[ThinLTO] Skip opt pipeline and summary wrapper pass on empty modules

Follow up to PR118508, to avoid unnecessary compile time for an empty
combind regular LTO module if all modules end up being ThinLTO only.

Author: teresajohnson

lld/test/ELF/lto/new-pass-manager.ll

@@ -9,3 +9,7 @@
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo() {
+  ret void
+}

llvm/lib/LTO/LTOBackend.cpp

@@ -351,6 +351,13 @@ static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
   MPM.run(Mod, MAM);
 }
 
+static bool isEmptyModule(const Module &Mod) {
+  // Module is empty if it has no functions, no globals, no inline asm and no
+  // named metadata (aliases and ifuncs require functions or globals so we
+  // don't need to check those explicitly).
+  return Mod.empty() && Mod.global_empty() && Mod.named_metadata_empty() && Mod.getModuleInlineAsm().empty();
+}
+
 bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
               bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
               const ModuleSummaryIndex *ImportSummary,
@@ -372,9 +379,16 @@ bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
                                /*EmbedBitcode*/ true, /*EmbedCmdline*/ true,
                                /*Cmdline*/ CmdArgs);
   }
-  // FIXME: Plumb the combined index into the new pass manager.
-  runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
-                 ImportSummary);
+  // No need to run any opt passes if the module is empty.
+  // In theory these passes should take almost no time for an empty
+  // module, however, this guards against doing any unnecessary summary-based
+  // analysis in the case of a ThinLTO build where this might be an empty
+  // regular LTO combined module, with a large combined index from ThinLTO.
+  if (!isEmptyModule(Mod)) {
+    // FIXME: Plumb the combined index into the new pass manager.
+    runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
+                   ImportSummary);
+  }
   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
 }
 
@@ -422,8 +436,14 @@ static void codegen(const Config &Conf, TargetMachine *TM,
   legacy::PassManager CodeGenPasses;
   TargetLibraryInfoImpl TLII(Triple(Mod.getTargetTriple()));
   CodeGenPasses.add(new TargetLibraryInfoWrapperPass(TLII));
-  CodeGenPasses.add(
-      createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
+  // No need to make index available if the module is empty.
+  // In theory these passes should not use the index for an empty
+  // module, however, this guards against doing any unnecessary summary-based
+  // analysis in the case of a ThinLTO build where this might be an empty
+  // regular LTO combined module, with a large combined index from ThinLTO.
+  if (!isEmptyModule(Mod))
+    CodeGenPasses.add(
+        createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
   if (Conf.PreCodeGenPassesHook)
     Conf.PreCodeGenPassesHook(CodeGenPasses);
   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,

llvm/test/Feature/load_plugin_error.ll

@@ -5,7 +5,7 @@
 
 ; RUN: opt %s -o %t.o
 ; RUN: not llvm-lto2 run -load-pass-plugin=%t/nonexistent.so %t.o -o %t \
-; RUN:     -r %t.o,test 2>&1 | \
+; RUN:     -r %t.o,test,plx 2>&1 | \
 ; RUN:   FileCheck %s
 
 ; CHECK: Could not load library {{.*}}nonexistent.so

llvm/test/Other/X86/lto-hot-cold-split.ll

@@ -10,3 +10,7 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; OLDPM-ANYLTO-POSTLINK-Os: HotColdSplittingPass
+
+define void @foo() {
+  ret void
+}