spirv_internals.md

Internals of SPIR-V

A SPIR-V module is a single unit of SPIR-V, most commonly found in a .spv file. Because SPIR-V is used in binary form, developers tend to reference the disassembled form instead (sometimes referred to as SPIR-V disassembly).

Small example of SPIR-V disassembly

     OpCapability Shader
     OpMemoryModel Logical Simple
     OpEntryPoint GLCompute %3 "main"
     OpExecutionMode %3 LocalSize 64 64 1
%1 = OpTypeVoid
%2 = OpTypeFunction %1 ; comment
%3 = OpFunction %1 None %2
%4 = OpLabel
     OpReturn
     OpFunctionEnd

The spec gives a full example of a simple GLSL shader to SPIR-V disassembly

A SPIR-V module is a single linear stream of words. The full physical layout of the module is well described in the spec. Here is a visual diagram showing the layout.

Reading SPIR-V Disassembly

SPIRV-Tools has a great breakdown of the syntax found in SPIR-V disassembly.

It is also very useful to use the spec when looking at SPIR-V disassembly.

Here is an example from a random set of SPIR-V disassembly line

%16 = OpTypeInt 32 1
%48 = OpVariable %47 Function
%54 = OpLoad %16 %48
%56 = OpSLessThan %25 %54 %55

Here is another example using

OpExecutionMode %3 LocalSize 64 64 1