Index: include/llvm/Object/ArchiveWriter.h
===================================================================
--- include/llvm/Object/ArchiveWriter.h
+++ include/llvm/Object/ArchiveWriter.h
@@ -41,6 +41,7 @@
 Error writeArchive(StringRef ArcName, ArrayRef<NewArchiveMember> NewMembers,
                    bool WriteSymtab, object::Archive::Kind Kind,
                    bool Deterministic, bool Thin,
+                   uint64_t CutoffForSym64 = 1ULL << 32,
                    std::unique_ptr<MemoryBuffer> OldArchiveBuf = nullptr);
 }
 
Index: lib/Object/ArchiveWriter.cpp
===================================================================
--- lib/Object/ArchiveWriter.cpp
+++ lib/Object/ArchiveWriter.cpp
@@ -451,7 +451,7 @@
 Error llvm::writeArchive(StringRef ArcName,
                          ArrayRef<NewArchiveMember> NewMembers,
                          bool WriteSymtab, object::Archive::Kind Kind,
-                         bool Deterministic, bool Thin,
+                         bool Deterministic, bool Thin, uint64_t CutoffForSym64,
                          std::unique_ptr<MemoryBuffer> OldArchiveBuf) {
   assert((!Thin || !isBSDLike(Kind)) && "Only the gnu format has a thin mode");
 
@@ -484,7 +484,7 @@
     // If LastOffset isn't going to fit in a 32-bit varible we need to switch
     // to 64-bit. Note that the file can be larger than 4GB as long as the last
     // member starts before the 4GB offset.
-    if (LastOffset >> 32 != 0)
+    if (LastOffset >= CutoffForSym64)
       Kind = object::Archive::K_GNU64;
   }
 
Index: test/Object/archive-GNU64-write.test
===================================================================
--- test/Object/archive-GNU64-write.test
+++ test/Object/archive-GNU64-write.test
@@ -2,11 +2,12 @@
 # REQUIRES: system-linux
 
 # RUN: yaml2obj %s > %t
-# RUN: dd if=%t of=%t bs=1 count=0 seek=2200M
+# RUN: dd if=%t of=%t bs=1 count=0 seek=1M
 # RUN: rm -f %t.lib
 # RUN: cp %t %t2
-# RUN: llvm-ar cr %t.lib %t %t2 %p/Inputs/trivial-object-test.elf-x86-64
+# RUN: llvm-ar -sym64-power-of-two=19 cr %t.lib %t %t2 %p/Inputs/trivial-object-test.elf-x86-64
 # RUN: llvm-nm --print-armap %t.lib | FileCheck %s
+# RUN: grep SYM64 %t.lib
 
 # Delete temp files. They are too large.
 # RUN: rm -f %t %t2 %t.lib
@@ -28,9 +29,9 @@
 # CHECK:      Archive map
 # CHECK-NEXT: main in trivial-object-test.elf-x86-64
 
-# CHECK:    archive-SYM64-write.test.tmp:
+# CHECK:    archive-GNU64-write.test.tmp:
 
-# CHECK:    archive-SYM64-write.test.tmp2:
+# CHECK:    archive-GNU64-write.test.tmp2:
 
 # CHECK:    trivial-object-test.elf-x86-64:
 # CHECK-NEXT:                     U SomeOtherFunction
Index: tools/llvm-ar/llvm-ar.cpp
===================================================================
--- tools/llvm-ar/llvm-ar.cpp
+++ tools/llvm-ar/llvm-ar.cpp
@@ -99,6 +99,8 @@
                          clEnumValN(DARWIN, "darwin", "darwin"),
                          clEnumValN(BSD, "bsd", "bsd")));
 
+static cl::opt<int> Sym64PowerOfTwo("sym64-power-of-two", cl::Hidden);
+
 static std::string Options;
 
 // Provide additional help output explaining the operations and modifiers of
@@ -683,9 +685,16 @@
     break;
   }
 
-  Error E =
-      writeArchive(ArchiveName, NewMembersP ? *NewMembersP : NewMembers, Symtab,
-                   Kind, Deterministic, Thin, std::move(OldArchiveBuf));
+  // The cutoff should always be 2^32 in practice but for testing we'd like it
+  // to be smaller. The Sym64PowerOfTwo flag allows us to change the cutoff to
+  // speed things up.
+  uint64_t CutoffForSym64 = 1ULL << 32;
+  if (Sym64PowerOfTwo != 0)
+    CutoffForSym64 = 1 << (Sym64PowerOfTwo.getValue() - 1);
+
+  Error E = writeArchive(ArchiveName, NewMembersP ? *NewMembersP : NewMembers,
+                         Symtab, Kind, Deterministic, Thin, CutoffForSym64,
+                         std::move(OldArchiveBuf));
   failIfError(std::move(E), ArchiveName);
 }