Set PrepareForThinLTO flag when using ThinLTO

The LLVM PassManager has a PrepareForThinLTO flag, which is intended
when compilation occurs in conjunction with linking by ThinLTO. The
flag has two effects:

 * The NameAnonGlobal pass is run after all other passes, which
   ensures that all globals have a name.
 * In optimized builds, a number of late passes (mainly related to
   vectorization and unrolling) are disabled, on the rationale that
   these a) will increase codesize of the intermediate artifacts
   and b) will be run by ThinLTO again anyway.

This patch enables the use of PrepareForThinLTO if Thin or ThinLocal
linking is used.

The background for this change is the CI failure in #49479, which
we assume to be caused by the NameAnonGlobal pass not being run.
As this changes which passes LLVM runs, this might have performance
(or other) impact, so we want to land this separately.

src/librustc_llvm/ffi.rs

@@ -1651,6 +1651,7 @@ extern "C" {
                                                MergeFunctions: bool,
                                                SLPVectorize: bool,
                                                LoopVectorize: bool,
+                                               PrepareForThinLTO: bool,
                                                PGOGenPath: *const c_char,
                                                PGOUsePath: *const c_char);
     pub fn LLVMRustAddLibraryInfo(PM: PassManagerRef,

src/librustc_trans/back/lto.rs

@@ -482,7 +482,7 @@ fn run_pass_manager(cgcx: &CodegenContext,
             llvm::CodeGenOptLevel::None => llvm::CodeGenOptLevel::Less,
             level => level,
         };
-        with_llvm_pmb(llmod, config, opt_level, &mut |b| {
+        with_llvm_pmb(llmod, config, opt_level, false, &mut |b| {
             if thin {
                 if !llvm::LLVMRustPassManagerBuilderPopulateThinLTOPassManager(b, pm) {
                     panic!("this version of LLVM does not support ThinLTO");

src/librustc_trans/back/write.rs

@@ -547,7 +547,8 @@ unsafe fn optimize(cgcx: &CodegenContext,
             llvm::LLVMRustAddAnalysisPasses(tm, fpm, llmod);
             llvm::LLVMRustAddAnalysisPasses(tm, mpm, llmod);
             let opt_level = config.opt_level.unwrap_or(llvm::CodeGenOptLevel::None);
-            with_llvm_pmb(llmod, &config, opt_level, &mut |b| {
+            let prepare_for_thin_lto = cgcx.lto == Lto::Thin || cgcx.lto == Lto::ThinLocal;
+            with_llvm_pmb(llmod, &config, opt_level, prepare_for_thin_lto, &mut |b| {
                 llvm::LLVMPassManagerBuilderPopulateFunctionPassManager(b, fpm);
                 llvm::LLVMPassManagerBuilderPopulateModulePassManager(b, mpm);
             })
@@ -2042,6 +2043,7 @@ pub fn run_assembler(cgcx: &CodegenContext, handler: &Handler, assembly: &Path,
 pub unsafe fn with_llvm_pmb(llmod: ModuleRef,
                             config: &ModuleConfig,
                             opt_level: llvm::CodeGenOptLevel,
+                            prepare_for_thin_lto: bool,
                             f: &mut FnMut(llvm::PassManagerBuilderRef)) {
     use std::ptr;
 
@@ -2069,6 +2071,7 @@ pub unsafe fn with_llvm_pmb(llmod: ModuleRef,
         config.merge_functions,
         config.vectorize_slp,
         config.vectorize_loop,
+        prepare_for_thin_lto,
         pgo_gen_path.as_ref().map_or(ptr::null(), |s| s.as_ptr()),
         pgo_use_path.as_ref().map_or(ptr::null(), |s| s.as_ptr()),
     );

src/rustllvm/PassWrapper.cpp

@@ -428,13 +428,16 @@ extern "C" void LLVMRustAddAnalysisPasses(LLVMTargetMachineRef TM,
 
 extern "C" void LLVMRustConfigurePassManagerBuilder(
     LLVMPassManagerBuilderRef PMBR, LLVMRustCodeGenOptLevel OptLevel,
-    bool MergeFunctions, bool SLPVectorize, bool LoopVectorize,
+    bool MergeFunctions, bool SLPVectorize, bool LoopVectorize, bool PrepareForThinLTO,
     const char* PGOGenPath, const char* PGOUsePath) {
   // Ignore mergefunc for now as enabling it causes crashes.
   // unwrap(PMBR)->MergeFunctions = MergeFunctions;
   unwrap(PMBR)->SLPVectorize = SLPVectorize;
   unwrap(PMBR)->OptLevel = fromRust(OptLevel);
   unwrap(PMBR)->LoopVectorize = LoopVectorize;
+#if LLVM_VERSION_GE(4, 0)
+  unwrap(PMBR)->PrepareForThinLTO = PrepareForThinLTO;
+#endif
 
 #ifdef PGO_AVAILABLE
   if (PGOGenPath) {