GLSL maximum number of instructions

Question

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.

Answer 1

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

Answer 2

There is a limit on the maximum amount of instructions a shader can have. As far as I know, it varies from GPU to GPU. If your shader is too large, compilation will generate an error.