Index: llvm/lib/MC/WinCOFFObjectWriter.cpp
===================================================================
--- llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
@@ -131,6 +132,8 @@
   using symbol_map = DenseMap<MCSymbol const *, COFFSymbol *>;
   using section_map = DenseMap<MCSection const *, COFFSection *>;
 
+  using symbol_list = DenseSet<COFFSymbol *>;
+
   std::unique_ptr<MCWinCOFFObjectTargetWriter> TargetObjectWriter;
 
   // Root level file contents.
@@ -143,6 +146,8 @@
   section_map SectionMap;
   symbol_map SymbolMap;
 
+  symbol_list WeakDefaults;
+
   bool UseBigObj;
 
   bool EmitAddrsigSection = false;
@@ -205,6 +210,7 @@
                         MCValue Target, uint64_t &FixedValue) override;
 
   void createFileSymbols(MCAssembler &Asm);
+  void setWeakDefaultNames();
   void assignSectionNumbers();
   void assignFileOffsets(MCAssembler &Asm, const MCAsmLayout &Layout);
 
@@ -376,6 +382,7 @@
         WeakDefault->Data.SectionNumber = COFF::IMAGE_SYM_ABSOLUTE;
       else
         WeakDefault->Section = Sec;
+      WeakDefaults.insert(WeakDefault);
       Local = WeakDefault;
     }
 
@@ -863,6 +870,47 @@
   }
 }
 
+void WinCOFFObjectWriter::setWeakDefaultNames() {
+  if (WeakDefaults.empty())
+    return;
+
+  // If multiple object files use a weak symbol (either with a regular
+  // defined default, or an absolute zero symbol as default), the defaults
+  // cause duplicate definitions unless their names are made unique. Look
+  // for a defined extern symbol, that isn't comdat - that should be unique
+  // unless there are other duplicate definitions. And if none is found,
+  // allow picking a comdat symbol, as that's still better than nothing.
+
+  COFFSymbol *Unique = nullptr;
+  for (bool AllowComdat : {false, true}) {
+    for (auto &Sym : Symbols) {
+      // Don't include the names of the defaults themselves
+      if (WeakDefaults.count(Sym.get()))
+        continue;
+      // Only consider external symbols
+      if (Sym->Data.StorageClass != COFF::IMAGE_SYM_CLASS_EXTERNAL)
+        continue;
+      // Only consider symbols defined in a section or that are absolute
+      if (!Sym->Section && Sym->Data.SectionNumber != COFF::IMAGE_SYM_ABSOLUTE)
+        continue;
+      if (!AllowComdat && Sym->Section &&
+          Sym->Section->Header.Characteristics & COFF::IMAGE_SCN_LNK_COMDAT)
+        continue;
+      Unique = Sym.get();
+      break;
+    }
+    if (Unique)
+      break;
+  }
+  // If we didn't find any unique symbol to use for the names, just skip this.
+  if (!Unique)
+    return;
+  for (auto *Sym : WeakDefaults) {
+    Sym->Name.append(".");
+    Sym->Name.append(Unique->Name);
+  }
+}
+
 static bool isAssociative(const COFFSection &Section) {
   return Section.Symbol->Aux[0].Aux.SectionDefinition.Selection ==
          COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE;
@@ -961,6 +1009,7 @@
   Header.NumberOfSections = Sections.size();
   Header.NumberOfSymbols = 0;
 
+  setWeakDefaultNames();
   assignSectionNumbers();
   createFileSymbols(Asm);
 
Index: llvm/test/MC/COFF/weak-alias-local.s
===================================================================
--- llvm/test/MC/COFF/weak-alias-local.s
+++ llvm/test/MC/COFF/weak-alias-local.s
@@ -33,7 +33,7 @@
 // CHECK-NEXT:   }
 // CHECK-NEXT: }
 // CHECK-NEXT: Symbol {
-// CHECK-NEXT:   Name: .weak.a.default
+// CHECK-NEXT:   Name: .weak.a.default{{$}}
 // CHECK-NEXT:   Value: 4
 // CHECK-NEXT:   Section: .data (2)
 // CHECK-NEXT:   BaseType: Null (0x0)
Index: llvm/test/MC/COFF/weak-name.s
===================================================================
--- /dev/null
+++ llvm/test/MC/COFF/weak-name.s
@@ -0,0 +1,35 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s -defsym case=0 -o %t.case0.o
+// RUN: llvm-readobj --symbols %t.case0.o | FileCheck %s --check-prefix=CHECK-CASE0
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s -defsym case=1 -o %t.case1.o
+// RUN: llvm-readobj --symbols %t.case1.o | FileCheck %s --check-prefix=CHECK-CASE1
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-win32 %s -defsym case=2 -o %t.case2.o
+// RUN: llvm-readobj --symbols %t.case2.o | FileCheck %s --check-prefix=CHECK-CASE2
+
+// Test that we prefer a non-comdat symbol for naming weak default symbols,
+// if such a symbol is available.
+
+        .section .text$comdat1,"xr",discard,comdat1
+        .globl   comdat1
+comdat1:
+        call     undeffunc
+
+        .weak    weaksym
+
+        .section .text$comdat2,"xr",discard,comdat2
+        .globl   comdat2
+comdat2:
+        call     undeffunc2
+
+.if case == 0
+        .text
+        .globl   regular
+regular:
+        call     undeffunc3
+.elseif case == 1
+        .globl   abssym
+abssym = 42
+.endif
+
+// CHECK-CASE0: Name: .weak.weaksym.default.regular
+// CHECK-CASE1: Name: .weak.weaksym.default.abssym
+// CHECK-CASE2: Name: .weak.weaksym.default.comdat1
Index: llvm/test/MC/COFF/weak-val.s
===================================================================
--- llvm/test/MC/COFF/weak-val.s
+++ llvm/test/MC/COFF/weak-val.s
@@ -23,7 +23,7 @@
 // CHECK-NEXT:   }
 // CHECK-NEXT: }
 // CHECK-NEXT: Symbol {
-// CHECK-NEXT:   Name: .weak.b.default
+// CHECK-NEXT:   Name: .weak.b.default{{$}}
 // CHECK-NEXT:   Value: 4
 // CHECK-NEXT:   Section: .data (2)
 // CHECK-NEXT:   BaseType: Null (0x0)
Index: llvm/test/MC/COFF/weak.s
===================================================================
--- llvm/test/MC/COFF/weak.s
+++ llvm/test/MC/COFF/weak.s
@@ -59,7 +59,7 @@
 // CHECK-NEXT: }
 
 // CHECK:      Symbol {
-// CHECK:        Name:                .weak._test_weak.default
+// CHECK:        Name:                .weak._test_weak.default._main
 // CHECK-NEXT:   Value:               0
 // CHECK-NEXT:   Section:             IMAGE_SYM_ABSOLUTE (-1)
 // CHECK-NEXT:   BaseType:            Null
@@ -83,7 +83,7 @@
 // CHECK-NEXT: }
 
 // CHECK:      Symbol {
-// CHECK:        Name: .weak._test_weak_alias.default
+// CHECK:        Name: .weak._test_weak_alias.default._main
 // CHECK-NEXT:   Value: 0
 // CHECK-NEXT:   Section: .text
 // CHECK-NEXT:   BaseType: Null