glMemoryBarrier — defines a barrier ordering memory transactions
void glMemoryBarrier(
|
GLbitfield barriers); |
void glMemoryBarrierByRegion(
|
GLbitfield barriers); |
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.
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.
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.
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.
| 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
|
- | - | - | - | - | - | - | - | - | - | - | ✔ |
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. http://opencontent.org/openpub/.