diff --git a/llvm/lib/Support/TarWriter.cpp b/llvm/lib/Support/TarWriter.cpp
index 6c62f8861afd4c..c7a744f0fc98cc 100644
--- a/llvm/lib/Support/TarWriter.cpp
+++ b/llvm/lib/Support/TarWriter.cpp
@@ -131,7 +131,17 @@ static bool splitUstar(StringRef Path, StringRef &Prefix, StringRef &Name) {
     return true;
   }
 
-  size_t Sep = Path.rfind('/', sizeof(UstarHeader::Prefix) + 1);
+  // tar 1.13 and earlier unconditionally look at the tar header interpreted
+  // as an 'oldgnu_header', which has an 'isextended' byte at offset 482 in the
+  // header, corresponding to offset 137 in the prefix. That's the version of
+  // tar in gnuwin, so only use 137 of the 155 bytes in the prefix. This means
+  // we'll need a pax header after 237 bytes of path instead of after 255,
+  // but in return paths up to 237 bytes work with gnuwin, instead of just
+  // 137 bytes of directory + 100 bytes of basename previously.
+  // (tar-1.13 also doesn't support pax headers, but in practice all paths in
+  // llvm's test suite are short enough for that to not matter.)
+  const int MaxPrefix = 137;
+  size_t Sep = Path.rfind('/', MaxPrefix + 1);
   if (Sep == StringRef::npos)
     return false;
   if (Path.size() - Sep - 1 >= sizeof(UstarHeader::Name))
diff --git a/llvm/unittests/Support/TarWriterTest.cpp b/llvm/unittests/Support/TarWriterTest.cpp
index bd67e036518f28..5bbc6615ea5d23 100644
--- a/llvm/unittests/Support/TarWriterTest.cpp
+++ b/llvm/unittests/Support/TarWriterTest.cpp
@@ -81,30 +81,30 @@ TEST_F(TarWriterTest, Basics) {
 }
 
 TEST_F(TarWriterTest, LongFilename) {
-  std::string x154(154, 'x');
-  std::string x155(155, 'x');
+  std::string x136(136, 'x');
+  std::string x137(137, 'x');
   std::string y99(99, 'y');
   std::string y100(100, 'y');
 
-  UstarHeader Hdr1 = createUstar("", x154 + "/" + y99);
-  EXPECT_EQ("/" + x154, StringRef(Hdr1.Prefix));
+  UstarHeader Hdr1 = createUstar("", x136 + "/" + y99);
+  EXPECT_EQ("/" + x136, StringRef(Hdr1.Prefix));
   EXPECT_EQ(y99, StringRef(Hdr1.Name));
 
-  UstarHeader Hdr2 = createUstar("", x155 + "/" + y99);
+  UstarHeader Hdr2 = createUstar("", x137 + "/" + y99);
   EXPECT_EQ("", StringRef(Hdr2.Prefix));
   EXPECT_EQ("", StringRef(Hdr2.Name));
 
-  UstarHeader Hdr3 = createUstar("", x154 + "/" + y100);
+  UstarHeader Hdr3 = createUstar("", x136 + "/" + y100);
   EXPECT_EQ("", StringRef(Hdr3.Prefix));
   EXPECT_EQ("", StringRef(Hdr3.Name));
 
-  UstarHeader Hdr4 = createUstar("", x155 + "/" + y100);
+  UstarHeader Hdr4 = createUstar("", x137 + "/" + y100);
   EXPECT_EQ("", StringRef(Hdr4.Prefix));
   EXPECT_EQ("", StringRef(Hdr4.Name));
 
   std::string yz = "yyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzz";
-  UstarHeader Hdr5 = createUstar("", x154 + "/" + yz);
-  EXPECT_EQ("/" + x154, StringRef(Hdr5.Prefix));
+  UstarHeader Hdr5 = createUstar("", x136 + "/" + yz);
+  EXPECT_EQ("/" + x136, StringRef(Hdr5.Prefix));
   EXPECT_EQ(yz, StringRef(Hdr5.Name));
 }