Index: llvm/include/llvm/Object/IRSymtab.h
===================================================================
--- llvm/include/llvm/Object/IRSymtab.h
+++ llvm/include/llvm/Object/IRSymtab.h
@@ -59,6 +59,9 @@
 /// table.
 struct Module {
   Word Begin, End;
+
+  /// The index of the first Uncommon for this Module.
+  Word UncBegin;
 };
 
 /// This is equivalent to an IR comdat.
@@ -82,7 +85,8 @@
   Word Flags;
   enum FlagBits {
     FB_visibility, // 2 bits
-    FB_undefined = FB_visibility + 2,
+    FB_has_uncommon = FB_visibility + 2,
+    FB_undefined,
     FB_weak,
     FB_common,
     FB_indirect,
@@ -94,10 +98,6 @@
     FB_unnamed_addr,
     FB_executable,
   };
-
-  /// The index into the Uncommon table, or -1 if this symbol does not have an
-  /// Uncommon.
-  Word UncommonIndex;
 };
 
 /// This data structure contains rarely used symbol fields and is optionally
@@ -247,15 +247,9 @@
 /// Reader::module_symbols().
 class Reader::SymbolRef : public Symbol {
   const storage::Symbol *SymI, *SymE;
+  const storage::Uncommon *UncI;
   const Reader *R;
 
-public:
-  SymbolRef(const storage::Symbol *SymI, const storage::Symbol *SymE,
-            const Reader *R)
-      : SymI(SymI), SymE(SymE), R(R) {
-    read();
-  }
-
   void read() {
     if (SymI == SymE)
       return;
@@ -265,16 +259,24 @@
     ComdatIndex = SymI->ComdatIndex;
     Flags = SymI->Flags;
 
-    uint32_t UncI = SymI->UncommonIndex;
-    if (UncI != -1u) {
-      const storage::Uncommon &Unc = R->Uncommons[UncI];
-      CommonSize = Unc.CommonSize;
-      CommonAlign = Unc.CommonAlign;
-      COFFWeakExternFallbackName = R->str(Unc.COFFWeakExternFallbackName);
+    if (Flags & (1 << storage::Symbol::FB_has_uncommon)) {
+      CommonSize = UncI->CommonSize;
+      CommonAlign = UncI->CommonAlign;
+      COFFWeakExternFallbackName = R->str(UncI->COFFWeakExternFallbackName);
     }
   }
+
+public:
+  SymbolRef(const storage::Symbol *SymI, const storage::Symbol *SymE,
+            const storage::Uncommon *UncI, const Reader *R)
+      : SymI(SymI), SymE(SymE), UncI(UncI), R(R) {
+    read();
+  }
+
   void moveNext() {
     ++SymI;
+    if (Flags & (1 << storage::Symbol::FB_has_uncommon))
+      ++UncI;
     read();
   }
 
@@ -282,15 +284,16 @@
 };
 
 inline Reader::symbol_range Reader::symbols() const {
-  return {SymbolRef(Symbols.begin(), Symbols.end(), this),
-          SymbolRef(Symbols.end(), Symbols.end(), this)};
+  return {SymbolRef(Symbols.begin(), Symbols.end(), Uncommons.begin(), this),
+          SymbolRef(Symbols.end(), Symbols.end(), nullptr, this)};
 }
 
 inline Reader::symbol_range Reader::module_symbols(unsigned I) const {
   const storage::Module &M = Modules[I];
   const storage::Symbol *MBegin = Symbols.begin() + M.Begin,
                         *MEnd = Symbols.begin() + M.End;
-  return {SymbolRef(MBegin, MEnd, this), SymbolRef(MEnd, MEnd, this)};
+  return {SymbolRef(MBegin, MEnd, Uncommons.begin() + M.UncBegin, this),
+          SymbolRef(MEnd, MEnd, nullptr, this)};
 }
 
 }
Index: llvm/lib/Object/IRSymtab.cpp
===================================================================
--- llvm/lib/Object/IRSymtab.cpp
+++ llvm/lib/Object/IRSymtab.cpp
@@ -34,8 +34,6 @@
   StringSaver Saver{Alloc};
 
   DenseMap<const Comdat *, unsigned> ComdatMap;
-  ModuleSymbolTable Msymtab;
-  SmallPtrSet<GlobalValue *, 8> Used;
   Mangler Mang;
   Triple TT;
 
@@ -59,18 +57,24 @@
   }
 
   Error addModule(Module *M);
-  Error addSymbol(ModuleSymbolTable::Symbol Sym);
+  Error addSymbol(const ModuleSymbolTable &Msymtab,
+                  const SmallPtrSet<GlobalValue *, 8> &Used,
+                  ModuleSymbolTable::Symbol Sym);
 
   Error build(ArrayRef<Module *> Mods);
 };
 
 Error Builder::addModule(Module *M) {
+  SmallPtrSet<GlobalValue *, 8> Used;
   collectUsedGlobalVariables(*M, Used, /*CompilerUsed*/ false);
 
-  storage::Module Mod;
-  Mod.Begin = Msymtab.symbols().size();
+  ModuleSymbolTable Msymtab;
   Msymtab.addModule(M);
-  Mod.End = Msymtab.symbols().size();
+
+  storage::Module Mod;
+  Mod.Begin = Syms.size();
+  Mod.End = Syms.size() + Msymtab.symbols().size();
+  Mod.UncBegin = Uncommons.size();
   Mods.push_back(Mod);
 
   if (TT.isOSBinFormatCOFF()) {
@@ -84,20 +88,25 @@
     }
   }
 
+  for (ModuleSymbolTable::Symbol Msym : Msymtab.symbols())
+    if (Error Err = addSymbol(Msymtab, Used, Msym))
+      return Err;
+
   return Error::success();
 }
 
-Error Builder::addSymbol(ModuleSymbolTable::Symbol Msym) {
+Error Builder::addSymbol(const ModuleSymbolTable &Msymtab,
+                         const SmallPtrSet<GlobalValue *, 8> &Used,
+                         ModuleSymbolTable::Symbol Msym) {
   Syms.emplace_back();
   storage::Symbol &Sym = Syms.back();
   Sym = {};
 
-  Sym.UncommonIndex = -1;
   storage::Uncommon *Unc = nullptr;
   auto Uncommon = [&]() -> storage::Uncommon & {
     if (Unc)
       return *Unc;
-    Sym.UncommonIndex = Uncommons.size();
+    Sym.Flags |= 1 << storage::Symbol::FB_has_uncommon;
     Uncommons.emplace_back();
     Unc = &Uncommons.back();
     *Unc = {};
@@ -193,15 +202,10 @@
   setStr(Hdr.SourceFileName, IRMods[0]->getSourceFileName());
   TT = Triple(IRMods[0]->getTargetTriple());
 
-  // This adds the symbols for each module to Msymtab.
   for (auto *M : IRMods)
     if (Error Err = addModule(M))
       return Err;
 
-  for (ModuleSymbolTable::Symbol Msym : Msymtab.symbols())
-    if (Error Err = addSymbol(Msym))
-      return Err;
-
   COFFLinkerOptsOS.flush();
   setStr(Hdr.COFFLinkerOpts, COFFLinkerOpts);