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Kacper Michajłow
24914d60df
It only wrapped math.h and mathematics.h since the fallbacks went away. Include math.h and mathematics.h directly where needed. internal.h keeps both so that the many translation units relying on them transitively keep building. Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
71 lines
2.7 KiB
Plaintext
71 lines
2.7 KiB
Plaintext
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/internal.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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