From 2aa29ee7c150994d5be4acf9d13a678d97f07816 Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Sun, 20 Mar 2016 22:56:31 +0000
Subject: [PATCH] [COFF] Remove undefined behavior from
 ObjectFile::createWeakExternal

LLD type-punned an integral type and a pointer type using a pointer
field.  This is problematic because the pointer type has alignment
greater than some of the integral values.

This would be less problematic if a union was used but it turns out the
integral values are only present for a short, transient, amount of time.

Let's remove this undefined behavior by skipping the punning altogether
by storing the state in a separate memory location: a vector which
informs us which symbols to process for weak externs.

llvm-svn: 263918
---
 lld/COFF/InputFiles.cpp | 21 +++++++--------------
 lld/COFF/InputFiles.h   |  1 -
 2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
index bc866b61ed4d..8ff6b9ceb187 100644
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -149,7 +149,7 @@ void ObjectFile::initializeSymbols() {
   uint32_t NumSymbols = COFFObj->getNumberOfSymbols();
   SymbolBodies.reserve(NumSymbols);
   SparseSymbolBodies.resize(NumSymbols);
-  llvm::SmallVector<Undefined *, 8> WeakAliases;
+  llvm::SmallVector<std::pair<Undefined *, uint32_t>, 8> WeakAliases;
   int32_t LastSectionNumber = 0;
   for (uint32_t I = 0; I < NumSymbols; ++I) {
     // Get a COFFSymbolRef object.
@@ -167,8 +167,10 @@ void ObjectFile::initializeSymbols() {
     if (Sym.isUndefined()) {
       Body = createUndefined(Sym);
     } else if (Sym.isWeakExternal()) {
-      Body = createWeakExternal(Sym, AuxP);
-      WeakAliases.push_back((Undefined *)Body);
+      Body = createUndefined(Sym);
+      uint32_t TagIndex =
+          static_cast<const coff_aux_weak_external *>(AuxP)->TagIndex;
+      WeakAliases.emplace_back((Undefined *)Body, TagIndex);
     } else {
       Body = createDefined(Sym, AuxP, IsFirst);
     }
@@ -179,8 +181,8 @@ void ObjectFile::initializeSymbols() {
     I += Sym.getNumberOfAuxSymbols();
     LastSectionNumber = Sym.getSectionNumber();
   }
-  for (Undefined *U : WeakAliases)
-    U->WeakAlias = SparseSymbolBodies[(uintptr_t)U->WeakAlias];
+  for (auto WeakAlias : WeakAliases)
+    WeakAlias.first->WeakAlias = SparseSymbolBodies[WeakAlias.second];
 }
 
 Undefined *ObjectFile::createUndefined(COFFSymbolRef Sym) {
@@ -189,15 +191,6 @@ Undefined *ObjectFile::createUndefined(COFFSymbolRef Sym) {
   return new (Alloc) Undefined(Name);
 }
 
-Undefined *ObjectFile::createWeakExternal(COFFSymbolRef Sym, const void *AuxP) {
-  StringRef Name;
-  COFFObj->getSymbolName(Sym, Name);
-  auto *U = new (Alloc) Undefined(Name);
-  auto *Aux = (const coff_aux_weak_external *)AuxP;
-  U->WeakAlias = (Undefined *)(uintptr_t)Aux->TagIndex;
-  return U;
-}
-
 Defined *ObjectFile::createDefined(COFFSymbolRef Sym, const void *AuxP,
                                    bool IsFirst) {
   StringRef Name;
diff --git a/lld/COFF/InputFiles.h b/lld/COFF/InputFiles.h
index 6a263fbaddf6..76a13c987e19 100644
--- a/lld/COFF/InputFiles.h
+++ b/lld/COFF/InputFiles.h
@@ -147,7 +147,6 @@ private:
 
   Defined *createDefined(COFFSymbolRef Sym, const void *Aux, bool IsFirst);
   Undefined *createUndefined(COFFSymbolRef Sym);
-  Undefined *createWeakExternal(COFFSymbolRef Sym, const void *Aux);
 
   std::unique_ptr<COFFObjectFile> COFFObj;
   llvm::BumpPtrAllocator Alloc;