diff --git a/src/p_local.h b/src/p_local.h
index 0e06ad72b0..cb404eb8cf 100644
--- a/src/p_local.h
+++ b/src/p_local.h
@@ -240,10 +240,11 @@ fixed_t P_AproxDistance (fixed_t dx, fixed_t dy);
 
 inline int P_PointOnLineSide (fixed_t x, fixed_t y, const line_t *line)
 {
-	const SDWORD result = DMulScale32 (y-line->v1->y, line->dx, line->v1->x-x, line->dy);
-	return (compatflags2 & COMPATF2_POINTONLINE)
-		? result >= 0
-		: result > 0;
+	extern int P_VanillaPointOnLineSide(fixed_t x, fixed_t y, const line_t* line);
+
+	return compatflags2 & COMPATF2_POINTONLINE
+		? P_VanillaPointOnLineSide(x, y, line)
+		: DMulScale32 (y-line->v1->y, line->dx, line->v1->x-x, line->dy) > 0;
 }
 
 //==========================================================================
@@ -257,10 +258,11 @@ inline int P_PointOnLineSide (fixed_t x, fixed_t y, const line_t *line)
 
 inline int P_PointOnDivlineSide (fixed_t x, fixed_t y, const divline_t *line)
 {
-	const SDWORD result = DMulScale32 (y-line->y, line->dx, line->x-x, line->dy);
+	extern int P_VanillaPointOnDivlineSide(fixed_t x, fixed_t y, const divline_t* line);
+
 	return (compatflags2 & COMPATF2_POINTONLINE)
-		? result >= 0
-		: result > 0;
+		? P_VanillaPointOnDivlineSide(x, y, line)
+		: (DMulScale32 (y-line->y, line->dx, line->x-x, line->dy) > 0);
 }
 
 //==========================================================================
diff --git a/src/p_maputl.cpp b/src/p_maputl.cpp
index 3a4a4c6a04..0a39460e7d 100644
--- a/src/p_maputl.cpp
+++ b/src/p_maputl.cpp
@@ -1523,3 +1523,92 @@ static AActor *RoughBlockCheck (AActor *mo, int index, void *param)
 	}
 	return NULL;
 }
+
+//===========================================================================
+//
+// P_VanillaPointOnLineSide
+// P_PointOnLineSide() from the initial Doom source code release
+//
+//===========================================================================
+
+int P_VanillaPointOnLineSide(fixed_t x, fixed_t y, const line_t* line)
+{
+	fixed_t	dx;
+	fixed_t	dy;
+	fixed_t	left;
+	fixed_t	right;
+
+	if (!line->dx)
+	{
+		if (x <= line->v1->x)
+			return line->dy > 0;
+
+		return line->dy < 0;
+	}
+	if (!line->dy)
+	{
+		if (y <= line->v1->y)
+			return line->dx < 0;
+
+		return line->dx > 0;
+	}
+
+	dx = (x - line->v1->x);
+	dy = (y - line->v1->y);
+
+	left = FixedMul ( line->dy>>FRACBITS , dx );
+	right = FixedMul ( dy , line->dx>>FRACBITS );
+
+	if (right < left)
+		return 0;		// front side
+	return 1;			// back side
+}
+
+//===========================================================================
+//
+// P_VanillaPointOnDivlineSide
+// P_PointOnDivlineSide() from the initial Doom source code release
+//
+//===========================================================================
+
+int P_VanillaPointOnDivlineSide(fixed_t x, fixed_t y, const divline_t* line)
+{
+	fixed_t	dx;
+	fixed_t	dy;
+	fixed_t	left;
+	fixed_t	right;
+
+	if (!line->dx)
+	{
+		if (x <= line->x)
+			return line->dy > 0;
+
+		return line->dy < 0;
+	}
+	if (!line->dy)
+	{
+		if (y <= line->y)
+			return line->dx < 0;
+
+		return line->dx > 0;
+	}
+
+	dx = (x - line->x);
+	dy = (y - line->y);
+
+	// try to quickly decide by looking at sign bits
+	if ( (line->dy ^ line->dx ^ dx ^ dy)&0x80000000 )
+	{
+		if ( (line->dy ^ dx) & 0x80000000 )
+			return 1;		// (left is negative)
+		return 0;
+	}
+
+	left = FixedMul ( line->dy>>8, dx>>8 );
+	right = FixedMul ( dy>>8 , line->dx>>8 );
+
+	if (right < left)
+		return 0;		// front side
+	return 1;			// back side
+}
+