Index: llvm/lib/Object/ArchiveWriter.cpp
===================================================================
--- llvm/lib/Object/ArchiveWriter.cpp
+++ llvm/lib/Object/ArchiveWriter.cpp
@@ -286,22 +286,12 @@
     print<uint32_t>(Out, Kind, Val);
 }
 
-static void writeSymbolTable(raw_ostream &Out, object::Archive::Kind Kind,
-                             bool Deterministic, ArrayRef<MemberData> Members,
-                             StringRef StringTable) {
-  // We don't write a symbol table on an archive with no members -- except on
-  // Darwin, where the linker will abort unless the archive has a symbol table.
-  if (StringTable.empty() && !isDarwin(Kind))
-    return;
-
-  unsigned NumSyms = 0;
-  for (const MemberData &M : Members)
-    NumSyms += M.Symbols.size();
-
-  unsigned Size = 0;
-  unsigned OffsetSize = is64BitKind(Kind) ? sizeof(uint64_t) : sizeof(uint32_t);
-
-  Size += OffsetSize; // Number of entries
+static uint64_t computeSymbolTableSize(object::Archive::Kind Kind,
+                                       uint64_t NumSyms, uint64_t OffsetSize,
+                                       StringRef StringTable,
+                                       uint32_t *Padding = nullptr) {
+  assert((OffsetSize == 4 || OffsetSize == 8) && "Unsupported OffsetSize");
+  uint64_t Size = OffsetSize; // Number of entries
   if (isBSDLike(Kind))
     Size += NumSyms * OffsetSize * 2; // Table
   else
@@ -313,10 +303,15 @@
   // least 4-byte aligned for 32-bit content.  Opt for the larger encoding
   // uniformly.
   // We do this for all bsd formats because it simplifies aligning members.
-  const Align Alignment(isBSDLike(Kind) ? 8 : 2);
-  unsigned Pad = offsetToAlignment(Size, Alignment);
+  uint32_t Pad = offsetToAlignment(Size, Align(isBSDLike(Kind) ? 8 : 2));
   Size += Pad;
+  if (Padding)
+    *Padding = Pad;
+  return Size;
+}
 
+static void writeSymbolTableHeader(raw_ostream &Out, object::Archive::Kind Kind,
+                                   bool Deterministic, uint64_t Size) {
   if (isBSDLike(Kind)) {
     const char *Name = is64BitKind(Kind) ? "__.SYMDEF_64" : "__.SYMDEF";
     printBSDMemberHeader(Out, Out.tell(), Name, now(Deterministic), 0, 0, 0,
@@ -325,6 +320,24 @@
     const char *Name = is64BitKind(Kind) ? "/SYM64" : "";
     printGNUSmallMemberHeader(Out, Name, now(Deterministic), 0, 0, 0, Size);
   }
+}
+
+static void writeSymbolTable(raw_ostream &Out, object::Archive::Kind Kind,
+                             bool Deterministic, ArrayRef<MemberData> Members,
+                             StringRef StringTable) {
+  // We don't write a symbol table on an archive with no members -- except on
+  // Darwin, where the linker will abort unless the archive has a symbol table.
+  if (StringTable.empty() && !isDarwin(Kind))
+    return;
+
+  unsigned NumSyms = 0;
+  for (const MemberData &M : Members)
+    NumSyms += M.Symbols.size();
+
+  uint64_t OffsetSize = is64BitKind(Kind) ? sizeof(uint64_t) : sizeof(uint32_t);
+  uint32_t Pad;
+  uint64_t Size = computeSymbolTableSize(Kind, NumSyms, OffsetSize, StringTable, &Pad);
+  writeSymbolTableHeader(Out, Kind, Deterministic, Size);
 
   uint64_t Pos = Out.tell() + Size;
 
@@ -581,15 +594,23 @@
   if (WriteSymtab) {
     uint64_t MaxOffset = 0;
     uint64_t LastOffset = MaxOffset;
+    uint64_t NumSyms = 0;
     for (const auto &M : Data) {
       // Record the start of the member's offset
       LastOffset = MaxOffset;
       // Account for the size of each part associated with the member.
       MaxOffset += M.Header.size() + M.Data.size() + M.Padding.size();
-      // We assume 32-bit symbols to see if 32-bit symbols are possible or not.
-      MaxOffset += M.Symbols.size() * 4;
+      NumSyms += M.Symbols.size();
     }
 
+    // We assume 32-bit offsets to see if 32-bit symbols are possible or not.
+    uint64_t SymtabSize = computeSymbolTableSize(Kind, NumSyms, sizeof(uint32_t), SymNamesBuf);
+    SmallString<0> SymtabHeaderBuf;
+    raw_svector_ostream SymtabHeader(SymtabHeaderBuf);
+    writeSymbolTableHeader(SymtabHeader, Kind, Deterministic, SymtabSize);
+
+    LastOffset += SymtabHeaderBuf.size() + SymtabSize;
+
     // The SYM64 format is used when an archive's member offsets are larger than
     // 32-bits can hold. The need for this shift in format is detected by
     // writeArchive. To test this we need to generate a file with a member that