diff --git a/llvm/include/llvm/MC/MCDirectives.h b/llvm/include/llvm/MC/MCDirectives.h
--- a/llvm/include/llvm/MC/MCDirectives.h
+++ b/llvm/include/llvm/MC/MCDirectives.h
@@ -46,6 +46,7 @@
   MCSA_WeakDefinition,          ///< .weak_definition (MachO)
   MCSA_WeakReference,           ///< .weak_reference (MachO)
   MCSA_WeakDefAutoPrivate,      ///< .weak_def_can_be_hidden (MachO)
+  MCSA_WeakAntiDep,             ///< .weak_anti_dep (COFF)
   MCSA_Memtag,                  ///< .memtag (ELF)
 };
 
diff --git a/llvm/include/llvm/MC/MCSymbol.h b/llvm/include/llvm/MC/MCSymbol.h
--- a/llvm/include/llvm/MC/MCSymbol.h
+++ b/llvm/include/llvm/MC/MCSymbol.h
@@ -102,6 +102,9 @@
   /// This symbol is private extern.
   mutable unsigned IsPrivateExtern : 1;
 
+  /// This symbol is weak external.
+  mutable unsigned IsWeakExternal : 1;
+
   /// LLVM RTTI discriminator. This is actually a SymbolKind enumerator, but is
   /// unsigned to avoid sign extension and achieve better bitpacking with MSVC.
   unsigned Kind : 3;
@@ -161,8 +164,8 @@
   MCSymbol(SymbolKind Kind, const StringMapEntry<bool> *Name, bool isTemporary)
       : IsTemporary(isTemporary), IsRedefinable(false), IsUsed(false),
         IsRegistered(false), IsExternal(false), IsPrivateExtern(false),
-        Kind(Kind), IsUsedInReloc(false), SymbolContents(SymContentsUnset),
-        CommonAlignLog2(0), Flags(0) {
+        IsWeakExternal(false), Kind(Kind), IsUsedInReloc(false),
+        SymbolContents(SymContentsUnset), CommonAlignLog2(0), Flags(0) {
     Offset = 0;
     FragmentAndHasName.setInt(!!Name);
     if (Name)
@@ -394,8 +397,10 @@
 
   MCFragment *getFragment(bool SetUsed = true) const {
     MCFragment *Fragment = FragmentAndHasName.getPointer();
-    if (Fragment || !isVariable())
+    if (Fragment || !isVariable() || isWeakExternal())
       return Fragment;
+    // If the symbol is a non-weak alias, get information about
+    // the aliasee. (Don't try to resolve weak aliases.)
     Fragment = getVariableValue(SetUsed)->findAssociatedFragment();
     FragmentAndHasName.setPointer(Fragment);
     return Fragment;
@@ -407,6 +412,8 @@
   bool isPrivateExtern() const { return IsPrivateExtern; }
   void setPrivateExtern(bool Value) { IsPrivateExtern = Value; }
 
+  bool isWeakExternal() const { return IsWeakExternal; }
+
   /// print - Print the value to the stream \p OS.
   void print(raw_ostream &OS, const MCAsmInfo *MAI) const;
 
diff --git a/llvm/include/llvm/MC/MCSymbolCOFF.h b/llvm/include/llvm/MC/MCSymbolCOFF.h
--- a/llvm/include/llvm/MC/MCSymbolCOFF.h
+++ b/llvm/include/llvm/MC/MCSymbolCOFF.h
@@ -23,8 +23,9 @@
     SF_ClassMask = 0x00FF,
     SF_ClassShift = 0,
 
-    SF_WeakExternal = 0x0100,
-    SF_SafeSEH = 0x0200,
+    SF_SafeSEH = 0x0100,
+    SF_WeakExternalCharacteristicsMask = 0x0E00,
+    SF_WeakExternalCharacteristicsShift = 9,
   };
 
 public:
@@ -45,11 +46,14 @@
     modifyFlags(StorageClass << SF_ClassShift, SF_ClassMask);
   }
 
-  bool isWeakExternal() const {
-    return getFlags() & SF_WeakExternal;
+  COFF::WeakExternalCharacteristics getWeakExternalCharacteristics() const {
+    return static_cast<COFF::WeakExternalCharacteristics>((getFlags() & SF_WeakExternalCharacteristicsMask) >>
+           SF_WeakExternalCharacteristicsShift);
   }
-  void setIsWeakExternal() const {
-    modifyFlags(SF_WeakExternal, SF_WeakExternal);
+  void setIsWeakExternal(COFF::WeakExternalCharacteristics Characteristics) const {
+    IsWeakExternal = true;
+    modifyFlags(Characteristics << SF_WeakExternalCharacteristicsShift,
+                SF_WeakExternalCharacteristicsMask);
   }
 
   bool isSafeSEH() const {
diff --git a/llvm/lib/MC/MCAsmStreamer.cpp b/llvm/lib/MC/MCAsmStreamer.cpp
--- a/llvm/lib/MC/MCAsmStreamer.cpp
+++ b/llvm/lib/MC/MCAsmStreamer.cpp
@@ -772,6 +772,9 @@
   case MCSA_Memtag:
     OS << "\t.memtag\t";
     break;
+  case MCSA_WeakAntiDep:
+    OS << "\t.weak_anti_dep\t";
+    break;
   }
 
   Symbol->print(OS, MAI);
diff --git a/llvm/lib/MC/MCELFStreamer.cpp b/llvm/lib/MC/MCELFStreamer.cpp
--- a/llvm/lib/MC/MCELFStreamer.cpp
+++ b/llvm/lib/MC/MCELFStreamer.cpp
@@ -216,6 +216,7 @@
   case MCSA_Invalid:
   case MCSA_IndirectSymbol:
   case MCSA_Exported:
+  case MCSA_WeakAntiDep:
     return false;
 
   case MCSA_NoDeadStrip:
diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -761,6 +761,9 @@
 }
 
 static bool canExpand(const MCSymbol &Sym, bool InSet) {
+  if (Sym.isWeakExternal())
+    return false;
+
   const MCExpr *Expr = Sym.getVariableValue();
   const auto *Inner = dyn_cast<MCSymbolRefExpr>(Expr);
   if (Inner) {
diff --git a/llvm/lib/MC/MCMachOStreamer.cpp b/llvm/lib/MC/MCMachOStreamer.cpp
--- a/llvm/lib/MC/MCMachOStreamer.cpp
+++ b/llvm/lib/MC/MCMachOStreamer.cpp
@@ -358,6 +358,7 @@
   case MCSA_LGlobal:
   case MCSA_Exported:
   case MCSA_Memtag:
+  case MCSA_WeakAntiDep:
     return false;
 
   case MCSA_Global:
diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -6371,7 +6371,7 @@
   case MCExpr::SymbolRef: {
     const MCSymbol &S =
         static_cast<const MCSymbolRefExpr *>(Value)->getSymbol();
-    if (S.isVariable())
+    if (S.isVariable() && !S.isWeakExternal())
       return isSymbolUsedInExpression(Sym, S.getVariableValue());
     return &S == Sym;
   }
diff --git a/llvm/lib/MC/MCParser/COFFAsmParser.cpp b/llvm/lib/MC/MCParser/COFFAsmParser.cpp
--- a/llvm/lib/MC/MCParser/COFFAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/COFFAsmParser.cpp
@@ -67,6 +67,7 @@
     addDirectiveHandler<&COFFAsmParser::ParseDirectiveLinkOnce>(".linkonce");
     addDirectiveHandler<&COFFAsmParser::ParseDirectiveRVA>(".rva");
     addDirectiveHandler<&COFFAsmParser::ParseDirectiveSymbolAttribute>(".weak");
+    addDirectiveHandler<&COFFAsmParser::ParseDirectiveSymbolAttribute>(".weak_anti_dep");
     addDirectiveHandler<&COFFAsmParser::ParseDirectiveCGProfile>(".cg_profile");
 
     // Win64 EH directives.
@@ -281,6 +282,7 @@
 bool COFFAsmParser::ParseDirectiveSymbolAttribute(StringRef Directive, SMLoc) {
   MCSymbolAttr Attr = StringSwitch<MCSymbolAttr>(Directive)
     .Case(".weak", MCSA_Weak)
+    .Case(".weak_anti_dep", MCSA_WeakAntiDep)
     .Default(MCSA_Invalid);
   assert(Attr != MCSA_Invalid && "unexpected symbol attribute directive!");
   if (getLexer().isNot(AsmToken::EndOfStatement)) {
diff --git a/llvm/lib/MC/MCWinCOFFStreamer.cpp b/llvm/lib/MC/MCWinCOFFStreamer.cpp
--- a/llvm/lib/MC/MCWinCOFFStreamer.cpp
+++ b/llvm/lib/MC/MCWinCOFFStreamer.cpp
@@ -115,7 +115,11 @@
   default: return false;
   case MCSA_WeakReference:
   case MCSA_Weak:
-    Symbol->setIsWeakExternal();
+    Symbol->setIsWeakExternal(COFF::IMAGE_WEAK_EXTERN_SEARCH_ALIAS);
+    Symbol->setExternal(true);
+    break;
+  case MCSA_WeakAntiDep:
+    Symbol->setIsWeakExternal(COFF::IMAGE_WEAK_EXTERN_ANTI_DEPENDENCY);
     Symbol->setExternal(true);
     break;
   case MCSA_Global:
diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -414,9 +414,9 @@
     Sym->Aux.resize(1);
     memset(&Sym->Aux[0], 0, sizeof(Sym->Aux[0]));
     Sym->Aux[0].AuxType = ATWeakExternal;
-    Sym->Aux[0].Aux.WeakExternal.TagIndex = 0;
+    Sym->Aux[0].Aux.WeakExternal.TagIndex = 0; // Filled in later
     Sym->Aux[0].Aux.WeakExternal.Characteristics =
-        COFF::IMAGE_WEAK_EXTERN_SEARCH_ALIAS;
+        cast<MCSymbolCOFF>(MCSym).getWeakExternalCharacteristics();
   } else {
     if (!Base)
       Sym->Data.SectionNumber = COFF::IMAGE_SYM_ABSOLUTE;
diff --git a/llvm/test/MC/COFF/addrsig.s b/llvm/test/MC/COFF/addrsig.s
--- a/llvm/test/MC/COFF/addrsig.s
+++ b/llvm/test/MC/COFF/addrsig.s
@@ -3,7 +3,7 @@
 // CHECK:      Name: .llvm_addrsig
 // CHECK-NEXT: VirtualSize: 0x0
 // CHECK-NEXT: VirtualAddress: 0x0
-// CHECK-NEXT: RawDataSize: 4
+// CHECK-NEXT: RawDataSize: 6
 // CHECK-NEXT: PointerToRawData:
 // CHECK-NEXT: PointerToRelocations: 0x0
 // CHECK-NEXT: PointerToLineNumbers: 0x0
@@ -46,6 +46,8 @@
 // CHECK-NEXT:   Sym: g3 (11)
 // CHECK-NEXT:   Sym: local (10)
 // CHECK-NEXT:   Sym: .data (2)
+// CHECK-NEXT:   Sym: weak_sym (12)
+// CHECK-NEXT:   Sym: .data (2)
 // CHECK-NEXT: ]
 
 .globl g1
@@ -64,3 +66,10 @@
 
 .data
 .Llocal:
+
+.weak weak_sym
+weak_sym:
+.addrsig_sym weak_sym
+
+.set .Lalias_weak_sym, weak_sym
+.addrsig_sym .Lalias_weak_sym
diff --git a/llvm/test/MC/COFF/alias.s b/llvm/test/MC/COFF/alias.s
--- a/llvm/test/MC/COFF/alias.s
+++ b/llvm/test/MC/COFF/alias.s
@@ -24,7 +24,7 @@
 // CHECK:        0x0 IMAGE_REL_I386_DIR32 external_aliased_to_local
 // CHECK:        0x4 IMAGE_REL_I386_DIR32 external1
 // CHECK:        0x8 IMAGE_REL_I386_DIR32 global_aliased_to_local
-// CHECK:        0xC IMAGE_REL_I386_DIR32 external2
+// CHECK:        0xC IMAGE_REL_I386_DIR32 weak_aliased_to_external
 // CHECK:      ]
 // CHECK:      Symbols [
 // CHECK-NEXT:   Symbol {
diff --git a/llvm/test/MC/COFF/weak-anti-dep.s b/llvm/test/MC/COFF/weak-anti-dep.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/COFF/weak-anti-dep.s
@@ -0,0 +1,68 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s | llvm-readobj --symbols - | FileCheck %s
+
+// CHECK:       Symbol {
+// CHECK-NEXT:    Name: .text
+// CHECK:       Symbol {
+// CHECK-NEXT:    Name: .data
+// CHECK:       Symbol {
+// CHECK-NEXT:    Name: .bss
+
+.weak_anti_dep a
+a = b
+
+// CHECK:       Symbol {
+// CHECK-NEXT:    Name: a
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: IMAGE_SYM_UNDEFINED (0)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: WeakExternal (0x69)
+// CHECK-NEXT:    AuxSymbolCount: 1
+// CHECK-NEXT:    AuxWeakExternal {
+// CHECK-NEXT:      Linked: b (8)
+// CHECK-NEXT:      Search: AntiDependency (0x4)
+// CHECK-NEXT:    }
+// CHECK-NEXT:  }
+// CHECK-NEXT:  Symbol {
+// CHECK-NEXT:    Name: b
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: IMAGE_SYM_UNDEFINED (0)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: External (0x2)
+// CHECK-NEXT:    AuxSymbolCount: 0
+// CHECK-NEXT:  }
+
+
+.weak_anti_dep r1
+.weak_anti_dep r2
+r1 = r2
+r2 = r1
+
+
+// CHECK:       Symbol {
+// CHECK-NEXT:    Name: r1
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: IMAGE_SYM_UNDEFINED (0)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: WeakExternal (0x69)
+// CHECK-NEXT:    AuxSymbolCount: 1
+// CHECK-NEXT:    AuxWeakExternal {
+// CHECK-NEXT:      Linked: r2 (11)
+// CHECK-NEXT:      Search: AntiDependency (0x4)
+// CHECK-NEXT:    }
+// CHECK-NEXT:  }
+// CHECK-NEXT:  Symbol {
+// CHECK-NEXT:    Name: r2
+// CHECK-NEXT:    Value: 0
+// CHECK-NEXT:    Section: IMAGE_SYM_UNDEFINED (0)
+// CHECK-NEXT:    BaseType: Null (0x0)
+// CHECK-NEXT:    ComplexType: Null (0x0)
+// CHECK-NEXT:    StorageClass: WeakExternal (0x69)
+// CHECK-NEXT:    AuxSymbolCount: 1
+// CHECK-NEXT:    AuxWeakExternal {
+// CHECK-NEXT:      Linked: r1 (9)
+// CHECK-NEXT:      Search: AntiDependency (0x4)
+// CHECK-NEXT:    }
+// CHECK-NEXT:  }