I'm making a game using XNA framework, so I use a lot functions that operate on vectors. (especially Vector2 (64bit struct)). What bothers me is that most of the methods are defined with ref and out parameters. Here is an example:
void Min(ref Vector2 value1, ref Vector2 value2, out Vector2 result)
which looks a bit strange too me. There is also another Min which is more obvious
public static Vector2 Min(Vector2 value1, Vector2 value2);
Basically, almost all the functions have overloads with refs and outs. Similar, other APIs.
What is the benefit of this design? XNA is optimized for performance, could it be a result? Say, Quaternion requires 128b where passing by ref less.
EDIT:
Here is a test code:
public class Game1 : Microsoft.Xna.Framework.Game
{
GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;
private Vector2 vec1 = new Vector2(1, 2);
private Vector2 vec2 = new Vector2(2, 3);
private Vector2 min;
private string timeRefOut1;
private string timeRefOut2;
private SpriteFont font;
public Game1()
{
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
refOut1();
refOut2();
}
private Vector2 refOut1()
{
Vector2 min = Vector2.Min(vec1, vec2);
return min;
}
private Vector2 refOut2()
{
Vector2.Min(ref vec1, ref vec2, out min);
return min;
}
protected override void Initialize()
{
const int len = 100000000;
Stopwatch stopWatch = new Stopwatch();
stopWatch.Start();
for (int i = 0; i < len; i++)
{
refOut1();
}
stopWatch.Stop();
timeRefOut1 = stopWatch.ElapsedMilliseconds.ToString();
stopWatch.Reset();
stopWatch.Start();
for (int i = 0; i < len; i++)
{
refOut2();
}
stopWatch.Stop();
timeRefOut2 = stopWatch.ElapsedMilliseconds.ToString();
base.Initialize();
}
protected override void LoadContent()
{
spriteBatch = new SpriteBatch(GraphicsDevice);
font = Content.Load<SpriteFont>("SpriteFont1");
}
protected override void Update(GameTime gameTime)
{
if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
this.Exit();
base.Update(gameTime);
}
protected override void Draw(GameTime gameTime)
{
GraphicsDevice.Clear(Color.CornflowerBlue);
spriteBatch.Begin();
spriteBatch.DrawString(font, timeRefOut1, new Vector2(200, 200), Color.White);
spriteBatch.DrawString(font, timeRefOut2, new Vector2(200, 300), Color.White);
spriteBatch.End();
// TODO: Add your drawing code here
base.Draw(gameTime);
}
}
The results:
Win 7 64bit, .Net 4. XNA 4.0
Also IL code
.method public hidebysig static void Min(valuetype Microsoft.Xna.Framework.Vector2& value1,
valuetype Microsoft.Xna.Framework.Vector2& value2,
[out] valuetype Microsoft.Xna.Framework.Vector2& result) cil managed
{
// Code size 69 (0x45)
.maxstack 3
IL_0000: ldarg.2
IL_0001: ldarg.0
IL_0002: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0007: ldarg.1
IL_0008: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_000d: blt.s IL_0017
IL_000f: ldarg.1
IL_0010: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0015: br.s IL_001d
IL_0017: ldarg.0
IL_0018: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_001d: stfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0022: ldarg.2
IL_0023: ldarg.0
IL_0024: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0029: ldarg.1
IL_002a: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_002f: blt.s IL_0039
IL_0031: ldarg.1
IL_0032: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0037: br.s IL_003f
IL_0039: ldarg.0
IL_003a: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_003f: stfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0044: ret
} // end of method Vector2::Min
and
.method public hidebysig static valuetype Microsoft.Xna.Framework.Vector2
Min(valuetype Microsoft.Xna.Framework.Vector2 value1,
valuetype Microsoft.Xna.Framework.Vector2 value2) cil managed
{
// Code size 80 (0x50)
.maxstack 3
.locals init (valuetype Microsoft.Xna.Framework.Vector2 V_0)
IL_0000: ldloca.s V_0
IL_0002: ldarga.s value1
IL_0004: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0009: ldarga.s value2
IL_000b: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0010: blt.s IL_001b
IL_0012: ldarga.s value2
IL_0014: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0019: br.s IL_0022
IL_001b: ldarga.s value1
IL_001d: ldfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0022: stfld float32 Microsoft.Xna.Framework.Vector2::X
IL_0027: ldloca.s V_0
IL_0029: ldarga.s value1
IL_002b: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0030: ldarga.s value2
IL_0032: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0037: blt.s IL_0042
IL_0039: ldarga.s value2
IL_003b: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0040: br.s IL_0049
IL_0042: ldarga.s value1
IL_0044: ldfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_0049: stfld float32 Microsoft.Xna.Framework.Vector2::Y
IL_004e: ldloc.0
IL_004f: ret
} // end of method Vector2::Min
Seems overhead is caused by temp Vector. Also I tried 1GHz WP 7.5 device:
Number of ticks for an order of magnitude smaller number of iterations.
Vector2 is a struct, which means that when it's returned as a value a copy is returned, rather than returning a reference to an existing structure. By using ref/out parameters you can avoid this copy so that the Vector created in the Min method is the exact vector in your result variable.
It's one of those micro optimization that normally would be discouraged, but in the game world it's done often enough, and in environments where performance matters enough, that it's worth the slightly less readable option.