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openglglsl

opengl shared memory layout and size

Given the following glsl declarations (this is just an example):

struct S{
    f16vec3 a;
    float16_t b;
    f16vec3_t c;
    float16_t d;
};
shared float16_t my_float_array[100];
shared S my_S_array[100];

I have the following questions:

  • How much shared memory will be used by a given declaration, in the above example for instance?
  • Which memory layout is used for variables in shared memory? std140, std430 or something else?
  • How does this play with bank conflicts?

I was able to get the total shared memory required by a program using glGetProgramBinary and skiping until the begining of the text part indicated by a line starting with "!!NV":

...
!!NVcp5.0
OPTION NV_shader_buffer_load;
OPTION NV_internal;
OPTION NV_gpu_program_fp64;
OPTION NV_shader_storage_buffer;
OPTION NV_bindless_texture;
OPTION NV_gpu_program5_mem_extended;
GROUP_SIZE 4 4 4;
SHARED_MEMORY 4480;
SHARED shared_mem[] = { program.sharedmem };
...

This is rather indirect though and does not tell much about the alignment/packing rules.

Solution

  • Which memory layout is used for variables in shared memory? std140, std430 or something else?

    It's implementation-defined.

    The layouts applied to buffer-backed storage matter because code external to the shader needs to be able to access it. shared variables cannot be externally to the shader, so the layout of such variables is functionally implementation-defined.

    In short, you cannot know for certain how much storage any particular shared variable declaration will consume. You can assume that it will take up at least as many bytes as is minimally required to store the data you asked to store. But that's about it.

    How does this play with bank conflicts?

    "Banks" are not a concept which OpenGL or GLSL recognizes. It is therefore implementation-defined.