extended_instruction_sets.md

Extended Instruction Sets

Extended instruction sets allow SPIR-V to at a core to be agnostic as many high-level shading languages describe rules around the same instructions differently.

Example: The accuracy for an instruction such as sin() might be different in GLSL shaders than it is in OpenCL kernels

Separate Spec

All extended instruction sets are specified in independent specifications as listed in the registry.

There are three things needed when referring to an extended instruction set

1. Reference the extended instruction set spec
2. Use the header in SPIRV-Headers
   • Example: GLSL.std.450.h
3. Be aware the header was generated from the grammar JSON file in SPIRV-Headers
   • Example: extinst.glsl.std.450.grammar.json

Example using extended instruction sets

Looking at a simple GLSL shader which uses sin()

#version 450

layout(location = 0) in float inputValue;

void main() {
    float pos = sin(inputValue);
    gl_Position = vec4(pos, 1.0, 1.0, 1.0);
}

produces the following SPIR-V disassembly

    %1 = OpExtInstImport "GLSL.std.450"

%float = OpTypeFloat 32
  %ptr = OpTypePointer Input %float
  %var = OpVariable %_ptr_Input_float Input

  %pos = OpVariable %_ptr_Function_float Function
   %11 = OpLoad %float %var
   %12 = OpExtInst %float %1 Sin %11
         OpStore %pos %12
   %22 = OpLoad %float %pos

Breaking this down:

• Need to import the name of the extended instruction set being added with OpExtInstImport
• Use OpExtInst
  • Set the Set field to the imported non-semantic extension
  • Set the Instruction to the non-semantic extension specific instruction being added
    • For GLSL.std.450 it is listed as being instruction 13 in the extended instruction set spec
    •