[ThinLTO] Optionally ignore empty index file

Summary: In order to simplify distributed build system integration, where actions may be scheduled before the Thin Link which determines the list of objects selected by the linker. The gold plugin currently will emit 0-sized index files for objects not selected by the link, to enable checking for expected output files by the build system. If the build system then schedules a backend action for these bitcode files, we want to be able to fall back to normal compilation instead of failing. Fallback is enabled under an option in LLVM (D28410), in which case a nullptr is returned from llvm::getModuleSummaryIndexForFile. Clang can just proceed with non-ThinLTO compilation in that case. I am investigating whether this can be addressed in our build system, but that is a longer term fix and so this enables a workaround in the meantime. Reviewers: mehdi_amini Subscribers: cfe-commits Differential Revision: https://reviews.llvm.org/D28362 llvm-svn: 291303
llvm · Jan 6, 2017 · cffeb54 · cffeb54
1 parent b0d70f8
commit cffeb54
Showing 2 changed files with 29 additions and 16 deletions.
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
@@ -861,21 +861,8 @@ void EmitAssemblyHelper::EmitAssemblyWithNewPassManager(
   }
 }
 
-static void runThinLTOBackend(const CodeGenOptions &CGOpts, Module *M,
+static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
                               std::unique_ptr<raw_pwrite_stream> OS) {
-  // If we are performing a ThinLTO importing compile, load the function index
-  // into memory and pass it into thinBackend, which will run the function
-  // importer and invoke LTO passes.
-  Expected<std::unique_ptr<ModuleSummaryIndex>> IndexOrErr =
-      llvm::getModuleSummaryIndexForFile(CGOpts.ThinLTOIndexFile);
-  if (!IndexOrErr) {
-    logAllUnhandledErrors(IndexOrErr.takeError(), errs(),
-                          "Error loading index file '" +
-                              CGOpts.ThinLTOIndexFile + "': ");
-    return;
-  }
-  std::unique_ptr<ModuleSummaryIndex> CombinedIndex = std::move(*IndexOrErr);
-
   StringMap<std::map<GlobalValue::GUID, GlobalValueSummary *>>
       ModuleToDefinedGVSummaries;
   CombinedIndex->collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
@@ -961,8 +948,26 @@ void clang::EmitBackendOutput(DiagnosticsEngine &Diags,
                               BackendAction Action,
                               std::unique_ptr<raw_pwrite_stream> OS) {
   if (!CGOpts.ThinLTOIndexFile.empty()) {
-    runThinLTOBackend(CGOpts, M, std::move(OS));
-    return;
+    // If we are performing a ThinLTO importing compile, load the function index
+    // into memory and pass it into runThinLTOBackend, which will run the
+    // function importer and invoke LTO passes.
+    Expected<std::unique_ptr<ModuleSummaryIndex>> IndexOrErr =
+        llvm::getModuleSummaryIndexForFile(CGOpts.ThinLTOIndexFile);
+    if (!IndexOrErr) {
+      logAllUnhandledErrors(IndexOrErr.takeError(), errs(),
+                            "Error loading index file '" +
+                            CGOpts.ThinLTOIndexFile + "': ");
+      return;
+    }
+    std::unique_ptr<ModuleSummaryIndex> CombinedIndex = std::move(*IndexOrErr);
+    // A null CombinedIndex means we should skip ThinLTO compilation
+    // (LLVM will optionally ignore empty index files, returning null instead
+    // of an error).
+    bool DoThinLTOBackend = CombinedIndex != nullptr;
+    if (DoThinLTOBackend) {
+      runThinLTOBackend(CombinedIndex.get(), M, std::move(OS));
+      return;
+    }
   }
 
   EmitAssemblyHelper AsmHelper(Diags, HeaderOpts, CGOpts, TOpts, LOpts, M);

diff --git a/clang/test/CodeGen/thinlto_backend.ll b/clang/test/CodeGen/thinlto_backend.ll
@@ -12,6 +12,14 @@
 ; RUN: %clang -O2 -o %t4.o -x ir %t1.o -c -fthinlto-index=bad.thinlto.bc 2>&1 | FileCheck %s -check-prefix=CHECK-ERROR1
 ; CHECK-ERROR1: Error loading index file 'bad.thinlto.bc'
 
+; Ensure we ignore empty index file under -ignore-empty-index-file, and run
+; non-ThinLTO compilation which would not import f2
+; RUN: touch %t4.thinlto.bc
+; RUN: %clang -O2 -o %t4.o -x ir %t1.o -c -fthinlto-index=%t4.thinlto.bc -mllvm -ignore-empty-index-file
+; RUN: llvm-nm %t4.o | FileCheck --check-prefix=CHECK-OBJ-IGNORE-EMPTY %s
+; CHECK-OBJ-IGNORE-EMPTY: T f1
+; CHECK-OBJ-IGNORE-EMPTY: U f2
+
 ; Ensure f2 was imported
 ; RUN: %clang -target x86_64-unknown-linux-gnu -O2 -o %t3.o -x ir %t1.o -c -fthinlto-index=%t.thinlto.bc
 ; RUN: llvm-nm %t3.o | FileCheck --check-prefix=CHECK-OBJ %s