Avoid branch prediction slowdowns in R_PointOnSide

src/r_main.c

@@ -249,7 +249,7 @@ static void FlipCam2_OnChange(void)
 //
 // killough 5/2/98: reformatted
 //
-INT32 R_PointOnSide(fixed_t x, fixed_t y, node_t *node)
+INT32 R_PointOnSide(fixed_t x, fixed_t y, node_t *restrict node)
 {
 	if (!node->dx)
 		return x <= node->x ? node->dy > 0 : node->dy < 0;
@@ -261,9 +261,10 @@ INT32 R_PointOnSide(fixed_t x, fixed_t y, node_t *node)
 	fixed_t dy = (y >> 1) - (node->y >> 1);
 
 	// Try to quickly decide by looking at sign bits.
-	if ((node->dy ^ node->dx ^ dx ^ dy) < 0)
-		return (node->dy ^ dx) < 0;  // (left is negative)
-	return FixedMul(dy, node->dx>>FRACBITS) >= FixedMul(node->dy>>FRACBITS, dx);
+	// also use a mask to avoid branch prediction
+	INT32 mask = (node->dy ^ node->dx ^ dx ^ dy) >> 31;
+	return (mask & ((node->dy ^ dx) < 0)) |  // (left is negative)
+		(~mask & (FixedMul(dy, node->dx>>FRACBITS) >= FixedMul(node->dy>>FRACBITS, dx)));
 }
 
 // killough 5/2/98: reformatted