On 25/06/15 17:27, Jeff Law wrote: > On 06/25/2015 06:28 AM, Richard Earnshaw wrote: >> On 24/06/15 17:47, Jeff Law wrote: >>> On 06/24/2015 03:18 AM, Alan Modra wrote: >>>> >>>> So in these examples we'd really like register moves to cost one >>>> insn. Hmm, at least, moves from hard regs ought to cost something. >>> The more I think about it, the more I think that's a reasonable step. >>> Nothing should have cost 0. >>> >>> Jeff >> >> It really means what you mean by cost here. I think rtx_cost is really >> talking about delta costs much of the time, so when recursing down >> >> plus (op1, op2) >> >> there's a cost from the plus, a cost from op1 and a cost from op2. >> >> I believe the idea behind reg being cost 0 is that when op1 and op2 are >> both registers in the above expression the overall cost is just the cost >> of the plus, with no additional cost coming from the operands. > Perhaps, but it's also the case on the PPC and a variety of other > architectures that there's no difference between a reg and some > constants. And on some architectures the PLUS is no different than a > logical operation or even a copy. >
Indeed, but that's why you need TARGET_RTX_COSTS to spot where such constants are used with no additional cost, or when more complex sub-operations can be folded into the basic cost. Just assuming all constants have zero cost would be equally wrong on most RISC processors, since it would be unusual for such chips to support any arbitrary constant in, say, a PLUS operation. R. > > > >> >> This leaves only one problem: if the entire expression is just >> >> reg >> >> then the overall cost becomes zero. We hit this problem because we only >> look at the source cost, not the overall insn cost. Mostly that's ok, >> but in the specific case of a move instruction it doesn't really >> generate the desired result. >> >> Perhaps the best thing to do is to use the OUTER code to spot the >> specific case where you've got a SET and return non-zero in that case. > Seems like it's worth an experiment. > > jeff
