Restore "[ThinLTO] Emit individual index files for distributed backends"

This restores commit r268627:
    Summary:
    When launching ThinLTO backends in a distributed build (currently
    supported in gold via the thinlto-index-only plugin option), emit
    an individual index file for each backend process as described here:
    http://lists.llvm.org/pipermail/llvm-dev/2016-April/098272.html

    ...

    Differential Revision: http://reviews.llvm.org/D19556

Address msan failures by avoiding std::prev on map.end(), the
theory is that this is causing issues due to some known UB problems
in __tree.

llvm-svn: 269059

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
index 70da5b33a58..68d7a714b6b 100644
--- a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
+++ b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
@@ -720,6 +720,29 @@ void ThinLTOCodeGenerator::crossModuleImport(Module &TheModule,
 }
 
 /**
+ * Compute the list of summaries needed for importing into module.
+ */
+void ThinLTOCodeGenerator::gatherImportedSummariesForModule(
+    StringRef ModulePath, ModuleSummaryIndex &Index,
+    std::map<std::string, GVSummaryMapTy> &ModuleToSummariesForIndex) {
+  auto ModuleCount = Index.modulePaths().size();
+
+  // Collect for each module the list of function it defines (GUID -> Summary).
+  StringMap<GVSummaryMapTy> ModuleToDefinedGVSummaries(ModuleCount);
+  Index.collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
+
+  // Generate import/export list
+  StringMap<FunctionImporter::ImportMapTy> ImportLists(ModuleCount);
+  StringMap<FunctionImporter::ExportSetTy> ExportLists(ModuleCount);
+  ComputeCrossModuleImport(Index, ModuleToDefinedGVSummaries, ImportLists,
+                           ExportLists);
+
+  llvm::gatherImportedSummariesForModule(ModulePath, ModuleToDefinedGVSummaries,
+                                         ImportLists,
+                                         ModuleToSummariesForIndex);
+}
+
+/**
  * Perform internalization.
  */
 void ThinLTOCodeGenerator::internalize(Module &TheModule,