Jeff Law <jeffreya...@gmail.com> writes: > The other issue that's been in the back of my mind is costing. But I > think the model here is combine without regards to cost.
No, it does take costing into account. For size, it's the usual "sum up the before and after insn costs and see which one is lower". For speed, the costs are weighted by execution frequency, so e.g. two insns of cost 4 in the same block can be combined into a single instruction of cost 8, but a hoisted invariant can only be combined into a loop body instruction if the loop body instruction's cost doesn't increase significantly. This is done by rtl_ssa::changes_are_worthwhile. Thanks, Richard
