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. 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. R.
