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GLMEMORYBARRIER(3G) | [FIXME: manual] | GLMEMORYBARRIER(3G) |
NAME¶
glMemoryBarrier - defines a barrier ordering memory transactionsC SPECIFICATION¶
void
glMemoryBarrier(GLbitfield barriers);
void
glMemoryBarrierByRegion(GLbitfield barriers);
PARAMETERS¶
barriersSpecifies 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_BITIf 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.
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 / Feature Name | 2.0 | 2.1 | 3.0 | 3.1 | 3.2 | 3.3 | 4.0 | 4.1 | 4.2 | 4.3 | 4.4 | 4.5 |
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 Group08/26/2014 | [FIXME: source] |