glMemoryBarrier

glMemoryBarrier: man4/glMemoryBarrier.xml

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: 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.

1. 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 glBufferSubData will invalidate shader caches implicitly as required.
2. 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.
3. 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.
4. 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.
5. 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.

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.