From c78f65775abf72ae6786b416d60436ff7c989d0b Mon Sep 17 00:00:00 2001
From: Rafael Espindola <rafael.espindola@gmail.com>
Date: Fri, 18 Mar 2011 05:12:38 +0000
Subject: [PATCH] Simplify the computation of undefined symbols. Instead of
 walking functions and initializers, just report the declarations present in
 the module.

The motivation is to open the way for using the lazy module parsing,
which should speed up clients that just want a symbol list (nm, ar).

This is slightly less precise, but since both -strip-dead-prototypes
and -globaldce are part of the standard pipeline, this shouldn't
change the result for clang/dragonegg produced binaries.

Any decl in an IL file was also put there because a FE expected it
to be necessary, so this should not be a problem for "-O0 -emit-llvm".

As a sanity check, I have bootstrapped clang on linux and built
firefox on both linux and darwin. A clang bootstrap on darwin
with LTO fails with or without this patch because, ironically,
the linker doesn't like the combination of dead_strip and LTO
when building libLTO.so :-)

llvm-svn: 127870
---
 llvm/tools/lto/LTOModule.cpp | 46 ++----------------------------------
 llvm/tools/lto/LTOModule.h   |  2 --
 2 files changed, 2 insertions(+), 46 deletions(-)

diff --git a/llvm/tools/lto/LTOModule.cpp b/llvm/tools/lto/LTOModule.cpp
index 4e9e74f49849..4cfe9ad166e1 100644
--- a/llvm/tools/lto/LTOModule.cpp
+++ b/llvm/tools/lto/LTOModule.cpp
@@ -178,16 +178,6 @@ void LTOModule::setTargetTriple(const char *triple) {
 void LTOModule::addDefinedFunctionSymbol(Function *f, Mangler &mangler) {
   // add to list of defined symbols
   addDefinedSymbol(f, mangler, true);
-
-  // add external symbols referenced by this function.
-  for (Function::iterator b = f->begin(); b != f->end(); ++b) {
-    for (BasicBlock::iterator i = b->begin(); i != b->end(); ++i) {
-      for (unsigned count = 0, total = i->getNumOperands();
-           count != total; ++count) {
-        findExternalRefs(i->getOperand(count), mangler);
-      }
-    }
-  }
 }
 
 // Get string that data pointer points to.
@@ -328,12 +318,6 @@ void LTOModule::addDefinedDataSymbol(GlobalValue *v, Mangler &mangler) {
       }
     }
   }
-
-  // add external symbols referenced by this data.
-  for (unsigned count = 0, total = v->getNumOperands();
-       count != total; ++count) {
-    findExternalRefs(v->getOperand(count), mangler);
-  }
 }
 
 
@@ -343,10 +327,6 @@ void LTOModule::addDefinedSymbol(GlobalValue *def, Mangler &mangler,
   if (def->getName().startswith("llvm."))
     return;
 
-  // ignore available_externally
-  if (def->hasAvailableExternallyLinkage())
-    return;
-
   // string is owned by _defines
   SmallString<64> Buffer;
   mangler.getNameWithPrefix(Buffer, def, false);
@@ -471,28 +451,6 @@ void LTOModule::addPotentialUndefinedSymbol(GlobalValue *decl,
 }
 
 
-
-// Find external symbols referenced by VALUE. This is a recursive function.
-void LTOModule::findExternalRefs(Value *value, Mangler &mangler) {
-  if (GlobalValue *gv = dyn_cast<GlobalValue>(value)) {
-    if (!gv->hasExternalLinkage())
-      addPotentialUndefinedSymbol(gv, mangler);
-    // If this is a variable definition, do not recursively process
-    // initializer.  It might contain a reference to this variable
-    // and cause an infinite loop.  The initializer will be
-    // processed in addDefinedDataSymbol().
-    return;
-  }
-
-  // GlobalValue, even with InternalLinkage type, may have operands with
-  // ExternalLinkage type. Do not ignore these operands.
-  if (Constant *c = dyn_cast<Constant>(value)) {
-    // Handle ConstantExpr, ConstantStruct, ConstantArry etc.
-    for (unsigned i = 0, e = c->getNumOperands(); i != e; ++i)
-      findExternalRefs(c->getOperand(i), mangler);
-  }
-}
-
 namespace {
   class RecordStreamer : public MCStreamer {
   public:
@@ -687,7 +645,7 @@ bool LTOModule::ParseSymbols() {
 
   // add functions
   for (Module::iterator f = _module->begin(); f != _module->end(); ++f) {
-    if (f->isDeclaration())
+    if (f->isDeclaration() || f->hasAvailableExternallyLinkage())
       addPotentialUndefinedSymbol(f, mangler);
     else
       addDefinedFunctionSymbol(f, mangler);
@@ -696,7 +654,7 @@ bool LTOModule::ParseSymbols() {
   // add data
   for (Module::global_iterator v = _module->global_begin(),
          e = _module->global_end(); v !=  e; ++v) {
-    if (v->isDeclaration())
+    if (v->isDeclaration() || v->hasAvailableExternallyLinkage())
       addPotentialUndefinedSymbol(v, mangler);
     else
       addDefinedDataSymbol(v, mangler);
diff --git a/llvm/tools/lto/LTOModule.h b/llvm/tools/lto/LTOModule.h
index 303151293b28..0b64a902f916 100644
--- a/llvm/tools/lto/LTOModule.h
+++ b/llvm/tools/lto/LTOModule.h
@@ -82,8 +82,6 @@ private:
                                                     bool isFunction);
     void                    addPotentialUndefinedSymbol(llvm::GlobalValue* decl, 
                                                         llvm::Mangler &mangler);
-    void                    findExternalRefs(llvm::Value* value, 
-                                                llvm::Mangler& mangler);
     void                    addDefinedFunctionSymbol(llvm::Function* f, 
                                                         llvm::Mangler &mangler);
     void                    addDefinedDataSymbol(llvm::GlobalValue* v,