glMemoryBarrier

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 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 binding.

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 , glTexSubImage1D , 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* , 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 via glBufferSubData , glCopyBufferSubData , or glGetBufferSubData , or to buffer object memory mapped by glMapBuffer or glMapBufferRange after the barrier will 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_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.

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.

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.

glMemoryBarrierByRegion behaves as described above for glMemoryBarrier , with two differences:

First, it narrows the region under consideration so that only reads and writes of prior fragment shaders that are invoked for a smaller region of the framebuffer will be completed/reflected prior to subsequent reads and writes of following fragment shaders. The size of the region is implementation-dependent and may be as small as one framebuffer pixel.

Second, it only applies to memory transactions that may be read by or written by a fragment shader. Therefore, only the barrier bits are supported.

When barriers is GL_ALL_BARRIER_BITS , shader memory accesses will be synchronized relative to all these barrier bits, but not to other barrier bits specific to glMemoryBarrier . This implies that reads and writes for scatter/gather-like algorithms may or may not be completed/reflected after a glMemoryBarrierByRegion command. However, for uses such as deferred shading, where a linked list of visible surfaces with the head at a framebuffer address may be constructed, and the entirety of the list is only dependent on previous executions at that framebuffer address, glMemoryBarrierByRegion may be significantly more efficient than glMemoryBarrier .

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.

GL_INVALID_VALUE is generated if barriers is not the special value GL_ALL_BARRIER_BITS , and has any bits set other than those described above for glMemoryBarrier or glMemoryBarrierByRegion respectively.

glBindImageTexture , glBufferData , glMapBuffer , glMapBufferRange , glFlushMappedBufferRange , memoryBarrier

Copyright 2011-2014 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. .