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.
Johann
