Generate tables for libcanard table coding system #30
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This allows the entry point to be inlined at the caller site, paving the way for various optimizations. <msg>_encode now calls the non-inlined _<msg>_encode function in the .c file which does the preliminary work then calls __<msg>_encode to encode the actual type, if compound (or e.g. canardEncodeScalar).
Allows the processing of 0-length messages (which is true of some important ones in the DSDL) to be optimized away.
If the message structure is compatible, the compiler generates a table which describes the structure. If enabled, an interpreter in libcanard is then used to follow the table description and encode or decode the message. This occupies substantially fewer bytes per message as the table is much smaller than the C function previously used. This translates to a large reduction in program memory usage, especially if many different message types need to be coded. The call to the interpreter (along with a pointer to the message and the associated table) is inlined whereever e.g. <msg>_encode is called. The existing function to encode/decode is still generated in case table coding is not enabled in libcanard or the structure is not compatible. This commit sets up all the hooks to generate the tables and call the interpreter functions. However, no tables are generated as the generation function is a simple stub that always fails.
Each table has a four byte header, plus some number of entries (4 bytes). Each field consumes one entry. The maximum encoded message length in bytes, size of the C message struct in chars, and number of table entries all are limited to 65535 for table coding to be possible.
There is no additional overhead for nesting.
Static arrays have a 2-entry header (8 bytes), plus the entries describing the contents of the array. Array contents cannot consist of more than 256 entries (including header entries and entries describing nested types). The array length cannot exceed 65535.
Dynamic arrays have a 3-entry header (12 bytes), plus the entries describing the contents of the array. Array contents cannot consist of more than 256 entries (including header entries and entries describing nested types). The maximum array length cannot exceed 65535.
Unions have a 2-entry header (8 bytes), plus 1 entry for each field, plus the entries describing each field. The maximum number of fields is 255. Each field cannot consist of more than 255 entries (including header entries and entries describing nested types). This completes the list of DSDL types.
This was referenced Jul 26, 2024
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