diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -134,14 +134,19 @@
 ;;
 check-generated-output)
     clean
-    echo "+++ Checking the output of the generated scripts"
+    echo "+++ Checking the output of the generator scripts"
     mkdir -p ${BUILD_DIR}
+    # Reject patches that introduce non-ASCII characters or hard tabs.
+    ! grep -r '[^ -~]' libcxx/include/
+    # Reject patches that don't update the generated output correctly.
     python3 libcxx/utils/generate_feature_test_macro_components.py
     python3 libcxx/utils/generate_header_inclusion_tests.py
     python3 libcxx/utils/generate_header_tests.py
     git diff | tee ${BUILD_DIR}/generated_output.patch
     # Check if the diffs are empty, fail otherwise.
     ! grep -q '^--- a' ${BUILD_DIR}/generated_output.patch
+    # Check that no dependency cycles have been introduced.
+    python3 libcxx/utils/graph_header_deps.py >/dev/null
 ;;
 generic-cxx03)
     export CC=clang
diff --git a/libcxx/utils/graph_header_deps.py b/libcxx/utils/graph_header_deps.py
--- a/libcxx/utils/graph_header_deps.py
+++ b/libcxx/utils/graph_header_deps.py
@@ -10,6 +10,7 @@
 import argparse
 import os
 import re
+import sys
 
 
 def is_config_header(h):
@@ -60,15 +61,19 @@
     local_includes = []
     system_includes = []
     linecount = 0
-    with open(fname, 'r') as f:
-        for line in f.readlines():
-            linecount += 1
-            m = re.match(r'\s*#\s*include\s+"([^"]*)"', line)
-            if m is not None:
-                local_includes.append(m.group(1))
-            m = re.match(r'\s*#\s*include\s+<([^>]*)>', line)
-            if m is not None:
-                system_includes.append(m.group(1))
+    with open(fname, 'r', encoding='ascii') as f:
+        try:
+            for line in f.readlines():
+                linecount += 1
+                m = re.match(r'\s*#\s*include\s+"([^"]*)"', line)
+                if m is not None:
+                    local_includes.append(m.group(1))
+                m = re.match(r'\s*#\s*include\s+<([^>]*)>', line)
+                if m is not None:
+                    system_includes.append(m.group(1))
+        except UnicodeDecodeError as e:
+            sys.stderr.write('Non-ASCII detected in %s\n' % fname)
+            raise
 
     fully_qualified_includes = [
         locate_header_file(h, options.search_dirs)
@@ -204,7 +209,7 @@
     for fname, entry in graph.items():
         for h in entry.includes:
             if transitively_includes(graph, h, fname):
-                print('Cycle detected between %s and %s' % (fname, h))
+                sys.stderr.write('Cycle detected between %s and %s\n' % (fname, h))
                 no_cycles_detected = False
     assert no_cycles_detected