ref: ef256921f31cfe2a3a8452fe30a3c0d96b416347
parent: a9a3ca3c0d8cea7fe24724a8f9bc17aa3c367677
author: Simon Howard <fraggle@soulsphere.org>
date: Fri Jul 10 23:38:49 EDT 2015
doom: Avoid overflow for spawn angle calculation. Integer overflow is undefined and this breaks when using Clang with -O2 optimization turned on. This fixes #572 (thanks to David Majnemer for insight into fixing this bug).
--- a/src/doom/g_game.c
+++ b/src/doom/g_game.c
@@ -1168,26 +1168,26 @@
fixed_t xa, ya;
signed int an;
- an = (ANG45 * ((signed int) mthing->angle / 45));
- // Right-shifting a negative signed integer is implementation-defined,
- // so divide instead.
- an /= 1 << ANGLETOFINESHIFT;
+ // This calculation overflows in Vanilla Doom, but here we deliberately
+ // avoid integer overflow as it is undefined behavior, so the value of
+ // 'an' will always be positive.
+ an = (ANG45 >> ANGLETOFINESHIFT) * ((signed int) mthing->angle / 45);
switch (an)
{
- case -4096:
+ case 4096: // -4096:
xa = finetangent[2048]; // finecosine[-4096]
ya = finetangent[0]; // finesine[-4096]
break;
- case -3072:
+ case 5120: // -3072:
xa = finetangent[3072]; // finecosine[-3072]
ya = finetangent[1024]; // finesine[-3072]
break;
- case -2048:
+ case 6144: // -2048:
xa = finesine[0]; // finecosine[-2048]
ya = finetangent[2048]; // finesine[-2048]
break;
- case -1024:
+ case 7168: // -1024:
xa = finesine[1024]; // finecosine[-1024]
ya = finetangent[3072]; // finesine[-1024]
break;
@@ -1195,7 +1195,6 @@
case 1024:
case 2048:
case 3072:
- case 4096:
xa = finecosine[an];
ya = finesine[an];
break;