that ART accelerator is nothing but a combination of prefetch, instruction cache and data cache
in the recent commit, steve is rather careful and basically disable ART disable only prefetch, leaving the instruction cache and data cache in place
https://github.com/stevstrong/Arduino_S ... a12f57e320
accordingly simply disabling prefetch is the best practice to reduce power consumption while still keeping adequate performance with the instruction and data cache. but accordingly if the 3 (prefetch + instr cache + data cache) on vs all 3 off
the bogo mflops
viewtopic.php?p=550#p550
can be significantly different
Beginning Whetstone benchmark at 144 MHz ... -O3 overclocked
ART enabled (prefetch + instr + data cache) on
C Converted Single Precision Whetstones:185.20 Mflops
ART disabled (prefetch + instr + data cache) off
C Converted Single Precision Whetstones:100.09 Mflops
note that this isn't steve's codes, i deliberately turn the 3 of them on or off to make a comparison. i think the bogo mflops is reflective of the effect of the cache and prefetch, the performance difference is nearly doubled for an (heavily) overclocked processor
but as it stands ART is cache and prefetch, this means that if your codes results in a lot of cache miss and prefetch miss as well, you may be running off flash without any assistance, no cache, no prefetch etc and hence *slow*.
of course this still won't explain those 'weird' cases e.g. if you have ART slower than if there is no ART. my guess is that one'd need to review the generated assembly or binary codes if one really want to get to the bottom of it. e.g. could the codes work in such a way that it causes a cache miss + prefetch miss every loop? and the other thing is could ART have an unintended effect of accelerating the microseconds or milliseconds timing code, e.g. with ART it 'runs faster' (i.e. each ms is shorter, counts more ms) while without ART it 'runs slower' (each ms is longer, count less ms) this would throw the ball out of the water, eitherway these mflops are for entertainment purpose, don't take them too seriously
one of the use of these bogo mflops is that it respond rather linearly to your sysclocks speed, hence while trying to set the rcc prescalers, this spoof bogo mflops gives a good feel of whether you set the pll prescalers correctly
