[Libre-soc-bugs] [Bug 238] POWER Compressed Formal Standard writeup

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Wed Nov 18 23:17:19 GMT 2020


--- Comment #38 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
jacob spotted during the meeting today that it is r1 that is used as the SP by
convention, not r31.

we therefore need at the minimum the option to select 2 regs for RA/RT on LD/ST

we also want to maximise imm length and given that the current sradi/srawi is
*already* using sub-sub-encoding taking up bits 1-4 it actually doesn't help
with LD/ST as much as it would to instead steal space from e.g. cmpdi/cmpwi.

if we stole space from cmpi we would get 2x or 4x the amount of benefit than if
we tried to steal from sradi/srawi because those share a sub-sub-encoding where
the cmpis have one sub-encoding *each*.

i will mull this over however please do feel free to try some shuffles

do watch out for the priority on the number of occurrences of each of the imm
instructions, from the analysis of /bin/bash that jacob did

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