Add script to diff two code size reports

This takes 2 JSON files containing the code size information and
computed the difference between them. It then pretty-prints the sizes
for files that are different along with the difference and percentage
difference in size.

Signed-off-by: David Horstmann <david.horstmann@arm.com>

Author: davidhorstmann-arm

scripts/diff_code_size.py

@@ -0,0 +1,78 @@
+#!/usr/bin/env python3
+"""Examine 2 code size reports and print the difference between them.
+"""
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+
+import json
+import sys
+
+def load_sizes(json_file):
+    with open(json_file) as f:
+        json_sizes = f.read()
+
+    sizes = json.loads(json_sizes)
+    return sizes
+
+def generate_diff_table(sizes_a, sizes_b):
+    table = []
+    total_size_a = 0
+    total_size_b = 0
+
+    for file in sizes_a:
+        size_a = (sizes_a[file]['text']
+                  + sizes_a[file]['data']
+                  + sizes_a[file]['bss'])
+        total_size_a += size_a
+
+        if file in sizes_b:
+            size_b = (sizes_b[file]['text']
+                      + sizes_b[file]['data']
+                      + sizes_b[file]['bss'])
+            size_diff = size_b - size_a
+
+            if size_diff != 0:
+                table.append((file.split('.')[0], size_a, size_b, size_diff,
+                              (size_diff * 100.0 / size_a)))
+        else:
+            # This file is only in A, so there's a code size decrease
+            table.append((file.split('.')[0], size_a, 0, -size_a, -100.0))
+
+    for file in sizes_b:
+        size_b = (sizes_b[file]['text']
+                  + sizes_b[file]['data']
+                  + sizes_b[file]['bss'])
+        total_size_b += size_b
+
+        if file not in sizes_a:
+            # This file is only in B, so there's a code size increase
+            table.append((file.split('.')[0], 0, size_b, size_b, 100.0))
+
+    total_size_diff = total_size_b - total_size_a
+    table.append(('TOTAL', total_size_a, total_size_b, total_size_diff,
+                  (total_size_diff * 100.0 / total_size_a)))
+
+    return table
+
+def display_diff_table(table):
+    table_line_format = '{:<40} {:>8} {:>8} {:>+8} {:>+8.2f}%'
+
+    print('{:<40} {:>8} {:>8} {:>8} {:>9}'.format(
+        'Module', 'Old', 'New', 'Delta', '% Delta'))
+
+    for line in table:
+        print(table_line_format.format(*line))
+
+def main():
+    if len(sys.argv) < 3:
+        print('Error: Less than 2 JSON files supplied.', file=sys.stderr)
+        sys.exit(1)
+
+    sizes_a = load_sizes(sys.argv[1])
+    sizes_b = load_sizes(sys.argv[2])
+
+    display_diff_table(generate_diff_table(sizes_a, sizes_b))
+
+if __name__ == '__main__':
+    main()