Vertex data default value?

Question

Vertex Layout in CPP

D3D11_INPUT_ELEMENT_DESC tangentspacevertexlayout[] =

   {

       {"POSITION",0,DXGI_FORMAT_R32G32B32_FLOAT,0,0, D3D11_INPUT_PER_VERTEX_DATA,0},

       {"TEXCOORD",0, DXGI_FORMAT_R32G32_FLOAT,   0,12, D3D11_INPUT_PER_VERTEX_DATA,0},

     …

But The HLSL code is DIFFERENT in June.2010 DXSDK and the new metro samples.

A. In Metro DX tutorial:
VS_INPUT  { float3 position;}

mul(float4(position,1.0f),mWorld)

----------------------

B. In DXSDK  2010:
VS_INPUT  { float4 position;} //position.w is default 1.0 ?

mul(position,mWorld)

---------------------

A has 2 more instructions than B,

mov r1.xyz, v0.xyzx 

mov r1.w, l(1.000000) 

The results are the same.  Which one is better,  Is B faster?

Are all devices support the default value 1.0 for undefined vector data?

Answer 1

B is faster, but does metro support B? 

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NEU_ShieldEdge

Answer 2

B is OK in metro as I tested on my NV GTX570