diff --git a/Tools/unicode/gen_expat_table.py b/Tools/unicode/gen_expat_table.py
new file mode 100644
index 000000000000000..ff08fb4c87dab39
--- /dev/null
+++ b/Tools/unicode/gen_expat_table.py
@@ -0,0 +1,107 @@
+import sys
+
+
+def info(msg, *args):
+    print(msg % args, file=sys.stderr)
+
+def fail(msg, *args):
+    info(msg, *args)
+    sys.exit(2)
+
+# ASCII characters that can appear in a well-formed XML document
+# except the characters "$@\^`{}~".
+compulsory_chars = (set(b'\t\n\r') | set(range(32, 127))) - set(br'$@\^`{}~')
+
+def check_compatibility(encoding):
+    for i in compulsory_chars:
+        c = chr(i)
+        b = bytes([i])
+        try:
+            decoded = b.decode(encoding)
+        except UnicodeDecodeError:
+            info('Cannot decode byte %#04x (%a)', i, c)
+            return False
+        if decoded != c:
+            info('Incompatible encoding: %#04x (%a) -> %a', i, c, decoded)
+            return False
+    return True
+
+def create_table(encoding):
+    mapping = [-1] * 256
+
+    non_bmp = None
+    for i in range(0x110000):
+        char = chr(i)
+        try:
+            encoded = char.encode(encoding)
+        except UnicodeEncodeError:
+            continue
+        if i >= 0x10000 and non_bmp is None:
+            non_bmp = i
+            info('Non-BMP character: %r (U+%04X)', char, i)
+        length = len(encoded)
+        k = encoded[0]
+        v = mapping[k]
+        if length == 1:
+            if v == -1:
+                mapping[k] = i
+            elif v < 0:
+                fail('Ambiguous mapping for %#04x: '
+                    '%r (U+%04X) and %d-byte sequence',
+                    k, c, i, -v)
+            else:
+                info('Ambiguous mapping for %#04x: '
+                    '%r (U+%04X) and %r (U+%04X)',
+                    k, chr(v), v, char, i)
+        else:
+            if length > 4:
+                fail('Too long encoding for %r (U+%04X): %r',
+                    char, i, encoded)
+            if v == -1:
+                mapping[k] = -length
+            elif v != -length:
+                if v < 0:
+                    fail('Ambiguous mapping for %#04x: '
+                        '%d-byte sequence and %d-byte sequence %r',
+                        k, -v, length, encoded)
+                else:
+                    fail('Ambiguous mapping for %#04x: '
+                        '%r (U+%04X) and %d-byte sequence %r',
+                        k, chr(v), v, length, encoded)
+    return mapping
+
+def print_table(mapping):
+    print(' '*8 + '_expat_decoding_table=(', end='')
+    if mapping[:128] == list(range(128)):
+        print('*range(128),')
+        start = 128
+    else:
+        print()
+        start = 0
+    line = ''
+    pos = 0
+    i = start
+    while True:
+        if i % 8 == 0:
+            if len(line) > 68:
+                print(' '*12 + line[:pos].rstrip())
+                line = line[pos:]
+            pos = len(line)
+        if i == 256:
+            break
+        v = mapping[i]
+        if v >= 0:
+            v = hex(v)
+        line += f'{v}, '
+        i += 1
+    print(' '*12 + line.rstrip().rstrip(',') + '),')
+
+
+encoding = sys.argv[1]
+
+if not check_compatibility(encoding):
+    print(' '*8 + '_expat_decoding_table=False,')
+    sys.exit(1)
+
+mapping = create_table(encoding)
+print_table(mapping)