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 GLSL.


SSBOs are a lot like Uniform Buffer Objects. Shader storage blocks are defined almost identically to uniform blocks. SSBOs are bound to SSBO binding points, just as UBOs are bound to UBO binding points. And so forth.

The major differences between them are:

  1. SSBOs are typically much larger. The smallest required UBO size is 16KB; the smallest required SSBO size is 16MB, and typical sizes will be on the order of the size of GPU memory.
  2. SSBOs are writable, even atomically; UBOs are uniform​s. SSBOs have the same memory characteristics as Image Load Store operations, so they need appropriate memory barriers.
  3. SSBOs can have unbounded storage, up to the buffer range bound; UBOs must have a specific, fixed storage size. This means that you can have an array of arbitrary length in an SSBO. 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.

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

Atomic operations

There are special atomic functions that can be applied to buffer variables (and to compute shader shared​ variables). They only take uint​ or int​ types, but these can be members of aggregates (structs/arrays) or vector elements (ie: you can atomically access uvec3.x​).

All of the atomic functions return the original value.

The atomic operations are (where "nint" can be int​ or uint​):

nint atomicAdd(inout nint mem​, nint data​)
nint atomicMin(inout nint mem​, nint data​)
nint atomicMax(inout nint mem​, nint data​)
nint atomicAnd (inout nint mem​, nint data​)
nint atomicOr(inout nint mem​, nint data​)
nint atomicXor(inout nint mem​, nint data​)
nint atomicExchange(inout nint mem​, nint data​)
nint atomicCompSwap(inout nint mem​, nint compare​, nint data​)

The last function bears some explanation. It will swap mem​ with data​ if compare​ is equal to the current value of mem​.