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NAME
glMemoryBarrier - defines a barrier ordering memory transactions
C SPECIFICATION
void glMemoryBarrier(GLbitfield barriers);
void glMemoryBarrierByRegion(GLbitfield barriers);
PARAMETERS
barriers
Specifies the barriers to insert.
For glMemoryBarrier, must be a bitwise combination of any 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.
For glMemoryBarrier, must be a bitwise combination of any of
GL_ATOMIC_COUNTER_BARRIER_BIT, or GL_FRAMEBUFFER_BARRIER_BIT,
GL_SHADER_IMAGE_ACCESS_BARRIER_BIT, GL_SHADER_STORAGE_BARRIER_BIT.
GL_TEXTURE_FETCH_BARRIER_BIT, or GL_UNIFORM_BARRIER_BIT.
If the special value GL_ALL_BARRIER_BITS is specified, all supported barriers for the
corresponding command will be inserted.
DESCRIPTION FOR 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.
• 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.
• 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.
DESCRIPTION FOR GLMEMORYBARRIERBYREGION
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
• GL_ATOMIC_COUNTER_BARRIER_BIT
• GL_FRAMEBUFFER_BARRIER_BIT
• GL_SHADER_IMAGE_ACCESS_BARRIER_BIT
• GL_SHADER_STORAGE_BARRIER_BIT
• GL_TEXTURE_FETCH_BARRIER_BIT
• GL_UNIFORM_BARRIER_BIT
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.
NOTES
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 is not the special value GL_ALL_BARRIER_BITS,
and has any bits set other than those described above for glMemoryBarrier or
glMemoryBarrierByRegion respectively.
VERSION SUPPORT
┌────────────────────────┬───────────────────────────────────────────────────────────────────────┐
│ │ OpenGL Version │
├────────────────────────┼─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┤
│Function │ 2.0 │ 2.1 │ 3.0 │ 3.1 │ 3.2 │ 3.3 │ 4.0 │ 4.1 │ 4.2 │ 4.3 │ 4.4 │ 4.5 │
│/ │ │ │ │ │ │ │ │ │ │ │ │ │
│Feature │ │ │ │ │ │ │ │ │ │ │ │ │
│Name │ │ │ │ │ │ │ │ │ │ │ │ │
├────────────────────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┤
│glMemoryBarrier │ - │ - │ - │ - │ - │ - │ - │ - │ ✔ │ ✔ │ ✔ │ ✔ │
├────────────────────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┤
│glMemoryBarrierByRegion │ - │ - │ - │ - │ - │ - │ - │ - │ - │ - │ - │ ✔ │
└────────────────────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┘
SEE ALSO
glBindImageTexture(), glBufferData(), glMapBuffer(), glMapBufferRange(),
glFlushMappedBufferRange(), memoryBarrier()
COPYRIGHT
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.
http://opencontent.org/openpub/.
COPYRIGHT
Copyright © 2011-2014 Khronos Group
[FIXME: source] 01/03/2018 GLMEMORYBARRIER(3G)