From 039887a0ac9bd02cfd84e4792d3d21a78bc06a7f Mon Sep 17 00:00:00 2001
From: Paul Sokolovsky
Date: Sun, 2 Nov 2014 02:39:41 +0200
Subject: py: Fix bug with right-shifting small ints by large amounts.

Undefined behavior in C, needs explicit check.
---
 py/runtime.c | 5 +++++
 1 file changed, 5 insertions(+)

(limited to 'py/runtime.c')

diff --git a/py/runtime.c b/py/runtime.c
index f6f34be94..e225ba8b0 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;
-- 
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