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Re: [rfc] slightly better reload constant rematerialization
On Thu, Jan 20, 2005 at 02:46:47PM -0500, Ian Lance Taylor wrote:
> The natural cost metrics to use would seem
> to be REGISTER_MOVE_COST and rtx_cost. Of course, as far as I know,
> we've never before said that those costs were commensurable.
Indeed they are not. The closest I can come up with is
COST_N_INSNS (REGISTER_MOVE_COST (x, y)) / 2
since the basis of REGISTER_MOVE_COST is defined to be 2 for
r->r moves, which I will equate to a latency of 1, which is
normally how COST_N_INSNS is defined. Except for -Os at which
point I have no idea how this works out.
> I kind of feel that a comparison model would work better, in which we
> can say "which is better: instruction sequence 1 or instruction
> sequence 2?" Then even in the current reductionist case of a single
> instruction or just part of one, we can still hopefully make more
> intelligent decisions rather than doing trial and error tinkering with
> cost values.
I don't think I understand. How do we compare the sequences if not
by some abstract cost metric?