Kyrill Tkachov <[email protected]> writes: > Hi all, > > This hunk that slightly reduces the cost of immediate moves doesn't > actually have any effect. In the whole of SPEC2006 it didn't make a > difference. In any case, I'd like to move to a point where we use > COSTS_N_INSNS units for our costs and not increment decrement them by > one.
I wonder whether that's always a good idea though? COSTS_N_INSNS exists to allow these kinds of fractional costs. It sounds like they're not useful in this particular case, but e.g. one case where they can be useful is if you want to say when optimising for size that two instructions are the same size but one is slightly more preferable for speed reasons. This might help prefer a 2-instruction shift/add sequence over a load-immediate followed by a general multiplication; both have the same size, but the shift/add is often faster. Giving multiplication a slightly higher cost than COSTS_N_INSNS (1) makes that clear. It's not perfect of course. Add too many fractional costs together and a carry will give you an extra full instruction at some fairly arbitrary point. Maybe size costs should be a (size, speed) pair. Thanks, Richard
