Index: llvm/utils/lit/lit/TestRunner.py
===================================================================
--- llvm/utils/lit/lit/TestRunner.py
+++ llvm/utils/lit/lit/TestRunner.py
@@ -365,7 +365,7 @@
     """executeBuiltinDiff - Compare files line by line."""
     args = expand_glob_expressions(cmd.args, cmd_shenv.cwd)[1:]
     try:
-        opts, args = getopt.gnu_getopt(args, "wbur", ["strip-trailing-cr"])
+        opts, args = getopt.gnu_getopt(args, "wbuU:r", ["strip-trailing-cr"])
     except getopt.GetoptError as err:
         raise InternalShellError(cmd, "Unsupported: 'diff':  %s" % str(err))
 
@@ -373,6 +373,7 @@
     ignore_all_space = False
     ignore_space_change = False
     unified_diff = False
+    num_context_lines = 3
     recursive_diff = False
     strip_trailing_cr = False
     for o, a in opts:
@@ -382,6 +383,16 @@
             ignore_space_change = True
         elif o == "-u":
             unified_diff = True
+        elif o.startswith("-U"):
+            unified_diff = True
+            try:
+                num_context_lines = int(a)
+                if num_context_lines < 0:
+                    raise ValueException
+            except:
+                raise InternalShellError(cmd,
+                                         "Error: invalid '-U' argument: {}\n"
+                                         .format(a))
         elif o == "-r":
             recursive_diff = True
         elif o == "--strip-trailing-cr":
@@ -433,12 +444,16 @@
         exitCode = 0
         if hasattr(difflib, 'diff_bytes'):
             # python 3.5 or newer
-            diffs = difflib.diff_bytes(difflib.unified_diff, filelines[0], filelines[1], filepaths[0].encode(), filepaths[1].encode())
+            diffs = difflib.diff_bytes(difflib.unified_diff, filelines[0],
+                                       filelines[1], filepaths[0].encode(),
+                                       filepaths[1].encode(),
+                                       n = num_context_lines)
             diffs = [diff.decode() for diff in diffs]
         else:
             # python 2.7
             func = difflib.unified_diff if unified_diff else difflib.context_diff
-            diffs = func(filelines[0], filelines[1], filepaths[0], filepaths[1])
+            diffs = func(filelines[0], filelines[1], filepaths[0], filepaths[1],
+                         n = num_context_lines)
 
         for diff in diffs:
             stdout.write(diff)
@@ -471,7 +486,8 @@
             filelines[idx]= [f(line) for line in lines]
 
         func = difflib.unified_diff if unified_diff else difflib.context_diff
-        for diff in func(filelines[0], filelines[1], filepaths[0], filepaths[1]):
+        for diff in func(filelines[0], filelines[1], filepaths[0], filepaths[1],
+                         n = num_context_lines):
             stdout.write(diff)
             exitCode = 1
         return exitCode
Index: llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-0.txt
===================================================================
--- /dev/null
+++ llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-0.txt
@@ -0,0 +1,3 @@
+# Check bad -U argument.
+# RUN: echo foo > %t
+# RUN: diff -U 30.1 %t %t
Index: llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-1.txt
===================================================================
--- /dev/null
+++ llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-1.txt
@@ -0,0 +1,3 @@
+# Check bad -U argument.
+# RUN: echo foo > %t
+# RUN: diff -U-1 %t %t
Index: llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified.txt
===================================================================
--- /dev/null
+++ llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified.txt
@@ -0,0 +1,34 @@
+# RUN: echo 1 > %t.foo
+# RUN: echo 2 >> %t.foo
+# RUN: echo 3 >> %t.foo
+# RUN: echo 4 >> %t.foo
+# RUN: echo 5 >> %t.foo
+# RUN: echo 6 foo >> %t.foo
+# RUN: echo 7 >> %t.foo
+# RUN: echo 8 >> %t.foo
+# RUN: echo 9 >> %t.foo
+# RUN: echo 10 >> %t.foo
+# RUN: echo 11 >> %t.foo
+
+# RUN: echo 1 > %t.bar
+# RUN: echo 2 >> %t.bar
+# RUN: echo 3 >> %t.bar
+# RUN: echo 4 >> %t.bar
+# RUN: echo 5 >> %t.bar
+# RUN: echo 6 bar >> %t.bar
+# RUN: echo 7 >> %t.bar
+# RUN: echo 8 >> %t.bar
+# RUN: echo 9 >> %t.bar
+# RUN: echo 10 >> %t.bar
+# RUN: echo 11 >> %t.bar
+
+# Default is 3 lines of context.
+# RUN: diff -u %t.foo %t.bar && false || true
+
+# Override default of 3 lines of context.
+# RUN: diff -U 2 %t.foo %t.bar && false || true
+# RUN: diff -U4 %t.foo %t.bar && false || true
+# RUN: diff -U0 %t.foo %t.bar && false || true
+
+# Fail so lit will print output.
+# RUN: false
Index: llvm/utils/lit/tests/max-failures.py
===================================================================
--- llvm/utils/lit/tests/max-failures.py
+++ llvm/utils/lit/tests/max-failures.py
@@ -8,7 +8,7 @@
 #
 # END.
 
-# CHECK: Failing Tests (27)
+# CHECK: Failing Tests (30)
 # CHECK: Failing Tests (1)
 # CHECK: Failing Tests (2)
 # CHECK: error: argument --max-failures: requires positive integer, but found '0'
Index: llvm/utils/lit/tests/shtest-shell.py
===================================================================
--- llvm/utils/lit/tests/shtest-shell.py
+++ llvm/utils/lit/tests/shtest-shell.py
@@ -149,6 +149,86 @@
 
 # CHECK: PASS: shtest-shell :: diff-r.txt
 
+# CHECK: FAIL: shtest-shell :: diff-unified-error-0.txt
+# CHECK: *** TEST 'shtest-shell :: diff-unified-error-0.txt' FAILED ***
+# CHECK: $ "diff" "-U" "30.1" "{{[^"]*}}" "{{[^"]*}}"
+# CHECK: # command stderr:
+# CHECK: Error: invalid '-U' argument: 30.1
+# CHECK: error: command failed with exit status: 127
+# CHECK: ***
+
+# CHECK: FAIL: shtest-shell :: diff-unified-error-1.txt
+# CHECK: *** TEST 'shtest-shell :: diff-unified-error-1.txt' FAILED ***
+# CHECK: $ "diff" "-U-1" "{{[^"]*}}" "{{[^"]*}}"
+# CHECK: # command stderr:
+# CHECK: Error: invalid '-U' argument: -1
+# CHECK: error: command failed with exit status: 127
+# CHECK: ***
+
+
+# CHECK: FAIL: shtest-shell :: diff-unified.txt
+
+# CHECK: *** TEST 'shtest-shell :: diff-unified.txt' FAILED ***
+
+# CHECK: $ "diff" "-u" "{{[^"]*}}.foo" "{{[^"]*}}.bar"
+# CHECK: # command output:
+# CHECK: @@ {{.*}} @@
+# CHECK-NEXT: 3
+# CHECK-NEXT: 4
+# CHECK-NEXT: 5
+# CHECK-NEXT: -6 foo
+# CHECK-NEXT: +6 bar
+# CHECK-NEXT: 7
+# CHECK-NEXT: 8
+# CHECK-NEXT: 9
+# CHECK-EMPTY:
+# CHECK-NEXT: error: command failed with exit status: 1
+# CHECK-NEXT: $ "true"
+
+# CHECK: $ "diff" "-U" "2" "{{[^"]*}}.foo" "{{[^"]*}}.bar"
+# CHECK: # command output:
+# CHECK: @@ {{.*}} @@
+# CHECK-NEXT: 4
+# CHECK-NEXT: 5
+# CHECK-NEXT: -6 foo
+# CHECK-NEXT: +6 bar
+# CHECK-NEXT: 7
+# CHECK-NEXT: 8
+# CHECK-EMPTY:
+# CHECK-NEXT: error: command failed with exit status: 1
+# CHECK-NEXT: $ "true"
+
+# CHECK: $ "diff" "-U4" "{{[^"]*}}.foo" "{{[^"]*}}.bar"
+# CHECK: # command output:
+# CHECK: @@ {{.*}} @@
+# CHECK-NEXT: 2
+# CHECK-NEXT: 3
+# CHECK-NEXT: 4
+# CHECK-NEXT: 5
+# CHECK-NEXT: -6 foo
+# CHECK-NEXT: +6 bar
+# CHECK-NEXT: 7
+# CHECK-NEXT: 8
+# CHECK-NEXT: 9
+# CHECK-NEXT: 10
+# CHECK-EMPTY:
+# CHECK-NEXT: error: command failed with exit status: 1
+# CHECK-NEXT: $ "true"
+
+# CHECK: $ "diff" "-U0" "{{[^"]*}}.foo" "{{[^"]*}}.bar"
+# CHECK: # command output:
+# CHECK: @@ {{.*}} @@
+# CHECK-NEXT: -6 foo
+# CHECK-NEXT: +6 bar
+# CHECK-EMPTY:
+# CHECK-NEXT: error: command failed with exit status: 1
+# CHECK-NEXT: $ "true"
+
+# CHECK: $ "false"
+
+# CHECK: ***
+
+
 # CHECK: FAIL: shtest-shell :: error-0.txt
 # CHECK: *** TEST 'shtest-shell :: error-0.txt' FAILED ***
 # CHECK: $ "not-a-real-command"
@@ -228,4 +308,4 @@
 # CHECK: PASS: shtest-shell :: sequencing-0.txt
 # CHECK: XFAIL: shtest-shell :: sequencing-1.txt
 # CHECK: PASS: shtest-shell :: valid-shell.txt
-# CHECK: Failing Tests (27)
+# CHECK: Failing Tests (30)