1 files changed, 33 insertions, 2 deletions
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index b39e881a2e7..393213f0810 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -30,11 +30,42 @@
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/IPO/FunctionAttrs.h"
 #include "llvm/Transforms/Utils/Cloning.h"
-#include "llvm/Transforms/Utils/ModuleUtils.h"
 using namespace llvm;
 
 namespace {
 
+// Produce a unique identifier for this module by taking the MD5 sum of the
+// names of the module's strong external symbols. This identifier is
+// normally guaranteed to be unique, or the program would fail to link due to
+// multiply defined symbols.
+//
+// If the module has no strong external symbols (such a module may still have a
+// semantic effect if it performs global initialization), we cannot produce a
+// unique identifier for this module, so we return the empty string, which
+// causes the entire module to be written as a regular LTO module.
+std::string getModuleId(Module *M) {
+  MD5 Md5;
+  bool ExportsSymbols = false;
+  for (auto &GV : M->global_values()) {
+    if (GV.isDeclaration() || GV.getName().startswith("llvm.") ||
+        !GV.hasExternalLinkage())
+      continue;
+    ExportsSymbols = true;
+    Md5.update(GV.getName());
+    Md5.update(ArrayRef<uint8_t>{0});
+  }
+
+  if (!ExportsSymbols)
+    return "";
+
+  MD5::MD5Result R;
+  Md5.final(R);
+
+  SmallString<32> Str;
+  MD5::stringifyResult(R, Str);
+  return ("$" + Str).str();
+}
+
 // Promote each local-linkage entity defined by ExportM and used by ImportM by
 // changing visibility and appending the given ModuleId.
 void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId) {
@@ -206,7 +237,7 @@ void forEachVirtualFunction(Constant *C, function_ref<void(Function *)> Fn) {
 void splitAndWriteThinLTOBitcode(
     raw_ostream &OS, raw_ostream *ThinLinkOS,
     function_ref<AAResults &(Function &)> AARGetter, Module &M) {
-  std::string ModuleId = getUniqueModuleId(&M);
+  std::string ModuleId = getModuleId(&M);
   if (ModuleId.empty()) {
     // We couldn't generate a module ID for this module, just write it out as a
     // regular LTO module.