Index: llvm/trunk/lib/Object/ArchiveWriter.cpp
===================================================================
--- llvm/trunk/lib/Object/ArchiveWriter.cpp
+++ llvm/trunk/lib/Object/ArchiveWriter.cpp
@@ -35,6 +35,15 @@
 
 using namespace llvm;
 
+// 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 has
+// an offset larger than 32-bits but this demands a very slow test. To speed
+// the test up we use this flag to pretend like the cutoff happens before
+// 32-bits and instead happens at some much smaller value.
+static cl::opt<int> Sym64Threshold("sym64-threshold", cl::Hidden,
+                                   cl::init(32));
+
 NewArchiveMember::NewArchiveMember(MemoryBufferRef BufRef)
     : Buf(MemoryBuffer::getMemBuffer(BufRef, false)),
       MemberName(BufRef.getBufferIdentifier()) {}
@@ -484,7 +493,7 @@
     // 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 >= (1ULL << Sym64Threshold))
       Kind = object::Archive::K_GNU64;
   }
 
Index: llvm/trunk/test/Object/archive-GNU64-write.test
===================================================================
--- llvm/trunk/test/Object/archive-GNU64-write.test
+++ llvm/trunk/test/Object/archive-GNU64-write.test
@@ -0,0 +1,39 @@
+# REQUIRES: llvm-64-bits
+# REQUIRES: system-linux
+
+# RUN: yaml2obj %s > %t
+# RUN: dd if=%t of=%t bs=1 count=0 seek=1M
+# RUN: rm -f %t.lib
+# RUN: cp %t %t2
+# RUN: llvm-ar -sym64-threshold=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
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         "00"
+    Size:            32
+
+# CHECK:      Archive map
+# CHECK-NEXT: main in trivial-object-test.elf-x86-64
+
+# CHECK:    archive-GNU64-write.test.tmp:
+
+# CHECK:    archive-GNU64-write.test.tmp2:
+
+# CHECK:    trivial-object-test.elf-x86-64:
+# CHECK-NEXT:                     U SomeOtherFunction
+# CHECK-NEXT:    0000000000000000 T main
+# CHECK-NEXT:                     U puts
Index: llvm/trunk/test/Object/archive-SYM64-write.test
===================================================================
--- llvm/trunk/test/Object/archive-SYM64-write.test
+++ llvm/trunk/test/Object/archive-SYM64-write.test
@@ -1,38 +0,0 @@
-# REQUIRES: llvm-64-bits
-# REQUIRES: system-linux
-
-# RUN: yaml2obj %s > %t
-# RUN: dd if=%t of=%t bs=1 count=0 seek=2200M
-# 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-nm --print-armap %t.lib | FileCheck %s
-
-# Delete temp files. They are too large.
-# RUN: rm -f %t %t2 %t.lib
-
-!ELF
-FileHeader:
-  Class:           ELFCLASS64
-  Data:            ELFDATA2LSB
-  Type:            ET_EXEC
-  Machine:         EM_X86_64
-Sections:
-  - Name:            .data
-    Type:            SHT_PROGBITS
-    Flags:           [ SHF_ALLOC ]
-    AddressAlign:    0x0000000000000001
-    Content:         "00"
-    Size:            32
-
-# CHECK:      Archive map
-# CHECK-NEXT: main in trivial-object-test.elf-x86-64
-
-# CHECK:    archive-SYM64-write.test.tmp:
-
-# CHECK:    archive-SYM64-write.test.tmp2:
-
-# CHECK:    trivial-object-test.elf-x86-64:
-# CHECK-NEXT:                     U SomeOtherFunction
-# CHECK-NEXT:    0000000000000000 T main
-# CHECK-NEXT:                     U puts