Index: test/tools/llvm-cov/multithreaded-report.test
===================================================================
--- test/tools/llvm-cov/multithreaded-report.test
+++ test/tools/llvm-cov/multithreaded-report.test
@@ -1,8 +1,5 @@
 # Test "report" command with and without multiple threads.
 
-# Temporarily disable the test on Windows as it doesn't support "diff -r".
-REQUIRES: shell
-
 RUN: llvm-cov report -num-threads=1 \
 RUN:   -path-equivalence=/tmp,%S/Inputs \
 RUN:   -instr-profile %S/Inputs/multithreaded_report/main.profdata \
Index: utils/lit/lit/TestRunner.py
===================================================================
--- utils/lit/lit/TestRunner.py
+++ utils/lit/lit/TestRunner.py
@@ -346,7 +346,7 @@
     """executeBuiltinDiff - Compare files line by line."""
     args = expand_glob_expressions(cmd.args, cmd_shenv.cwd)[1:]
     try:
-        opts, args = getopt.gnu_getopt(args, "wbu", ["strip-trailing-cr"])
+        opts, args = getopt.gnu_getopt(args, "wbur", ["strip-trailing-cr"])
     except getopt.GetoptError as err:
         raise InternalShellError(cmd, "Unsupported: 'diff':  %s" % str(err))
 
@@ -354,6 +354,7 @@
     ignore_all_space = False
     ignore_space_change = False
     unified_diff = False
+    recursive_diff = False
     strip_trailing_cr = False
     for o, a in opts:
         if o == "-w":
@@ -362,6 +363,8 @@
             ignore_space_change = True
         elif o == "-u":
             unified_diff = True
+        elif o == "-r":
+            recursive_diff = True
         elif o == "--strip-trailing-cr":
             strip_trailing_cr = True
         else:
@@ -370,6 +373,18 @@
     if len(args) != 2:
         raise InternalShellError(cmd, "Error:  missing or extra operand")
 
+    def recursivelyReadDir(path):
+        lines = []
+        for dir, subdirList, fileList in os.walk(path):
+            for file in fileList:
+                lines += "Filename: %s" % os.path.relpath(file, path)
+                with open(os.path.join(dir, file), 'r') as f:
+                    lines += f.readlines()
+            if not fileList:
+                # Add empty dirs for comparison as well.
+                lines.append('Empty dir "%s'"." % os.path.relpath(dir, path))
+        return lines
+
     stderr = StringIO()
     stdout = StringIO()
     exitCode = 0
@@ -378,8 +393,11 @@
             if not os.path.isabs(file):
                 file = os.path.realpath(os.path.join(cmd_shenv.cwd, file))
             filepaths.append(file)
-            with open(file, 'r') as f:
-                filelines.append(f.readlines())
+            if recursive_diff:
+                filelines.append(recursivelyReadDir(file))
+            else:
+                with open(file, 'r') as f:
+                    filelines.append(f.readlines())
 
         def compose2(f, g):
             return lambda x: f(g(x))
Index: utils/lit/tests/Inputs/shtest-shell/diff-r.txt
===================================================================
--- /dev/null
+++ utils/lit/tests/Inputs/shtest-shell/diff-r.txt
@@ -0,0 +1,54 @@
+# Check recursive diff ("diff -r").
+
+# Create two directories for further comparison.
+# RUN: rm -rf %t/dir1 %t/dir2
+# RUN: mkdir -p %t/dir1 %t/dir2
+
+# Create same files in both of the dirs.
+# RUN: echo "hello" > %t/dir1/f1
+# RUN: echo "hello" > %t/dir2/f1
+
+# Create same subdirectories with same contents.
+# RUN: mkdir -p %t/dir1/subdir %t/dir2/subdir
+# RUN: echo "12345" > %t/dir1/subdir/f01
+# RUN: echo "12345" > %t/dir2/subdir/f01
+# RUN: echo "xxx" > %t/dir1/subdir/f02
+# RUN: echo "xxx" > %t/dir2/subdir/f02
+
+# Create empty subdirectories with same names.
+# RUN: mkdir -p %t/dir1/empty_subdir %t/dir2/empty_subdir
+# RUN: diff -r %t/dir1 %t/dir2
+
+# Add two empty files with different names, "diff -r" should fail.
+# RUN: touch %t/dir1/dir1unique
+# RUN: touch %t/dir2/dir2unique
+# RUN: not diff -r %t/dir1 %t/dir2
+# RUN: rm %t/dir1/dir1unique %t/dir2/dir2unique
+
+# Same filenames but different content, "diff -r" should fail.
+# RUN: echo "00000" > %t/dir2/subdir/f01
+# RUN: not diff -r %t/dir1 %t/dir2
+# RUN: echo "12345" > %t/dir2/subdir/f01
+
+# An extra file in one of the directories, "dirr -r" should fail.
+# RUN: touch %t/dir2/extrafile
+# RUN: not diff -r %t/dir1 %t/dir2
+
+# Non-empty extra file, "diff -r" should fail.
+# RUN: echo "content" > %t/dir2/extrafile
+# RUN: not diff -r %t/dir1 %t/dir2
+# RUN: rm %t/dir2/extrafile
+
+# Empty extra directory, "diff -r" should fail.
+# RUN: mkdir -p %t/dir1/extra_subdir
+# RUN: not diff -r %t/dir1 %t/dir2
+
+# Non-empty extra directory, diff -r should fail.
+# RUN: mkdir -p %t/dir1/extra_subdir/dir
+# RUN: echo "1337" > %t/dir1/extra_subdir/dir/file
+# RUN: not diff -r %t/dir1 %t/dir2
+
+# Sanity check for the test directory.
+# RUN: rm -rf %t/dir1/extra_subdir
+# RUN: diff -r %t/dir1 %t/dir2
+# RUN: rm -rf %t/dir1 %t/dir2