Index: llvm/utils/lit/lit/builtin_commands/diff.py
===================================================================
--- llvm/utils/lit/lit/builtin_commands/diff.py
+++ llvm/utils/lit/lit/builtin_commands/diff.py
@@ -24,10 +24,17 @@
         return path, sorted(child_trees)
 
 def compareTwoFiles(flags, filepaths):
+    # FIXME: How can we restart stdin if the encoding is wrong?  How can we
+    # read stdin with a different encoding or in binary mode in a way that's
+    # compatible with python 2 and 3?
     compare_bytes = False
     encoding = None
     filelines = []
+    has_stdin = False
     for file in filepaths:
+        if file == "-":
+            has_stdin = True
+            continue
         try:
             with open(file, 'r') as f:
                 filelines.append(f.readlines())
@@ -39,7 +46,15 @@
             except:
                 compare_bytes = True
 
+    if has_stdin and encoding:
+        sys.stderr.write("Unsupported: cannot compare '-' with encoding {}\n"
+                         .format(encoding))
+        sys.exit(1)
+
     if compare_bytes:
+        if has_stdin:
+            sys.stderr.write("Unsupported: cannot compare '-' as binary\n")
+            sys.exit(1)
         return compareTwoBinaryFiles(flags, filepaths)
     else:
         return compareTwoTextFiles(flags, filepaths, encoding)
@@ -71,7 +86,9 @@
 def compareTwoTextFiles(flags, filepaths, encoding):
     filelines = []
     for file in filepaths:
-        if encoding is None:
+        if file == "-":
+            filelines.append(sys.stdin.readlines())
+        elif encoding is None:
             with open(file, 'r') as f:
                 filelines.append(f.readlines())
         else:
@@ -205,10 +222,13 @@
     exitCode = 0
     try:
         for file in args:
-            if not os.path.isabs(file):
+            if file != "-" and not os.path.isabs(file):
                 file = os.path.realpath(os.path.join(os.getcwd(), file))
 
             if flags.recursive_diff:
+                if file == "-":
+                    sys.stderr.write("Error: cannot recursively compare '-'\n")
+                    sys.exit(1)
                 dir_trees.append(getDirTree(file))
             else:
                 filepaths.append(file)
Index: llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
===================================================================
--- llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
+++ llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
@@ -5,6 +5,16 @@
 # RUN: diff %t.foo %t.foo | FileCheck -allow-empty -check-prefix=EMPTY %s
 # RUN: diff -u %t.foo %t.bar | FileCheck %s && false || true
 
+# Check input pipe.
+# RUN: echo foo | diff -u - %t.foo
+# RUN: echo foo | diff -u %t.foo -
+# RUN: echo bar | diff -u %t.foo - && false || true
+# RUN: echo bar | diff -u - %t.foo && false || true
+
+# Check output and input pipes at the same time.
+# RUN: echo foo | diff - %t.foo | FileCheck -allow-empty -check-prefix=EMPTY %s
+# RUN: echo bar | diff -u %t.foo - | FileCheck %s && false || true
+
 # Fail so lit will print output.
 # RUN: false
 
Index: llvm/utils/lit/tests/Inputs/shtest-shell/diff-r-error-7.txt
===================================================================
--- /dev/null
+++ llvm/utils/lit/tests/Inputs/shtest-shell/diff-r-error-7.txt
@@ -0,0 +1,2 @@
+# diff -r currently cannot handle stdin.
+# RUN: diff -r - %t
Index: llvm/utils/lit/tests/Inputs/shtest-shell/diff-r-error-8.txt
===================================================================
--- /dev/null
+++ llvm/utils/lit/tests/Inputs/shtest-shell/diff-r-error-8.txt
@@ -0,0 +1,2 @@
+# diff -r currently cannot handle stdin.
+# RUN: diff -r %t -
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 (29)
 # CHECK: Failing Tests (1)
 # CHECK: Failing Tests (2)
 # CHECK: error: Option '--max-failures' requires positive integer
Index: llvm/utils/lit/tests/shtest-shell.py
===================================================================
--- llvm/utils/lit/tests/shtest-shell.py
+++ llvm/utils/lit/tests/shtest-shell.py
@@ -106,6 +106,51 @@
 # CHECK-NOT: error
 # CHECK: $ "true"
 
+# CHECK: $ "echo" "foo"
+# CHECK: $ "diff" "-u" "-" "{{[^"]*}}.foo"
+# CHECK-NOT: note
+# CHECK-NOT: error
+
+# CHECK: $ "echo" "foo"
+# CHECK: $ "diff" "-u" "{{[^"]*}}.foo" "-"
+# CHECK-NOT: note
+# CHECK-NOT: error
+
+# CHECK: $ "echo" "bar"
+# CHECK: $ "diff" "-u" "{{[^"]*}}.foo" "-"
+# CHECK: # command output:
+# CHECK: @@
+# CHECK-NEXT: -foo
+# CHECK-NEXT: +bar
+# CHECK: error: command failed with exit status: 1
+# CHECK: $ "true"
+
+# CHECK: $ "echo" "bar"
+# CHECK: $ "diff" "-u" "-" "{{[^"]*}}.foo"
+# CHECK: # command output:
+# CHECK: @@
+# CHECK-NEXT: -bar
+# CHECK-NEXT: +foo
+# CHECK: error: command failed with exit status: 1
+# CHECK: $ "true"
+
+# CHECK: $ "echo" "foo"
+# CHECK: $ "diff" "-" "{{[^"]*}}.foo"
+# CHECK-NOT: note
+# CHECK-NOT: error
+# CHECK: $ "FileCheck"
+# CHECK-NOT: note
+# CHECK-NOT: error
+
+# CHECK: $ "echo" "bar"
+# CHECK: $ "diff" "-u" "{{[^"]*}}.foo" "-"
+# CHECK: note: command had no output on stdout or stderr
+# CHECK: error: command failed with exit status: 1
+# CHECK: $ "FileCheck"
+# CHECK-NOT: note
+# CHECK-NOT: error
+# CHECK: $ "true"
+
 # CHECK: $ "false"
 
 # CHECK: ***
@@ -164,6 +209,20 @@
 # CHECK: File {{.*}}dir1{{.*}}extra_file is a regular empty file while file {{.*}}dir2{{.*}}extra_file is a directory
 # CHECK: error: command failed with exit status: 1
 
+# CHECK: FAIL: shtest-shell :: diff-r-error-7.txt
+# CHECK: *** TEST 'shtest-shell :: diff-r-error-7.txt' FAILED ***
+# CHECK: $ "diff" "-r" "-" "{{[^"]*}}"
+# CHECK: # command stderr:
+# CHECK: Error: cannot recursively compare '-'
+# CHECK: error: command failed with exit status: 1
+
+# CHECK: FAIL: shtest-shell :: diff-r-error-8.txt
+# CHECK: *** TEST 'shtest-shell :: diff-r-error-8.txt' FAILED ***
+# CHECK: $ "diff" "-r" "{{[^"]*}}" "-"
+# CHECK: # command stderr:
+# CHECK: Error: cannot recursively compare '-'
+# CHECK: error: command failed with exit status: 1
+
 # CHECK: PASS: shtest-shell :: diff-r.txt
 
 # CHECK: FAIL: shtest-shell :: error-0.txt
@@ -245,4 +304,4 @@
 # 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 (29)