py: Fix bug with right-shifting small ints by large amounts.

Undefined behavior in C, needs explicit check.

1 files changed, 5 insertions, 0 deletions

diff --git a/py/runtime.c b/py/runtime.c
index f6f34be940..e225ba8b05 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -337,6 +337,11 @@ mp_obj_t mp_binary_op(mp_uint_t op, mp_obj_t lhs, mp_obj_t rhs) {
                         nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "negative shift count"));
                     } else {
                         // standard precision is enough for right-shift
+                        if (rhs_val >= BITS_PER_WORD) {
+                            // Shifting to big amounts is underfined behavior
+                            // in C and is CPU-dependent; propagate sign bit.
+                            rhs_val = BITS_PER_WORD - 1;
+                        }
                         lhs_val >>= rhs_val;
                     }
                     break;