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204 lines
6.6 KiB
Text
204 lines
6.6 KiB
Text
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Orc Integration
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===============
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Sections
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--------
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- About Orc
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- Fast memcpy()
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- Normal Usage
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- Build Process
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- Testing
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- Orc Limitations
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About Orc
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---------
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Orc code can be in one of two forms: in .orc files that is converted
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by orcc to C code that calls liborc functions, or C code that calls
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liborc to create complex operations at runtime. The former is mostly
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for functions with predetermined functionality. The latter is for
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functionality that is determined at runtime, where writing .orc
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functions for all combinations would be prohibitive. Orc also has
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a fast memcpy and memset which are useful independently.
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Fast memcpy()
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-------------
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*** This part is not integrated yet. ***
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Orc has built-in functions orc_memcpy() and orc_memset() that work
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like memcpy() and memset(). These are meant for large copies only.
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A reasonable cutoff for using orc_memcpy() instead of memcpy() is
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if the number of bytes is generally greater than 100. DO NOT use
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orc_memcpy() if the typical is size is less than 20 bytes, especially
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if the size is known at compile time, as these cases are inlined by
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the compiler.
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(Example: sys/ximage/ximagesink.c)
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Add $(ORC_CFLAGS) to libgstximagesink_la_CFLAGS and $(ORC_LIBS) to
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libgstximagesink_la_LIBADD. Then, in the source file, add:
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#ifdef HAVE_ORC
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#include <orc/orc.h>
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#else
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#define orc_memcpy(a,b,c) memcpy(a,b,c)
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#endif
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Then switch relevant uses of memcpy() to orc_memcpy().
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The above example works whether or not Orc is enabled at compile
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time.
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Normal Usage
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------------
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The following lines are added near the top of Makefile.am for plugins
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that use Orc code in .orc files (this is for the volume plugin):
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ORC_BASE=volume
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include $(top_srcdir)/common/orc.mk
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Also add the generated source file to the plugin build:
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nodist_libgstvolume_la_SOURCES = $(ORC_SOURCES)
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And of course, add $(ORC_CFLAGS) to libgstvolume_la_CFLAGS, and
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$(ORC_LIBS) to libgstvolume_la_LIBADD.
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The value assigned to ORC_BASE does not need to be related to
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the name of the plugin.
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Advanced Usage
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--------------
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The Holy Grail of Orc usage is to programmatically generate Orc code
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at runtime, have liborc compile it into binary code at runtime, and
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then execute this code. Currently, the best example of this is in
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Schroedinger. An example of how this would be used is audioconvert:
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given an input format, channel position manipulation, dithering and
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quantizing configuration, and output format, a Orc code generator
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would create an OrcProgram, add the appropriate instructions to do
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each step based on the configuration, and then compile the program.
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Successfully compiling the program would return a function pointer
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that can be called to perform the operation.
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This sort of advanced usage requires structural changes to current
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plugins (e.g., audioconvert) and will probably be developed
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incrementally. Moreover, if such code is intended to be used without
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Orc as strict build/runtime requirement, two codepaths would need to
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be developed and tested. For this reason, until GStreamer requires
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Orc, I think it's a good idea to restrict such advanced usage to the
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cog plugin in -bad, which requires Orc.
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Build Process
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-------------
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The goal of the build process is to make Orc non-essential for most
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developers and users. This is not to say you shouldn't have Orc
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installed -- without it, you will get slow backup C code, just that
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people compiling GStreamer are not forced to switch from Liboil to
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Orc immediately.
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With Orc installed, the build process will use the Orc Compiler (orcc)
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to convert each .orc file into a temporary C source (tmp-orc.c) and a
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temporary header file (${name}orc.h if constructed from ${base}.orc).
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The C source file is compiled and linked to the plugin, and the header
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file is included by other source files in the plugin.
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If 'make orc-update' is run in the source directory, the files
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tmp-orc.c and ${base}orc.h are copied to ${base}orc-dist.c and
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${base}orc-dist.h respectively. The -dist.[ch] files are automatically
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disted via orc.mk. The -dist.[ch] files should be checked in to
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git whenever the .orc source is changed and checked in. Example
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workflow:
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edit .orc file
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... make, test, etc.
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make orc-update
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git add volume.orc volumeorc-dist.c volumeorc-dist.h
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git commit
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At 'make dist' time, all of the .orc files are compiled, and then
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copied to their -dist.[ch] counterparts, and then the -dist.[ch]
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files are added to the dist directory.
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Without Orc installed (or --disable-orc given to configure), the
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-dist.[ch] files are copied to tmp-orc.c and ${name}orc.h. When
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compiled Orc disabled, DISABLE_ORC is defined in config.h, and
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the C backup code is compiled. This backup code is pure C, and
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does not include orc headers or require linking against liborc.
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The common/orc.mk build method is limited by the inflexibility of
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automake. The file tmp-orc.c must be a fixed filename, using ORC_NAME
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to generate the filename does not work because it conflicts with
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automake's dependency generation. Building multiple .orc files
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is not possible due to this restriction.
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Testing
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-------
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If you create another .orc file, please add it to
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tests/orc/Makefile.am. This causes automatic test code to be
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generated and run during 'make check'. Each function in the .orc
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file is tested by comparing the results of executing the run-time
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compiled code and the C backup function.
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Orc Limitations
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---------------
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audioconvert
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Orc doesn't have a mechanism for generating random numbers, which
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prevents its use as-is for dithering. One way around this is to
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generate suitable dithering values in one pass, then use those
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values in a second Orc-based pass.
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Orc doesn't handle 64-bit float, for no good reason.
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Irrespective of Orc handling 64-bit float, it would be useful to
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have a direct 32-bit float to 16-bit integer conversion.
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audioconvert is a good candidate for programmatically generated
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Orc code.
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audioconvert enumerates functions in terms of big-endian vs.
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little-endian. Orc's functions are "native" and "swapped".
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Programmatically generating code removes the need to worry about
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this.
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Orc doesn't handle 24-bit samples. Fixing this is not a priority
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(for ds).
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videoscale
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Orc doesn't handle horizontal resampling yet. The plan is to add
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special sampling opcodes, for nearest, bilinear, and cubic
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interpolation.
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videotestsrc
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Lots of code in videotestsrc needs to be rewritten to be SIMD
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(and Orc) friendly, e.g., stuff that uses oil_splat_u8().
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A fast low-quality random number generator in Orc would be useful
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here.
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volume
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Many of the comments on audioconvert apply here as well.
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There are a bunch of FIXMEs in here that are due to misapplied
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patches.
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