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