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This patch adds more RAM optimizations to eLua:
- direct file memory mapping: files in ROMFS will be read directly from Flash,
without allocating any additional buffers. This doesn't help with RAM
consumption in itself, but enables the set of optimizations below.
- pseudo read-only strings. These are still TStrings, but the actual string
content can point directly to Flash. Original Lua strings are kept in
TStrings structures (lobject.h):
typedef union TString {
L_Umaxalign dummy; /* ensures maximum alignment for strings */
struct {
CommonHeader;
lu_byte reserved;
unsigned int hash;
size_t len;
} tsv;
} TString;
The actual string content comes right after the union TString above.
Pseudo RO strings have the same header, but instead of having the string
content after TString, they have a pointer that points to the actual
string content (which should exist in a RO memory (Flash) that is directly
accesbile from the MCU bus (like its internal Flash memory)). lua_newlstr
detects automatically if it should create a regular string or a pseudo RO
string by checking if the string pointer comes from the Flash region of the
MCU. This optimization works for both precompiled (.lc) files that exist in
ROMFS and for internal Lua strings (C code).
- functions in Flash: for precompiled (.lc) files that exist in ROMFS, the code
of the functions and a part of the debug information will be read directly
from Flash.
- ROMFS was changed to support files that are larger than 2**16 bytes and it
aligns all its files to an offset which is a multiple of 4 in order to prevent
data alignment issues with precompiled Lua code.
- the Lua bytecode dumper was changed to align all the instructions in a Lua
function and a part of the debug information to an offset which is a multiple
of 4. This might slightly increase the size of the precompiled Lua file.
These changes were succesfully checked against the Lua 5.1 test suite.
These changes were tested in eLua on LM3S and AVR32.