iosbitmapdecoding
Experimenting with bitmap decoding on ios
iosbitmapdecoding
Experimenting with bitmap decoding on ios
Inspired by this blog post...
- Really fast bitset decoding for average densities https://lemire.me/blog/2019/05/03/really-fast-bitset-decoding-for-average-densities/
Requirements
- A recent mac with Xcode installed
- A recent iOS device
Usage
- Open BitmapDecoding.xcodeproj in Xcode (macOS)
- Click on the "BitmapDecoding" project within Xcode, go to "Identity" and select a valid account under "Team".
- Plug your device (e.g., iPhone) in your mac.
- In Xcode, go to Window > Devices. This will open a new window. You should see your device in this new window. Don't lose this new window.
- In Xcode, go to Product > Destination and choose your device.
- In Xcode, go to Product > Build For > Running.
- You should see in Xcode under Products "BitmapDecoding.app", drag and drop it into the "Devices" window we opened previously.
- You should now see the app on your device. By default, iOS won't let you run it, so go under Settings > General. You will find a setting there to allow you to run the app.
- Run the app.
Using Instruments
Apple has a tool called Instruments as part of Xcode that can be used to record the number of instructions (FIXED_INSTRUCTIONS) and the number of cycles (FIXED_CYCLES). First build and upload your app. Connect your device to your mac and launch Instruments, select the "Counters" template and your device and app. Then go to File > Recording Options and click the + in the Events and Formulas section, and pick the events FIXED_INSTRUCTIONS and FIXED_CYCLES and sample by time.
For this app, I found that FIXED_INSTRUCTIONS goes up to 29346274130 whereas FIXED_CYCLES goes up to 9408002514 for an average of 3.12 instructions per cycle. TIt is much higher than on x64 where the highest I reach for part of the benchmark is 2.6 instructions per cycle.
Sample result
iPhone XR, Xcode 10.1
Measuring frequency
Median frequency detected: 2.131252 GHz (939 samples out of 1000).
129996 129996
basic_decoder timing: 47.445 nanoseconds per set bit
simdjson_decoder timing: 11.264 nanoseconds per set bit
basic_decoder: 101.117 cycles per set bit
simdjson_decoder: 24.007 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (964 samples out of 1000).
129996 129996
basic_decoder timing: 1.652 nanoseconds per set bit
simdjson_decoder timing: 1.200 nanoseconds per set bit
basic_decoder: 4.125 cycles per set bit
simdjson_decoder: 2.997 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (970 samples out of 1000).
129996 129996
basic_decoder timing: 1.652 nanoseconds per set bit
simdjson_decoder timing: 1.197 nanoseconds per set bit
basic_decoder: 4.125 cycles per set bit
simdjson_decoder: 2.989 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (917 samples out of 1000).
129996 129996
basic_decoder timing: 1.824 nanoseconds per set bit
simdjson_decoder timing: 1.294 nanoseconds per set bit
basic_decoder: 4.553 cycles per set bit
simdjson_decoder: 3.230 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (969 samples out of 1000).
129996 129996
basic_decoder timing: 1.641 nanoseconds per set bit
simdjson_decoder timing: 1.197 nanoseconds per set bit
basic_decoder: 4.097 cycles per set bit
simdjson_decoder: 2.989 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (962 samples out of 1000).
129996 129996
basic_decoder timing: 1.633 nanoseconds per set bit
simdjson_decoder timing: 1.194 nanoseconds per set bit
basic_decoder: 4.078 cycles per set bit
simdjson_decoder: 2.980 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (966 samples out of 1000).
129996 129996
basic_decoder timing: 1.629 nanoseconds per set bit
simdjson_decoder timing: 1.189 nanoseconds per set bit
basic_decoder: 4.068 cycles per set bit
simdjson_decoder: 2.970 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (950 samples out of 1000).
129996 129996
basic_decoder timing: 1.652 nanoseconds per set bit
simdjson_decoder timing: 1.180 nanoseconds per set bit
basic_decoder: 4.125 cycles per set bit
simdjson_decoder: 2.947 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (968 samples out of 1000).
129996 129996
basic_decoder timing: 1.655 nanoseconds per set bit
simdjson_decoder timing: 1.203 nanoseconds per set bit
basic_decoder: 4.131 cycles per set bit
simdjson_decoder: 3.003 cycles per set bit
Measuring frequency
Median frequency detected: 2.496610 GHz (967 samples out of 1000).
129996 129996
basic_decoder timing: 1.648 nanoseconds per set bit
simdjson_decoder timing: 1.174 nanoseconds per set bit
basic_decoder: 4.114 cycles per set bit
simdjson_decoder: 2.932 cycles per set bit
The Apple A12 has seemingly a rated frequency of 2.5GHz. So these nanoseconds translate into about 4.3 and 3.0 cycles per set bit. For comparison, an Intel Skylake get 4.7 and 3.4 cycles per set bit.
Credit
The app. reuses code by Benot Maison.