-----Original Message----- From: gcc-ow...@gcc.gnu.org [mailto:gcc-ow...@gcc.gnu.org] On Behalf Of Jeff Law Sent: Wednesday, June 24, 2015 10:36 AM To: gcc@gcc.gnu.org Subject: Re: set_src_cost lying comment
On 06/21/2015 11:57 PM, Alan Modra wrote: > set_src_cost says it is supposed to > /* Return the cost of moving X into a register, relative to the cost > of a register move. SPEED_P is true if optimizing for speed rather > than size. */ > > Now, set_src_cost of a register move (set (reg1) (reg2)), is zero. > Why? Well, set_src_cost is used just on the right hand side of a SET, > so the cost is that of (reg2), which is zero according to rtlanal.c > rtx_cost. targetm.rtx_costs doesn't get a chance to modify this. > > Now consider (set (reg1) (ior (reg2) (reg3))), for which set_src_cost > on rs6000 currently returns COSTS_N_INSNS(1). It seems to me that > this also ought to return zero, if the set_src_cost comment is to be > believed. I'd claim the right hand side of this expression costs the > same as a register move. A register move machine insn "mr reg1,reg2" > is encoded as "or reg1,reg2,reg2" on rs6000! >>Certainly seems inconsistent -- all the costing stuff should be revisited. >>The basic design for costing dates back to the m68k/vax era. >>I certainly agree that the cost of a move, logicals and arithmetic is >>essentially the same at the chip level for many processors. But a copy has >>other properties >>that make it "cheaper" -- namely we can often propagate it >>away or arrange for the source & dest of the copy to have the same hard >>register which achieves >>the same effect. I have seen for target like Microblaze, making the changes in the cost of move instructions not same at the chip level cost/latency improves the performance for Mibench/EEMBC benchmarks. Also changing the cost of branch instruction and sometimes making it zero also improves the performance for Mibench/EEMBC benchmarks. >>So one could argue that a copy should have cost 0 as it has a reasonable >>chance of just going away, while logicals, alu operations on the appropriate >>chips >>should have a cost of 1. Thanks & Regards Ajit jeff
