Change LLVM bitcode linking to use tree reduction to scale better

llvm/tools/gold/gold-plugin.cpp

Index: llvm/tools/gold/gold-plugin.cpp
===================================================================
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -99,6 +99,7 @@
 namespace options {
   enum generate_bc { BC_NO, BC_ALSO, BC_ONLY };
   static bool generate_api_file = false;
+  static bool gather_then_link = true; // @LOCALMOD
   static generate_bc generate_bc_file = BC_NO;
   static std::string bc_path;
   static std::string obj_path;
@@ -128,6 +129,10 @@
       triple = opt.substr(strlen("mtriple="));
     } else if (opt.startswith("obj-path=")) {
       obj_path = opt.substr(strlen("obj-path="));
+      // @LOCALMOD-BEGIN
+    } else if (opt == "no-gather-then-link") {
+      gather_then_link = false;
+      // @LOCALMOD-END
     } else if (opt == "emit-llvm") {
       generate_bc_file = BC_ONLY;
     } else if (opt == "also-emit-llvm") {
@@ -461,14 +466,20 @@
         DepLibs.push_back(soname);
       }
     } else {
-      lto_codegen_add_module(code_gen, M);
+      if (options::gather_then_link) {
+        lto_codegen_gather_module_for_link(code_gen, M);
+      } else {
+        lto_codegen_add_module(code_gen, M);
+      }
       cf.is_linked_in = true;
     }
   }
+
+  // With gather_then_link, the modules are disposed when linking.
+  if (!options::gather_then_link)
+    lto_module_dispose(M);
   // @LOCALMOD-END
 
-  lto_module_dispose(M);
-
   return LDPS_OK;
 }
 
@@ -480,6 +491,12 @@
   std::ofstream api_file;
   assert(code_gen);
 
+  // @LOCALMOD-BEGIN
+  if (options::gather_then_link) {
+    lto_codegen_link_gathered_modules_and_dispose(code_gen);
+  }
+  // @LOCALMOD-END
+
   if (options::generate_api_file) {
     api_file.open("apifile.txt", std::ofstream::out | std::ofstream::trunc);
     if (!api_file.is_open()) {