Hi Tom, Thanks for the reply, and sorry for responding so slowly.
Tom de Vries <vr...@codesourcery.com> writes: > On 05/25/2011 03:44 PM, Richard Sandiford wrote: >> Sorry for being so late. I was just curious... >> >> Tom de Vries <vr...@codesourcery.com> writes: >>> The init cost of an iv will in general not be zero. It will be >>> exceptional that the iv register happens to be initialized with the >>> proper value at no cost. In general, there will at the very least be a >>> regcopy or a const set. >>> >>> 2011-05-05 Tom de Vries <t...@codesourcery.com> >>> >>> PR target/45098 >>> * tree-ssa-loop-ivopts.c (determine_iv_cost): Prevent >>> cost_base.cost == 0. >>> Index: gcc/tree-ssa-loop-ivopts.c >>> =================================================================== >>> --- gcc/tree-ssa-loop-ivopts.c (revision 173380) >>> +++ gcc/tree-ssa-loop-ivopts.c (working copy) >>> @@ -4688,6 +4688,8 @@ determine_iv_cost (struct ivopts_data *d >>> >>> base = cand->iv->base; >>> cost_base = force_var_cost (data, base, NULL); >>> + if (cost_base.cost == 0) >>> + cost_base.cost = COSTS_N_INSNS (1); >>> cost_step = add_cost (TYPE_MODE (TREE_TYPE (base)), data->speed); >>> >>> cost = cost_step + adjust_setup_cost (data, cost_base.cost); >> >> ...why does this reasoning apply only to this call to force_var_cost? >> >> Richard > > force_var_cost is described as estimating the cost of forcing expression expr > into a variable. If expr is already a var, this definition is ambiguous. > If we can use the var directly, the cost is zero, but if we need a regcopy, it > should be the cost of a regcopy. > > What is special for an iv, is that we know that it is not only used but also > modified. If a var is used in or after the loop, we need a regcopy to init the > iv with that var. If that var is not used in or after the loop, we can use > that > var as iv. The patch above is a heuristic that estimates that the latter > situation is the less frequent one. > > In general, we don't have such specific information, and the the cost of zero > is > a good choice then. > > We could add a parameter to force_var_cost that indicates this choice, that > would perhaps be a better fix. > > As for the reasoning related to the const set, that is something that indeed > holds more general, and could be implemented in force_var_cost, which is what > you're suggesting if I understand you correctly. It was actually a genuine question. I honestly wasn't sure whether (and why) this was the only site at which a reg copy should be counted. However... > The tentative patch below explores these last 2 ideas. ...this makes things _much_ clearer to me FWIW. Thanks. Richard
