On Thu, Apr 16, 2020 at 7:28 PM Sasha Krassovsky <samo...@uw.edu> wrote: > @Jim I saw you were from SiFive - I noticed that modifying the costs for > integer multiplies in the riscv_tune_info structs didn’t affect the generated > code. Could this be why?
rtx_costs is used for instruction selection. For instance, choosing whether to use a shift and add sequence as opposed to a multiply depends on rtx_cost. rtx_cost is not used for instruction scheduling. This uses the latency info from the pipeline model, e.g. generic.md. It looks like I didn't read your first message closely enough and should have mentioned this earlier. Changing multiply rtx_cost does affect code generation. Just try a testcase multiplying by a number of small prime factors, and you will see that which ones use shift/add and which ones use multiply depends on the multiply cost in the riscv_tune_info structs. This also factors into the optimization that turns divide by constant into a multiply. When this happens depends on the relative values of the multiply cost and the divide cost. Jim
