[LTO] Store target attributes as vector of strings.

[fhahn]

Summary

The target features are obtained as a list of features/attributes.
Instead of storing them in a single string, store the vector. This
matches lto::Config's behavior and simplifies the transition to
lto::backend().

[tejohnson]

lgtm

llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h

struct LTOCodeGenerator {
   void setDebugInfo(lto_debug_model);
   void setCodePICModel(Optional<Reloc::Model> Model) { RelocModel = Model; }
 
   /// Set the file type to be emitted (assembly or object code).
   /// The default is CGFT_ObjectFile.
   void setFileType(CodeGenFileType FT) { FileType = FT; }
 
   void setCpu(StringRef MCpu) { this->MCpu = std::string(MCpu); }
-  void setAttr(StringRef MAttr) { this->MAttr = std::string(MAttr); }
+  void setAttrs(std::vector<std::string> MAttrs) { this->MAttrs = MAttrs; }
   void setOptLevel(unsigned OptLevel);
 
   void setShouldInternalize(bool Value) { ShouldInternalize = Value; }
   void setShouldEmbedUselists(bool Value) { ShouldEmbedUselists = Value; }
 
   /// Restore linkage of globals
   ///
   /// When set, the linkage of globals will be restored prior to code
private:
   bool HasVerifiedInput = false;
   Optional<Reloc::Model> RelocModel;
   StringSet<> MustPreserveSymbols;
   StringSet<> AsmUndefinedRefs;
   StringMap<GlobalValue::LinkageTypes> ExternalSymbols;
   std::vector<std::string> CodegenOptions;
   std::string FeatureStr;
   std::string MCpu;
-  std::string MAttr;
+  std::vector<std::string> MAttrs;
   std::string NativeObjectPath;
   TargetOptions Options;
   CodeGenOpt::Level CGOptLevel = CodeGenOpt::Default;
   const Target *MArch = nullptr;
   std::string TripleStr;
   unsigned OptLevel = 2;
   lto_diagnostic_handler_t DiagHandler = nullptr;
   void *DiagContext = nullptr;

llvm/lib/LTO/LTOCodeGenerator.cpp

bool LTOCodeGenerator::determineTarget() {
   MArch = TargetRegistry::lookupTarget(TripleStr, ErrMsg);
   if (!MArch) {
     emitError(ErrMsg);
     return false;
   }
 
   // Construct LTOModule, hand over ownership of module and target. Use MAttr as
   // the default set of features.
-  SubtargetFeatures Features(MAttr);
+  SubtargetFeatures Features(join(MAttrs, ""));
   Features.getDefaultSubtargetFeatures(Triple);
   FeatureStr = Features.getString();
   // Set a default CPU for Darwin triples.
   if (MCpu.empty() && Triple.isOSDarwin()) {
     if (Triple.getArch() == llvm::Triple::x86_64)
       MCpu = "core2";
     else if (Triple.getArch() == llvm::Triple::x86)
       MCpu = "yonah";

llvm/tools/llvm-lto/llvm-lto.cpp

int main(int argc, char **argv) {
   // Add all the dso symbols to the table of symbols to expose.
   for (unsigned i = 0; i < KeptDSOSyms.size(); ++i)
     CodeGen.addMustPreserveSymbol(KeptDSOSyms[i]);
 
   // Set cpu and attrs strings for the default target/subtarget.
   CodeGen.setCpu(codegen::getMCPU().c_str());
 
   CodeGen.setOptLevel(OptLevel - '0');
-
-  auto MAttrs = codegen::getMAttrs();
-  if (!MAttrs.empty()) {
-    std::string attrs = join(MAttrs, ",");
-    CodeGen.setAttr(attrs);
-  }
+  CodeGen.setAttrs(codegen::getMAttrs());
 
   if (auto FT = codegen::getExplicitFileType())
     CodeGen.setFileType(FT.getValue());
 
   if (!OutputFilename.empty()) {
     if (SaveLinkedModuleFile) {
       std::string ModuleFilename = OutputFilename;
       ModuleFilename += ".linked.bc";

llvm/tools/lto/lto.cpp

 
 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(LibLTOCodeGenerator, lto_code_gen_t)
 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(ThinLTOCodeGenerator, thinlto_code_gen_t)
 DEFINE_SIMPLE_CONVERSION_FUNCTIONS(LTOModule, lto_module_t)
 
 // Convert the subtarget features into a string to pass to LTOCodeGenerator.
 static void lto_add_attrs(lto_code_gen_t cg) {
   LTOCodeGenerator *CG = unwrap(cg);
-  auto MAttrs = codegen::getMAttrs();
-  if (!MAttrs.empty()) {
-    std::string attrs = join(MAttrs, ",");
-    CG->setAttr(attrs);
-  }
+  CG->setAttrs(codegen::getMAttrs());
 
   if (OptLevel < '0' || OptLevel > '3')
     report_fatal_error("Optimization level must be between 0 and 3");
   CG->setOptLevel(OptLevel - '0');
   CG->setFreestanding(EnableFreestanding);
 }
 
 extern const char* lto_get_version() {