From b41971e187c9faa6877423b042a8d939b41693ab Mon Sep 17 00:00:00 2001
From: Justin Ethier <justin.ethier@gmail.com>
Date: Mon, 10 Sep 2018 13:29:30 -0400
Subject: [PATCH] Issue #276 - Improve overflow detection for multiplication

---
 runtime.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/runtime.c b/runtime.c
index ae98b2ac..ad46248c 100644
--- a/runtime.c
+++ b/runtime.c
@@ -3050,17 +3050,14 @@ static int Cyc_checked_sub(int x, int y, int *result)
   return ((((*result ^ x) & ~(*result ^ y)) >> 30) != 0);
 }
 
-// Code from http://stackoverflow.com/q/1815367/101258
 static int Cyc_checked_mul(int x, int y, int *result)
 {
-//  *result = x * y;
-//  return (*result != 0 && (*result)/x != y) || // Overflow
-//         (*result > CYC_FIXNUM_MAX) ||
-//         (*result < CYC_FIXNUM_MIN);
+  // Avoid undefined behavior by detecting overflow prior to multiplication
+  // Based on code from Hacker's Delight and CHICKEN scheme
   uint xu, yu, c;
-  c = 1UL<<31UL;
-  xu = x < 0 ? -1 : x;
-  yu = y < 0 ? -1 : y;
+  c = (1UL<<30UL) - 1;
+  xu = x < 0 ? -x : x;
+  yu = y < 0 ? -y : y;
 
   if (yu != 0 && xu > (c / yu)) return 1; // Overflow