[ThinLTO] Handle -emit-llvm* in ThinLTO backends

Summary:
Use PreCodeGenModuleHook to invoke the correct writer when emitting LLVM
IR, returning false to skip codegen from within thinBackend.

Reviewers: pcc, mehdi_amini

Subscribers: Prazek, cfe-commits

Differential Revision: https://reviews.llvm.org/D31534

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299274 91177308-0d34-0410-b5e6-96231b3b80d8

Author: teresajohnson

lib/CodeGen/BackendUtil.cpp

@@ -994,9 +994,30 @@ static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
   Conf.MAttrs = TOpts.Features;
   Conf.RelocModel = getRelocModel(CGOpts);
   Conf.CGOptLevel = getCGOptLevel(CGOpts);
-  Conf.CGFileType = getCodeGenFileType(Action);
   initTargetOptions(Conf.Options, CGOpts, TOpts, LOpts, HeaderOpts);
   Conf.SampleProfile = std::move(SampleProfile);
+  switch (Action) {
+  case Backend_EmitNothing:
+    Conf.PreCodeGenModuleHook = [](size_t Task, const Module &Mod) {
+      return false;
+    };
+    break;
+  case Backend_EmitLL:
+    Conf.PreCodeGenModuleHook = [&](size_t Task, const Module &Mod) {
+      M->print(*OS, nullptr, CGOpts.EmitLLVMUseLists);
+      return false;
+    };
+    break;
+  case Backend_EmitBC:
+    Conf.PreCodeGenModuleHook = [&](size_t Task, const Module &Mod) {
+      WriteBitcodeToFile(M, *OS, CGOpts.EmitLLVMUseLists);
+      return false;
+    };
+    break;
+  default:
+    Conf.CGFileType = getCodeGenFileType(Action);
+    break;
+  }
   if (Error E = thinBackend(
           Conf, 0, AddStream, *M, *CombinedIndex, ImportList,
           ModuleToDefinedGVSummaries[M->getModuleIdentifier()], ModuleMap)) {

test/CodeGen/thinlto-emit-llvm.c

@@ -0,0 +1,10 @@
+// Test to ensure -emit-llvm and -emit-llvm-bc work when invoking the
+// ThinLTO backend path.
+// RUN: %clang -O2 %s -flto=thin -c -o %t.o
+// RUN: llvm-lto -thinlto -o %t %t.o
+// RUN: %clang_cc1 -O2 -x ir %t.o -fthinlto-index=%t.thinlto.bc -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -O2 -x ir %t.o -fthinlto-index=%t.thinlto.bc -emit-llvm-bc -o - | llvm-dis -o - | FileCheck %s
+
+// CHECK: define void @foo()
+void foo() {
+}