[LTO] Fix handling of mixed (regular and thin) mode LTO

[tejohnson]

Summary

In runThinLTO we start the task numbering for ThinLTO backend
tasks depending on whether there was also a regular LTO object
(CombinedModule). However, the CombinedModule is moved at
the end of runRegularLTO, so we need to save this information and
pass it into runThinLTO. Otherwise the AddOutput callback to the client
will use the same task number for both the regular LTO object
and the first ThinLTO object, which in gold-plugin caused only
one to be end up in the output filename array and therefore passed
back to gold for the final native link.

[mehdi_amini]

Can you write the test with llvm-lto2?

[tejohnson]

Add llvm-lto2 test

[mehdi_amini]

LGTM

include/llvm/LTO/LTO.h

void addSymbolToGlobalRes(object::IRObjectFile *Obj,
                             unsigned Partition);
 
   Error addRegularLTO(std::unique_ptr<InputFile> Input,
                       ArrayRef<SymbolResolution> Res);
   Error addThinLTO(std::unique_ptr<InputFile> Input,
                    ArrayRef<SymbolResolution> Res);
 
   Error runRegularLTO(AddOutputFn AddOutput);
-  Error runThinLTO(AddOutputFn AddOutput);
+  Error runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO);
 
   mutable bool CalledGetMaxTasks = false;
 };
 
 /// The resolution for a symbol. The linker must provide a SymbolResolution for
 /// each global symbol based on its internal resolution of that symbol.
 struct SymbolResolution {
   SymbolResolution()

lib/LTO/LTO.cpp

 }
 
 unsigned LTO::getMaxTasks() const {
   CalledGetMaxTasks = true;
   return RegularLTO.ParallelCodeGenParallelismLevel + ThinLTO.ModuleMap.size();
 }
 
 Error LTO::run(AddOutputFn AddOutput) {
+  // Save the status of having a regularLTO combined module, as
+  // this is needed for generating the ThinLTO Task ID, and
+  // the CombinedModule will be moved at the end of runRegularLTO.
+  bool HasRegularLTO = RegularLTO.CombinedModule != nullptr;
   // Invoke regular LTO if there was a regular LTO module to start with.
-  if (RegularLTO.CombinedModule)
+  if (HasRegularLTO)
     if (auto E = runRegularLTO(AddOutput))
       return E;
-  return runThinLTO(AddOutput);
+  return runThinLTO(AddOutput, HasRegularLTO);
 }
 
 Error LTO::runRegularLTO(AddOutputFn AddOutput) {
   // Make sure commons have the right size/alignment: we kept the largest from
   // all the prevailing when adding the inputs, and we apply it here.
   const DataLayout &DL = RegularLTO.CombinedModule->getDataLayout();
   for (auto &I : RegularLTO.Commons) {
     if (!I.second.Prevailing)
return [=](Config &Conf, ModuleSummaryIndex &CombinedIndex,
              const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
              AddOutputFn AddOutput) {
     return llvm::make_unique<WriteIndexesThinBackend>(
         Conf, CombinedIndex, ModuleToDefinedGVSummaries, OldPrefix, NewPrefix,
         ShouldEmitImportsFiles, LinkedObjectsFile);
   };
 }
 
-Error LTO::runThinLTO(AddOutputFn AddOutput) {
+Error LTO::runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO) {
   if (ThinLTO.ModuleMap.empty())
     return Error();
 
   if (Conf.CombinedIndexHook && !Conf.CombinedIndexHook(ThinLTO.CombinedIndex))
     return Error();
 
   // Collect for each module the list of function it defines (GUID ->
   // Summary).
Error LTO::runThinLTO(AddOutputFn AddOutput, bool HasRegularLTO) {
   std::unique_ptr<ThinBackendProc> BackendProc = ThinLTO.Backend(
       Conf, ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries, AddOutput);
 
   // Partition numbers for ThinLTO jobs start at 1 (see comments for
   // GlobalResolution in LTO.h). Task numbers, however, start at
   // ParallelCodeGenParallelismLevel if an LTO module is present, as tasks 0
   // through ParallelCodeGenParallelismLevel-1 are reserved for parallel code
   // generation partitions.
-  unsigned Task = RegularLTO.CombinedModule
-                      ? RegularLTO.ParallelCodeGenParallelismLevel
-                      : 0;
+  unsigned Task =
+      HasRegularLTO ? RegularLTO.ParallelCodeGenParallelismLevel : 0;
   unsigned Partition = 1;
 
   for (auto &Mod : ThinLTO.ModuleMap) {
     if (Error E = BackendProc->start(Task, Mod.second, ImportLists[Mod.first],
                                      ExportLists[Mod.first],
                                      ResolvedODR[Mod.first], ThinLTO.ModuleMap))
       return E;
 
     ++Task;
     ++Partition;
   }
 
   return BackendProc->wait();
 }

test/tools/gold/X86/Inputs/mixed_lto.ll

+target triple = "x86_64-unknown-linux-gnu"
+declare i32 @g()
+define i32 @main() {
+  call i32 @g()
+  ret i32 0
+}

test/tools/gold/X86/mixed_lto.ll

+; Test mixed-mode LTO (mix of regular and thin LTO objects)
+; RUN: opt %s -o %t.o
+; RUN: opt -module-summary %p/Inputs/mixed_lto.ll -o %t2.o
+
+; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:     -shared \
+; RUN:     --plugin-opt=thinlto \
+; RUN:     --plugin-opt=-import-instr-limit=0 \
+; RUN:     -o %t3.o %t2.o %t.o
+; RUN: nm %t3.o | FileCheck %s
+
+; CHECK-DAG: T main
+; CHECK-DAG: T g
+
+target triple = "x86_64-unknown-linux-gnu"
+define i32 @g() {
+  ret i32 0
+}