The condition-code re-use issue is the point, where, IMO, the link to the
subreg-lowering 2.) shows up. After, e.g., breaking down a HI mode "sub"
operation into two QI mode "sub" and "sub-with-carry"s at expand, I consider
it to be extremely difficult to make the mid-end smart enough to identify
that a the end of the QI "sub-with-carry" the condition code is set according
to the corresponding HImode substract operation.
As a last resort, ou can always use peephole2's to remove unnecessary
subtracts on the HImode values.
(parallel [
(use (operands[0]))
(set (operands[0]) (minus:HI (operands[1]) (operands[2]))
(note "please delete the entire embracing parallel instruction before
register life-time analysis by a new pass: It pretends to use operands 1 and
2 while in fact this instruction does nothing except from giving hints to
GCSE.")
])
This seems define_insn_and_split, but it is a lot more complex than what
you probably can do...
IIRC, s390 does use add with carry and subtract with borrow instructions
effectively (alc and slb in IBM360^W s390-ese). Search the archives on
google or gmane.
- combine -re-run
No way, combine is too expensive... Its simplification engine is fine,
but a lot of things ought to be redone from scratch so that it becomes a
serious instruction selection pass.
Paolo
