GLAPI/glMemoryBarrier

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glMemoryBarrier
Core in version 4.4
Core since version 4.2
Core ARB extension ARB_shader_image_load_store

glMemoryBarrier: defines a barrier ordering memory transactions

Function Definition

 void glMemoryBarrier(GLbitfield barriers​);
barriers
Specifies the barriers to insert. Must be a bitwise combination of GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT​, GL_ELEMENT_ARRAY_BARRIER_BIT​, GL_UNIFORM_BARRIER_BIT​, GL_TEXTURE_FETCH_BARRIER_BIT​, GL_SHADER_IMAGE_ACCESS_BARRIER_BIT​, GL_COMMAND_BARRIER_BIT​, GL_PIXEL_BUFFER_BARRIER_BIT​, GL_TEXTURE_UPDATE_BARRIER_BIT​, GL_BUFFER_UPDATE_BARRIER_BIT​, GL_FRAMEBUFFER_BARRIER_BIT​, GL_TRANSFORM_FEEDBACK_BARRIER_BIT​, GL_QUERY_BUFFER_BARRIER_BIT​, GL_ATOMIC_COUNTER_BARRIER_BIT​, GL_CLIENT_MAPPED_BUFFER_BARRIER_BIT​, or GL_SHADER_STORAGE_BARRIER_BIT​. If the special value GL_ALL_BARRIER_BITS​ is specified, all supported barriers will be inserted.

Description

glMemoryBarrier defines a barrier ordering the memory transactions issued prior to the command relative to those issued after the barrier. For the purposes of this ordering, memory transactions performed by shaders are considered to be issued by the rendering command that triggered the execution of the shader. barriers​ is a bitfield indicating the set of operations that are synchronized with shader stores.

The key to understand here is that the bits you use represent how you intend to use the data written from an image load/store shader operation. If you write to an image in a rendering command, then read from it as a texture, you use GL_TEXTURE_FETCH_BARRIER_BIT​. If you instead attach that image to an FBO and use blending to do a read/modify/write operation, then you want to use the GL_FRAMEBUFFER_BARRIER_BIT​ barrier.

The bits used in barriers​ are as follows:

GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT​
If set, vertex data sourced from buffer objects after the barrier will reflect data written by shaders prior to the barrier. The set of buffer objects affected by this bit is derived from the buffer object bindings used for generic vertex attributes derived from the GL_VERTEX_ATTRIB_ARRAY_BUFFER​ bindings.
GL_ELEMENT_ARRAY_BARRIER_BIT​
If set, vertex array indices sourced from buffer objects after the barrier will reflect data written by shaders prior to the barrier. The buffer objects affected by this bit are derived from the GL_ELEMENT_ARRAY_BUFFER​ binding.
GL_UNIFORM_BARRIER_BIT​
Shader uniforms sourced from buffer objects after the barrier will reflect data written by shaders prior to the barrier.
GL_TEXTURE_FETCH_BARRIER_BIT​
Texture fetches from shaders, including fetches from buffer object memory via buffer textures, after the barrier will reflect data written by shaders prior to the barrier.
GL_SHADER_IMAGE_ACCESS_BARRIER_BIT​
Memory accesses using shader image load, store, and atomic built-in functions issued after the barrier will reflect data written by shaders prior to the barrier. Additionally, image stores and atomics issued after the barrier will not execute until all memory accesses (e.g., loads, stores, texture fetches, vertex fetches) initiated prior to the barrier complete.
GL_COMMAND_BARRIER_BIT​
Command data sourced from buffer objects by Draw*Indirect commands after the barrier will reflect data written by shaders prior to the barrier. The buffer objects affected by this bit are derived from the GL_DRAW_INDIRECT_BUFFER​ and GL_DISPATCH_INDIRECT_BUFFER​ bindings.
GL_PIXEL_BUFFER_BARRIER_BIT​
Reads and writes of buffer objects via the GL_PIXEL_PACK_BUFFER​ and GL_PIXEL_UNPACK_BUFFER​ bindings (via glReadPixels, glTexSubImage2D, etc.) after the barrier will reflect data written by shaders prior to the barrier. Additionally, buffer object writes issued after the barrier will wait on the completion of all shader writes initiated prior to the barrier.
GL_TEXTURE_UPDATE_BARRIER_BIT​
Writes to a texture via glTex(Sub)Image*, glCopyTex(Sub)Image*, glClearTex*Image, glCompressedTex(Sub)Image*, and reads via glGetTexImage after the barrier will reflect data written by shaders prior to the barrier. Additionally, texture writes from these commands issued after the barrier will not execute until all shader writes initiated prior to the barrier complete.
GL_BUFFER_UPDATE_BARRIER_BIT​
Reads or writes to buffer objects via any OpenGL API functions that allow modifying their contents reflect data written by shaders prior to the barrier. Additionally, writes via these commands issued after the barrier will wait on the completion of any shader writes to the same memory initiated prior to the barrier.
GL_QUERY_BUFFER_BARRIER_BIT​
Writes of buffer objects via the GL_QUERY_BUFFER​ binding after the barrier will reflect data written by shaders prior to the barrier. Additionally, buffer object writes issued after the barrier will wait on the completion of all shader writes initiated prior to the barrier.
GL_CLIENT_MAPPED_BUFFER_BARRIER_BIT​
Access by the client to persistent mapped regions of buffer objects will reflect data written by shaders prior to the barrier. Note that this may cause additional synchronization operations.
GL_FRAMEBUFFER_BARRIER_BIT​
Reads and writes via framebuffer object attachments after the barrier will reflect data written by shaders prior to the barrier. Additionally, framebuffer writes issued after the barrier will wait on the completion of all shader writes issued prior to the barrier.
GL_TRANSFORM_FEEDBACK_BARRIER_BIT​
Writes via transform feedback bindings after the barrier will reflect data written by shaders prior to the barrier. Additionally, transform feedback writes issued after the barrier will wait on the completion of all shader writes issued prior to the barrier.
GL_ATOMIC_COUNTER_BARRIER_BIT​
Accesses to atomic counters after the barrier will reflect writes prior to the barrier.
GL_SHADER_STORAGE_BARRIER_BIT​
Accesses to shader storage blocks after the barrier will reflect writes prior to the barrier.


If barriers​ is GL_ALL_BARRIER_BITS​, shader memory accesses will be synchronized relative to all the operations described above.

Implementations may cache buffer object and texture image memory that could be written by shaders in multiple caches; for example, there may be separate caches for texture, vertex fetching, and one or more caches for shader memory accesses. Implementations are not required to keep these caches coherent with shader memory writes. Stores issued by one invocation may not be immediately observable by other pipeline stages or other shader invocations because the value stored may remain in a cache local to the processor executing the store, or because data overwritten by the store is still in a cache elsewhere in the system. When glMemoryBarrier is called, the GL flushes and/or invalidates any caches relevant to the operations specified by the barriers​ parameter to ensure consistent ordering of operations across the barrier.

To allow for independent shader invocations to communicate by reads and writes to a common memory address, image variables in the OpenGL Shading Language may be declared as "coherent". Buffer object or texture image memory accessed through such variables may be cached only if caches are automatically updated due to stores issued by any other shader invocation. If the same address is accessed using both coherent and non-coherent variables, the accesses using variables declared as coherent will observe the results stored using coherent variables in other invocations. Using variables declared as "coherent" guarantees only that the results of stores will be immediately visible to shader invocations using similarly-declared variables; calling glMemoryBarrier is required to ensure that the stores are visible to other operations.

The following guidelines may be helpful in choosing when to use coherent memory accesses and when to use barriers.

  • Data that are read-only or constant may be accessed without using coherent variables or calling MemoryBarrier(). Updates to the read-only data via API calls such as BufferSubData will invalidate shader caches implicitly as required.
  • Data that are shared between shader invocations at a fine granularity (e.g., written by one invocation, consumed by another invocation) should use coherent variables to read and write the shared data.
  • Data written by one shader invocation and consumed by other shader invocations launched as a result of its execution ("dependent invocations") should use coherent variables in the producing shader invocation and call memoryBarrier() after the last write. The consuming shader invocation should also use coherent variables.
  • Data written to image variables in one rendering pass and read by the shader in a later pass need not use coherent variables or memoryBarrier(). Calling ```glMemoryBarrier`` with the SHADER_IMAGE_ACCESS_BARRIER_BIT​ set in barriers​ between passes is necessary.
  • Data written by the shader in one rendering pass and read by another mechanism (e.g., vertex or index buffer pulling) in a later pass need not use coherent variables or memoryBarrier(). Calling glMemoryBarrier with the appropriate bits set in barriers​ between passes is necessary.

Notes

glMemoryBarrier is available only if the GL version is 4.2 or higher.

GL_SHADER_STORAGE_BARRIER_BIT​ is available only if the GL version is 4.3 or higher.

GL_QUERY_BUFFER_BARRIER_BIT​ is available only if the GL version is 4.4 or higher.

Errors

GL_INVALID_VALUE​ is generated if barriers​ contains any unsupported bits, or is not the special value GL_ALL_BARRIER_BITS​.

See Also

glBindImageTexture, glBufferData, glMapBuffer, glMapBufferRange, glBindBufferRange

Copyright

Copyright © 2011-2013 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.