Revert localmod for bitcode linking speedup (different fix upstream).

Original localmod: https://codereview.chromium.org/10808021/
which fixes some quadratic behavior.

Xiaofei Wan fixed the quadratic behavior upstream differently, via
a series of commits around:
http://llvm.org/viewvc/llvm-project?view=revision&revision=181104

and it is actually faster:
~7 seconds to link pnacl-llc w/ just the upstream fix
~11 seconds to link with localmod
(used to be ~120 seconds w/ quadratic behavior)

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2883
TEST= trybots: http://chromegw.corp.google.com/i/tryserver.nacl/builders/nacl-toolchain-linux-pnacl-x86_64/builds/784
R=eliben@chromium.org

Review URL: https://codereview.chromium.org/22509002

5 files changed, 2 insertions, 103 deletions

diff --git a/tools/gold/gold-plugin.cpp b/tools/gold/gold-plugin.cpp
index 1e96762d10..2ea45dd9d3 100644
--- a/tools/gold/gold-plugin.cpp
+++ b/tools/gold/gold-plugin.cpp
@@ -96,7 +96,6 @@ namespace {
 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;
@@ -126,10 +125,6 @@ namespace options {
       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") {
@@ -463,19 +458,12 @@ static ld_plugin_status claim_file_hook(const ld_plugin_input_file *file,
         DepLibs.push_back(soname);
       }
     } else {
-      if (options::gather_then_link) {
-        lto_codegen_gather_module_for_link(code_gen, M);
-      } else {
-        lto_codegen_add_module(code_gen, M);
-      }
+      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;
 }
@@ -488,12 +476,6 @@ static ld_plugin_status all_symbols_read_hook(void) {
   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()) {
diff --git a/tools/lto/LTOCodeGenerator.cpp b/tools/lto/LTOCodeGenerator.cpp
index 5a597d001d..b716da5707 100644
--- a/tools/lto/LTOCodeGenerator.cpp
+++ b/tools/lto/LTOCodeGenerator.cpp
@@ -98,68 +98,6 @@ bool LTOCodeGenerator::addModule(LTOModule* mod, std::string& errMsg) {
   return ret;
 }
 
-// @LOCALMOD-BEGIN
-/// Add a module that will be merged with the final output module.
-/// The merging does not happen until linkGatheredModulesAndDispose().
-void LTOCodeGenerator::gatherModuleForLinking(LTOModule* mod) {
-  _gatheredModules.push_back(mod);
-}
-
-/// Merge all modules gathered from gatherModuleForLinking(), and
-/// destroy the source modules in the process.
-bool LTOCodeGenerator::linkGatheredModulesAndDispose(std::string& errMsg) {
-
-  // We gather the asm undefs earlier than addModule() does,
-  // since we delete the modules during linking, and would not be
-  // able to do this after linking.  The undefs vector contain lists
-  // of global variable names which are considered "used", which will be
-  // appended into the "llvm.compiler.used" list.  The names must be the
-  // same before linking as they are after linking, since we have switched
-  // the order.
-  for (unsigned i = 0, ei = _gatheredModules.size(); i != ei; ++i) {
-    const std::vector<const char*> &undefs =
-        _gatheredModules[i]->getAsmUndefinedRefs();
-    for (int j = 0, ej = undefs.size(); j != ej; ++j) {
-      _asmUndefinedRefs[undefs[j]] = 1;
-    }
-  }
-
-  // Tree-reduce the mods, re-using the incoming mods as scratch
-  // intermediate results.  Module i is linked with (i + stride), with i as
-  // the dest.  We begin with a stride of 1, and double each time.  E.g.,
-  // after the first round, only the even-indexed modules are still available,
-  // and after the second, only those with index that are a multiple of 4
-  // are available.  Eventually the Module with the content of all other modules
-  // will be Module 0.
-  // NOTE: we may be able to be smarter about linking if we did not do them
-  // pairwise using Linker::LinkModules.  We also disregard module sizes
-  // and try our best to keep the modules in order (linking adjacent modules).
-  for (unsigned stride = 1, len = _gatheredModules.size();
-       stride < len;
-       stride *= 2) {
-    for (unsigned i = 0; i + stride < len; i = i + (stride * 2)) {
-      if (Linker::LinkModules(_gatheredModules[i]->getLLVVMModule(),
-                              _gatheredModules[i+stride]->getLLVVMModule(),
-                              Linker::DestroySource, &errMsg)) {
-        errs() << "LinkModules " << i << " w/ " << i + stride << " failed...\n";
-        // We leak the memory in this case...
-        return true;
-      }
-      delete _gatheredModules[i+stride];
-    }
-  }
-
-  // Finally, link Node 0 with the Dest and delete Node 0.
-  if (_linker.linkInModule(_gatheredModules[0]->getLLVVMModule(), &errMsg)) {
-    errs() << "LinkModules Dst w/ _gatheredModules[0] failed...\n";
-    return true;
-  }
-  delete _gatheredModules[0];
-
-  return false;
-}
-// @LOCALMOD-END
-
 bool LTOCodeGenerator::setDebugInfo(lto_debug_model debug,
                                     std::string& errMsg) {
   switch (debug) {
diff --git a/tools/lto/LTOCodeGenerator.h b/tools/lto/LTOCodeGenerator.h
index 4cc3928340..5a78488e29 100644
--- a/tools/lto/LTOCodeGenerator.h
+++ b/tools/lto/LTOCodeGenerator.h
@@ -41,12 +41,6 @@ struct LTOCodeGenerator {
   ~LTOCodeGenerator();
 
   bool addModule(struct LTOModule*, std::string &errMsg);
-  // @LOCALMOD-BEGIN
-  // Alternative methods of adding modules, which delay merging modules until
-  // all modules are available.
-  void gatherModuleForLinking(struct LTOModule*);
-  bool linkGatheredModulesAndDispose(std::string &errMsg);
-  // @LOCALMOD-END
   bool setDebugInfo(lto_debug_model, std::string &errMsg);
   bool setCodePICModel(lto_codegen_model, std::string &errMsg);
 
@@ -94,9 +88,6 @@ private:
   std::vector<char*>          _codegenOptions;
   std::string                 _mCpu;
   std::string                 _nativeObjectPath;
-
-  // @LOCALMOD
-  std::vector<LTOModule*> _gatheredModules;
 };
 
 #endif // LTO_CODE_GENERATOR_H
diff --git a/tools/lto/lto.cpp b/tools/lto/lto.cpp
index 1915acbdae..11d54c6a9d 100644
--- a/tools/lto/lto.cpp
+++ b/tools/lto/lto.cpp
@@ -204,16 +204,6 @@ bool lto_codegen_add_module(lto_code_gen_t cg, lto_module_t mod) {
   return cg->addModule(mod, sLastErrorString);
 }
 
-// @LOCALMOD-BEGIN
-void lto_codegen_gather_module_for_link(lto_code_gen_t cg, lto_module_t mod) {
-  cg->gatherModuleForLinking(mod);
-}
-
-bool lto_codegen_link_gathered_modules_and_dispose(lto_code_gen_t cg) {
-  return cg->linkGatheredModulesAndDispose(sLastErrorString);
-}
-// @LOCALMOD-END
-
 /// lto_codegen_set_debug_model - Sets what if any format of debug info should
 /// be generated. Returns true on error (check lto_get_error_message() for
 /// details).
diff --git a/tools/lto/lto.exports b/tools/lto/lto.exports
index 10d2fe03f6..596d2d6ec3 100644
--- a/tools/lto/lto.exports
+++ b/tools/lto/lto.exports
@@ -22,8 +22,6 @@ lto_module_is_object_file_in_memory
 lto_module_is_object_file_in_memory_for_target
 lto_module_dispose
 lto_codegen_add_module
-lto_codegen_gather_module_for_link
-lto_codegen_link_gathered_modules_and_dispose
 lto_codegen_add_must_preserve_symbol
 lto_codegen_compile
 lto_codegen_create