py/objint: In int.from_bytes, only create big-int if really needed.

dpgeorge · dpgeorge · commit 58bb73e01030 · 2017-05-06T10:29:09.000+10:00
This patch ensures that int.from_bytes only creates a big-int if necessary,
by checking the value for a small-int overflow as it's being parsed.
diff --git a/py/objint.c b/py/objint.c
@@ -407,26 +407,25 @@ STATIC mp_obj_t int_from_bytes(size_t n_args, const mp_obj_t *args) {
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[1], &bufinfo, MP_BUFFER_READ);
 
-    #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
-    // If result guaranteedly fits in small int, use that
-    if (bufinfo.len >= sizeof(mp_uint_t) || !MP_SMALL_INT_FITS(1 << (bufinfo.len * 8 - 1))) {
-        return mp_obj_int_from_bytes_impl(args[2] != MP_OBJ_NEW_QSTR(MP_QSTR_little), bufinfo.len, bufinfo.buf);
-    } else
-    #endif
-    {
-        const byte* buf = (const byte*)bufinfo.buf;
-        int delta = 1;
-        if (args[2] == MP_OBJ_NEW_QSTR(MP_QSTR_little)) {
-            buf += bufinfo.len - 1;
-            delta = -1;
-        }
+    const byte* buf = (const byte*)bufinfo.buf;
+    int delta = 1;
+    if (args[2] == MP_OBJ_NEW_QSTR(MP_QSTR_little)) {
+        buf += bufinfo.len - 1;
+        delta = -1;
+    }
 
-        mp_uint_t value = 0;
-        for (; bufinfo.len--; buf += delta) {
-            value = (value << 8) | *buf;
+    mp_uint_t value = 0;
+    size_t len = bufinfo.len;
+    for (; len--; buf += delta) {
+        #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
+        if (value > (MP_SMALL_INT_MAX >> 8)) {
+            // Result will overflow a small-int so construct a big-int
+            return mp_obj_int_from_bytes_impl(args[2] != MP_OBJ_NEW_QSTR(MP_QSTR_little), bufinfo.len, bufinfo.buf);
         }
-        return mp_obj_new_int_from_uint(value);
+        #endif
+        value = (value << 8) | *buf;
     }
+    return mp_obj_new_int_from_uint(value);
 }
 
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(int_from_bytes_fun_obj, 3, 4, int_from_bytes);
