[EMAIL PROTECTED] (Richard Kenner) writes: > 3) Write a new optimization pass enabled on targets which define > NOTICE_UPDATE_CC. I think this pass would be run just before > machine dependent reorg, although perhaps there is a better place > for it. Walk through the instructions, calling NOTICE_UPDATE_CC on > each one. When we find an instruction which sets CC_REG, check the > source of the set with the current CC status, just as > final_scan_insn does now. If the current CC status is the same, > delete the instruction which sets CC_REG. > > My gut feeling is that we shouldn't need to do this: the present > optimization passes should do this when we have an explicit register.
Well, in my proposal, I think they won't, because most of the instructions which set CC_REG (e.g., arithmetic instructions) will be marked as clobbering it. So the optimizers won't be able to do anything with that information. The point of NOTICE_UPDATE_CC is that it pulls out the information about how the condition code is set just when we need it. It is possible to mark each instruction with the affect it has on CC_REG, by changing it into a parallel set. Then the optimizers would be able to see the change to CC_REG, and do the appropriate combination. However, this would be a lot more work, and moreover it's not clear how effective it would be since so much of the compiler calls single_set and is not prepared to handle instructions with parallel sets. Ian
