[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291303 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/BackendUtil.cpp b/lib/CodeGen/BackendUtil.cpp
index aa08a454e4e0ac3119d2f9ffe57346c3b56612ed..d2ce6ea48e41997d59802b52eb51209a5846fc27 100644 (file)
--- a/lib/CodeGen/BackendUtil.cpp
+++ b/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/test/CodeGen/thinlto_backend.ll b/test/CodeGen/thinlto_backend.ll
index 89f4fc407fbce7eba6494af7efb5cf2db8b8a59e..0a5a685457e985cf0c88add564a669bb22cd5ae6 100644 (file)
--- a/test/CodeGen/thinlto_backend.ll
+++ b/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