glMemoryBarrier man page on RedHat
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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.
· 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.
· 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 MemoryBarrier() 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.
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 © 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 03/16/2013 GLMEMORYBARRIER(3G)
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