On Fri, Dec 6, 2024 at 2:17 PM Georg-Johann Lay <a...@gjlay.de> wrote: > > Am 06.12.24 um 13:23 schrieb Sam James: > > Georg-Johann Lay <a...@gjlay.de> writes: > > > >> This patch disables CRC lookup tables which consume quite some RAM. > > > > Given that -foptimize-crc is new, it may be useful to CC the pass > > authors in case they have input. > > CCing Mariam Arutunian > > >> Ok for trunk? > >> > >> Johann > > The problem is not in the new CRC pass, but because AVR is a very > limited hardware. Because AVR has no linear address space, .rodata > has to be placed in RAM, and most devices have just a few KiB of RAM, > or even less. > > An extension PR49857 to put such lookup tables in flash / program memory > has been rejected by the global maintainers as "too specific", ignoring > most of the constraints imposed by the requirement of using named > address-spaces. > > The point is that you cannot just put data in flash, one must also use > the correct instructions to access them, which is achieved by means > of avr specific named address-spaces like __flash. > > This would require a new target hook like proposed in PR49857, which > could put lookup-tables into a non-generic address-space provided: > > *) All respective data is put in the preferred address-space, and > > *) All accesses have to use the same address-space as of 1), > independent of what the rest of the code may look like. > > To date, only 3 lookup tables generated by GCC meet these criteria: > > 1) Lookup tables from tree-switch conversion. > > 2) Lookup tables from the current CRC work > > 3) vtables. Though g++ does not, and probably never will, support > named address spaces.
4) const global data I think generally this might be a sound optimization - but as you said it requires generating correct accesses in the first place which also means once anything takes the address of the data all pointer uses have to know beforehand. Thus application is likely quite limited for 4) at least. Maybe it's possible to perform half of the task by the linker via relaxation? Though I can easily guess it's somewhat difficult for AVR. Richard. > Johann > > > >
