11 Nov
2017
11 Nov
'17
5:58 a.m.
Hi Toby,
It's funny you should mention "1-bit
bytes" and PowerPC close
together, because the PowerPC has an architectural feature that I have
not seen discussed much - the 8 x 4 bit CR register set [...] and of
course the usual branch tests.
I have been curious about whether a compiler could make good use of
this facility.
POWER, PowerPC's predecessor, had this too, at least on the IBM RS/6000s
running AIX I used. A lot of the work was performance related, so I
spent much time looking at disassembly of the output of xlc, their cc.
It seemed to make use of more than one CR quite often, but never very
many of them. I recall thinking it could have done better. Certainly,
re-doing bits in assembler allowed that to be exploited as long as one
could track which CR was tracking what previous test and whether it was
still valid. Perhaps the compiler's architecture, probably not written
from scratch?, didn't adapt well to that possibility?
--
Cheers, Ralph.
https://plus.google.com/+RalphCorderoy