Unity compiled shader better understanding

Unity Engine
1

Hi there
I have several questions about compiled shaders and will be very thankful if you help.

// Vertex combos: 9
//   opengl - ALU: 22 to 34
//   d3d9 - ALU: 23 to 37
//   d3d11 - ALU: 8 to 15, TEX: 0 to 0, FLOW: 1 to 1
  1. What does “combos”, “ALU”, “TEX” and “FLOW” mean ?
  2. If “ALU” is Arithmetic logic unit, then why it is not constant and varies from one value “to” another ?
  3. How can I get instruction count used by shader ?

Thanks

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