Wilco Dijkstra <wilco.dijks...@arm.com> writes: > Hi Richard, > >> I'm just concerned that here we're using the same explanation but with >> different numbers. Why are the new numbers more right than the old ones >> (especially when it comes to code size, where the trade-off hasn't >> really changed)? > > Like all tuning/costing parameters, these values are never fixed but change > over time due to optimizations, micro architectures and workloads. > The previous values were out of date so that's why I retuned them by > benchmarking different values and choosing the best combinations. > >> It would be good to have more discussion of why certain numbers are >> too small or too high, and why 8 is the right pivot point for -Os. > > You mean add more discussion in the comment? That comment is already overly > large and too specific - it would be better to reduce it. The -Os value was > never > tuned, and 8 turns out to be faster and smaller than GCC's default.
The problem is that you're effectively asking for these values to be taken on faith without providing any analysis and without describing how you arrived at the new numbers. Did you try other values too? If so, how did they compare with the numbers that you finally chose? At least that would give an indication of where the boundaries are. For example, it's easier to believe that 8 is the right value for -Os if you say that you tried 9 and 7 as well, and they were worse than 8 by X% and Y%. This would also help anyone who wants to tweak the numbers again in future. BTW, which CPU did you use to do the experiments? Are the tuning parameters for that CPU already consistent with the new generic values? Thanks, Richard
