71 lines
2.7 KiB
Text
71 lines
2.7 KiB
Text
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Writing a table generator
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This documentation is preliminary.
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Parts of the API are not good and should be changed.
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Basic concepts
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A table generator consists of two files, *_tablegen.c and *_tablegen.h.
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The .h file will provide the variable declarations and initialization
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code for the tables, the .c calls the initialization code and then prints
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the tables as a header file using the tableprint.h helpers.
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Both of these files will be compiled for the host system, so to avoid
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breakage with cross-compilation neither of them may include, directly
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or indirectly, config.h or avconfig.h.
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This means that e.g. libavutil/mathematics.h is ok but libavutil/libm.h is not.
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Due to this, the .c file or Makefile may have to provide additional defines
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or stubs, though if possible this should be avoided.
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In particular, CONFIG_HARDCODED_TABLES should always be defined to 0.
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The .c file
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This file should include the *_tablegen.h and tableprint.h files and
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anything else it needs as long as it does not depend on config.h or
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avconfig.h.
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In addition to that it must contain a main() function which initializes
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all tables by calling the init functions from the .h file and then prints
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them.
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The printing code typically looks like this:
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write_fileheader();
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printf("static const uint8_t my_array[100] = {\n");
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write_uint8_t_array(my_array, 100);
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printf("};\n");
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This is the more generic form, in case you need to do something special.
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Usually you should instead use the short form:
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write_fileheader();
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WRITE_ARRAY("static const", uint8_t, my_array);
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write_fileheader() adds some minor things like a "this is a generated file"
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comment and some standard includes.
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tablegen.h defines some write functions for one- and two-dimensional arrays
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for standard types - they print only the "core" parts so they are easier
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to reuse for multi-dimensional arrays so the outermost {} must be printed
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separately.
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If there's no standard function for printing the type you need, the
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WRITE_1D_FUNC_ARGV macro is a very quick way to create one.
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See libavcodec/dv_tablegen.c for an example.
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The .h file
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This file should contain:
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- one or more initialization functions
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- the table variable declarations
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If CONFIG_HARDCODED_TABLES is set, the initialization functions should
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not do anything, and instead of the variable declarations the
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generated *_tables.h file should be included.
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Since that will be generated in the build directory, the path must be
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included, i.e.
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#include "libavcodec/example_tables.h"
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not
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#include "example_tables.h"
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Makefile changes
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To make the automatic table creation work, you must manually declare the
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new dependency.
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For this add a line similar to this:
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$(SUBDIR)example.o: $(SUBDIR)example_tables.h
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under the "ifdef CONFIG_HARDCODED_TABLES" section in the Makefile.
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