThinLTOBuffer) -> size_t;

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -891,7 +891,7 @@ fn execute_optimize_work_item<B: ExtraBackendMethods>(
     match lto_type {
         ComputedLtoType::No => finish_intra_module_work(cgcx, module, module_config),
         ComputedLtoType::Thin => {
-            let (name, thin_buffer) = B::prepare_thin(module);
+            let (name, thin_buffer) = B::prepare_thin(module, false);
             if let Some(path) = bitcode {
                 fs::write(&path, thin_buffer.data()).unwrap_or_else(|e| {
                     panic!("Error writing pre-lto-bitcode file `{}`: {}", path.display(), e);

compiler/rustc_codegen_ssa/src/traits/write.rs

@@ -56,7 +56,7 @@ pub trait WriteBackendMethods: 'static + Sized + Clone {
         module: ModuleCodegen<Self::Module>,
         config: &ModuleConfig,
     ) -> Result<CompiledModule, FatalError>;
-    fn prepare_thin(module: ModuleCodegen<Self::Module>) -> (String, Self::ThinBuffer);
+    fn prepare_thin(module: ModuleCodegen<Self::Module>, want_summary: bool) -> (String, Self::ThinBuffer);
     fn serialize_module(module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer);
 }
 

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -1492,7 +1492,7 @@ struct LLVMRustThinLTOBuffer {
 };
 
 extern "C" LLVMRustThinLTOBuffer*
-LLVMRustThinLTOBufferCreate(LLVMModuleRef M, bool is_thin) {
+LLVMRustThinLTOBufferCreate(LLVMModuleRef M, bool is_thin, bool emit_summary) {
   auto Ret = std::make_unique<LLVMRustThinLTOBuffer>();
   {
     auto OS = raw_string_ostream(Ret->data);
@@ -1510,7 +1510,10 @@ LLVMRustThinLTOBufferCreate(LLVMModuleRef M, bool is_thin) {
         PB.registerLoopAnalyses(LAM);
         PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
         ModulePassManager MPM;
-        MPM.addPass(ThinLTOBitcodeWriterPass(OS, &ThinLinkOS));
+        // We only pass ThinLinkOS to be filled in if we want the summary,
+        // because otherwise LLVM does extra work and may double-emit some
+        // errors or warnings.
+        MPM.addPass(ThinLTOBitcodeWriterPass(OS, emit_summary ? &ThinLinkOS : nullptr));
         MPM.run(*unwrap(M), MAM);
       } else {
         WriteBitcodeToFile(*unwrap(M), OS);