Maciej W. Rozycki (email@example.com) writes:
> > We added patterns to let our (old) GCC use the new rotates and
> > bit-insert/extracts, at least in simple cases. I'm not sure whether
> > we've put those in our 3.4 evolution tree yet, but if we have we
> > should push those out.
> As a side note, it makes me wonder where the borderline of the RISC
> actually is. Even Intel abandoned support for bit insert/extract
> instructions after an initial attempt for the i386. They figured out the
> implementation was too complicated. ;-)
It probably was... but MIPS uses register-to-register ALU operations
and no condition codes. The interface to the ALU is typically rather
simple. So adding some peculiar new 2- or 3-operand computation is
If the instruction is too complicated, of course, it might eventually
become a critical path and make the whole CPU slower. But
insert/extract - while elaborate to describe - involve only fairly
Remember: the point of RISC was never to have less instructions
(that's just a cute acronym) - the point was and is to define an
instruction set which is easy to implement as an efficient pipeline.
Of course, instructions still have to be *useful* to be added.
Insert/extract make a reasonable case for themselves, but actually
arrived in MIPS32 release 2 as part of a bunch of other bit-shuffle
instructions (also includes rotates and various byte-swaps) which -
together - help quite a bit to manipulate sub-word data in registers.