compressed auipc

[jnk0le]

standard 3bit rd`
without immediate, has a range of subsequent addi/lw with +-2KiB range, adding small imm multiplies it

use cases:

• callback heavy code

need to find such codebases (usually proprietary)
Results could be biased by current compilers not caring about callback code placement (no reason to), loading addresses from .rodata or as lui + addi

• addressing .rodata

sometimes .rodata is in close range of current pc (e.g. small embedded with like <=32K of FLASH)
not very useful if tp ends up as .rodata pointer instead of second gp

EDIT:

• PIC code: GOT table acceses are also auipc based

[jnk0le]

IIRC someone was talking about such instr on isa-dev long time ago

[jnk0le]

assuming that all sections land close to each other

• with larger constant it can also reach rest of .data and .bss

embedded: I don't expect sram placed so close to flash

wrt memory models:
medlow + no offset: compete with c.lui - no gain
medany + no offset: is the compiler allowed to emit lui instead of auipc anyway? if yes - no gain

about the constant: reserved encoding next to compressed loads/stores allows for upto 7 bits of imm. +- 260K addresable space

[jnk0le]

to make encoding symmetric with c.lui, load/store {b,bu,h,hu} encoding needs to change, so the 12th bit becomes sign extension.

15:13	12	11:10	9:7	6:2	1:0
101	imm[17]	c.auipc	rd'	imm[16:12]	00