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GLMEMORYBARRIER(3G)			  OpenGL Manual 		      GLMEMORYBARRIER(3G)

NAME
       glMemoryBarrier - defines a barrier ordering memory transactions

C SPECIFICATION
       void glMemoryBarrier(GLbitfield barriers);

PARAMETERS
       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_ATOMIC_COUNTER_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 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(), glTexSubImage(), 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.

       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.

       o   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.

       o   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.

       o   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.

       o   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 MemoryBarrier()
	   with the SHADER_IMAGE_ACCESS_BARRIER_BIT set in barriers between passes is necessary.

       o   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.

ERRORS
       GL_INVALID_VALUE is generated if barriers contains any bits other than those listed above,
       or is not the special value GL_ALL_BARRIER_BITS.

SEE ALSO
       glBindImageTexture(), glBufferData(), glMapBuffer(), glMapBufferRange(),
       glFlushMappedBufferRange()

COPYRIGHT
       Copyright (C) 2011 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/.

AUTHORS
       opengl.org

opengl.org				    06/10/2014			      GLMEMORYBARRIER(3G)
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