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GLSL maximum number of instructions

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glsl

Is there a maximum number of assembly language instructions to be loaded into the fragment program unit? I have an algorithm on to port from cpu to gpu and apparently it doesn't fit on the gpu.

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Alin Avatar asked Apr 11 '10 17:04

Alin

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1 Answers

There are several hard and soft limits, some of which are not immediately obvious:

  • Instruction slots: The total number of instructions that the hardware can accomodate in local memory.
  • Executed instructions: The maximum number of instructions that will execute (including instructions that run several times in a loop)
  • A single GLSL instruction can map to a dozen or more instructions
  • Several GLSL instructions can map to a single instruction depending on the optimizer's quality (e.g. multiply-add, dot, lerp)
  • Limited temp registers (only 32) may require more instructions than necessary on pre-SM4 hardware (no such problem with 4096).
  • Swizzling usually does not cost extra instructions nowadays, but does on some older hardware, and may in some situations on some hardware (esp. gl_FragColor is such a candidate)
  • Regardless of actual instructions, OpenGL 2.0 compatible hardware is limited to 8 dependent texture fetches (unlimited on hardware that can do OpenGL 2.1 or better)

You have these guaranteed minimums (most cards have more):

  • 512 instruction slots for vertex and pixel shaders on OpenGL 2.x (SM3) capable hardware
    • 65536 executed instructions
  • 4096 vertex and 65536 pixel shader instruction slots on 3.x (SM4) hardware
    • 65536 executed vertex shader instructions, unlimited pixel shader instructions
  • At least 24 dynamic branches possible on 2.x (SM3) hardware
  • Fully dynamic branching (no limits) on SM4 hardware
  • Only conditional move available on SM2.x, everything else must be accomodated by code duplication and loop unrolling, or must fail
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Damon Avatar answered Oct 21 '22 07:10

Damon
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