On Sun, Jul 16, 2017 at 12:51 AM, Segher Boessenkool <seg...@kernel.crashing.org> wrote: > Hi! > > On Wed, Jul 12, 2017 at 03:15:09PM +0200, Georg-Johann Lay wrote: >> the current cost computations in rtlanal.c and maybe other places >> suffer from the fact that they are hiding parts of the expressions >> from the back-end, like SET_DESTs of single_set or the anatomy of >> PARALELLs. >> >> Would it be in order to have a hook like the one attached? >> >> I am aware of that, in an ideal world, there wouldn't be more >> than one hook to get rtx costs. But well... > > The number of hooks is not a problem. The overall complexity of this > interface (between the backends and optimisers; a group of related > hooks) _is_ a problem. Also, the interface should be good for all > targets, easy to use, do one thing and do it well. > > Currently we have rtx_costs, which computes an estimated cost for any > expression. In most cases what we want to compute is the cost of an > instruction though! It can be a single set (in which case the > expression cost is a reasonable approximation), but it also can be > something else (like, a parallel). Also, on many targets at least, > it is *much* easier to compute the cost knowing that this is a valid > instruction. > > So I argue that we want to have a hook for insn cost.
And I think at previous GNU Cauldrons we agreed to that. > Now what should it take as input? An rtx_insn, or just the pattern > (as insn_rtx_cost does)? Is there any useful info on the other operands of an rtx_insn? If not then passing in the pattern (a rtx) might be somewhat more flexible. Of course it's then way easier to confuse rtx_cost and insn_cost ... > And if an rtx_insn, should that than be an > rtx_insn that is already linked into the insn chain (so we can see what > other insns generate its inputs, and which of its outputs are unused, > etc.)? > > One comment about your patch: > >> +/* A magic value to be returned by TARGET_INSN_COSTS to indicate that >> + the costs are not known or too complicated to work out there. */ >> +#define INSN_COSTS_UNKNOWN (-1234567) > > Why not just -1? And is 0 really so terrible; in the extremely rare > case we really want cost 0, it won't hurt much saying "unknown", as we > do currently. For "hook unimplemented", just set the hook to NULL. > > > Segher
