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Gonna joint the discussion. I've ported Lua to STM32 too, with 80-250k of RAM.
Had a test build with what I'm gonna call "ROM strings". My solution was:
1. `loadfile()` the target program on the PC Lua
2. Via some modifications to Lua, dump internal strings hash table along with the seed into a file.
3. "burn" the table into the microcontroller along with the Lua program
4. Modified Lua inside the microcontroller would take a pointer to the pre-build strings hash table and will use strings from there whenever possible. These strings are never freed (they're in ROM). It is easy to distinguish 'normal' strings from 'ROM' strings simply by looking at the address value of the pointer (ROM and RAM have different address spaces). Essentially there were two strings hash tables: one static in ROM and one dynamic in RAM. In the end it was too cumbersome to maintain and use, but it lowered RAM consumption approximately by half in my case.

Regards, Alexander Chernoskutov
@alexcherov (telegram)

On 26.07.2023 18:48, Roberto Ierusalimschy wrote:
Last time I looked at the source code, Lua didn't really have a concept of
"ROMability", in the sense that you can't save data structures in ROM/flash
and use them directly from there (without having to load then into RAM). In
eLua it's possible to use the compiled bytecode directly from flash and
there's also a "read-only string" type that keeps the *content* of the
string in flash (just the content, not the whole TString data structure)
and uses it from there. But all these require changes to the Lua source
Both would be interesting additions to next version. By "read-only
string" do you mean a string that is never freed? (All strings
in Lua are read only...)

-- Roberto