Hi, On Wed, 10 Jul 2013, David Given wrote:
> Michael Matz wrote: > [...] > > As you didn't adjust any cost I would guess the high value comes from the > > default implementation of address_cost, which simply uses arithmetic cost, > > and the MULT in there is quite expensive by default. > > > > See TARGET_ADDRESS_COST in several ports. > > Oddly, TARGET_ADDRESS_COST is never being called for my port, Bah, it's only used from some call sites (fwprop, postreload and loop-invariant), not generally to adjust the cost of (MEM addr) based on address_cost (addr). That might be a sensible change in itself I guess. > After having done a bunch of reading up on how costing works, and > deciphering the rather cryptic other ports, my understanding is: > > Costing is based entirely on analysis of the RTL, and is completely > irrelevant of what insns are selected. > > Therefore if my backend wants to support certain optimised addressing > modes, I need to insert code into my TARGET_RTX_COSTS hook that looks > for mem constructions which can be represented by such addressing modes, (or for when outer_code is MEM) > and encourages the compiler to select them by giving them a low cost. I > don't get any assistance from the patterns in the .md file. > > Have I got that right? With the current state of affairs, yes. Ciao, Michael.
