Shader Storage Buffer Object

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Shader Storage Buffer Object
Core in version 4.5
Core since version 4.3
Core ARB extension ARB_shader_storage_buffer_object

A Shader Storage Buffer Object is a Buffer Object that is used to store and retrieve data from within the OpenGL Shading Language.

SSBOs are a lot like Uniform Buffer Objects. Shader storage blocks are defined by Interface Block (GLSL)s in almost the same way as uniform blocks. Buffer objects that store SSBOs are bound to SSBO binding points, just as buffer objects for uniforms are bound to UBO binding points. And so forth.

The major differences between them are:

  1. SSBOs can be much larger. The OpenGL spec guarantees that UBOs can be up to 16KB in size (implementations can allow them to be bigger). The spec guarantees that SSBOs can be up to 128MB. Most implementations will let you allocate a size up to the limit of GPU memory.
  2. SSBOs are writable, even atomically; UBOs are uniforms. SSBOs reads and writes use incoherent memory accesses, so they need the appropriate barriers, just as Image Load Store operations.
  3. SSBOs can have variable storage, up to whatever buffer range was bound for that particular buffer; UBOs must have a specific, fixed storage size. This means that you can have an array of arbitrary length in an SSBO (at the end, rather). The actual size of the array, based on the range of the buffer bound, can be queried at runtime in the shader using the length function on the unbounded array variable.
  4. SSBO access, all things being equal, will likely be slower than UBO access. SSBOs generally are accesses like buffer textures, while UBO data is accessed through internal shader-accessible memory reads. At the very least, UBOs will be no slower than SSBOs.

Functionally speaking, SSBOs can be thought of as a much nicer interface to Buffer Textures when accessed via Image Load Store.

Shader specification

SSBOs are declared as interface blocks, using the buffer keyword. They have special layout qualifiers for specifying important aspects of them, such a memory layout and binding qualities.

Atomic operations

There are special atomic functions that can be applied to variables in storage blocks (these can also be applied to Compute Shader shared variables). These only operate on uint or int types, but these can be members of aggregates (structs/arrays) or vector elements (ie: you can atomically access uvec3.x).

V · E

All of the atomic functions return the original value. The term "nint" can be int or uint.

nint atomicAdd(inout nint mem​, nint data​)

Adds data​ to mem​.

nint atomicMin(inout nint mem​, nint data​)

The mem​'s value is no lower than data​.

nint atomicMax(inout nint mem​, nint data​)

The mem​'s value is no greater than data​.

nint atomicAnd (inout nint mem​, nint data​)

mem​ becomes the bitwise-and between mem​ and data​.

nint atomicOr(inout nint mem​, nint data​)

mem​ becomes the bitwise-or between mem​ and data​.

nint atomicXor(inout nint mem​, nint data​)

mem​ becomes the bitwise-xor between mem​ and data​.

nint atomicExchange(inout nint mem​, nint data​)

Sets mem​'s value to data​.

nint atomicCompSwap(inout nint mem​, nint compare​, nint data​)

If the current value of mem​ is equal to compare​, then mem​ is set to data​. Otherwise it is left unchanged.

OpenGL usage

To copy an array of variable size to the SSBO

int data[];
GLuint ssbo;
glGenBuffers(1, &ssbo);
glBufferData(GL_SHADER_STORAGE_BUFFER, GLsizeiptr size​, data​, GLenum usage);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 3, ssbo);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0); // unbind

Note that the data is bound to the Layout Qualifier 3. It can be assigned to an array in the shader like this:

 layout(std430, binding = 3) buffer layoutName
     int data_SSBO[];

There can only be one array of variable size per SSBO and it has to be the bottommost in the layout definition. So this is possible too:

layout(std430, binding = 2) buffer anotherLayoutName
    int some_int;
    float fixed_array[42];
    float variable_array[];

The data can be assigned via a struct like this:

struct ssbo_data_t
    int foo;
    float bar[42];
    float baz[MY_SIZE];
} ssbo_data;