How to run OpenMP parallel in android native code? - android-ndk

Question

I am currently using OpenMP's #pragma's to distribute Android app's native code for loop to parallel threads as follows:
#pragma omp parallel shared(variable1) shared(variable2) shared(variable3) private(array_1) private(array_1) private(array_1)
{
LOG(INFO) << "Running inside thread: " << omp_get_thread_num();
#pragma omp for
for (int c = 1; c <= itrloopLim; c++) {
...
}
}
I am compiling with -fopenmp, but after installing the .apk, the app crashes on runtime with the below error:
12-06 23:30:58.700 32133 32314 W System.err: TensorFlowLite: failed to load native library: dlopen failed: cannot locate symbol "__kmpc_end_critical" referenced by "/data/app/org.mlperf.inference-aKoP_FhySRDbKCU5EeEcmQ==/lib/arm64/libmlperf_jni.so"...
12-06 23:30:58.700 32133 32314 E lperf.inferenc: No implementation found for void org.mlperf.inference.MLPerfDriverWrapper.nativeDeleteBackend(long) (tried Java_org_mlperf_inference_MLPerfDriverWrapper_nativeDeleteBackend and Java_org_mlperf_inference_MLPerfDriverWrapper_nativeDeleteBackend__J)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: FATAL EXCEPTION: MLPerf.Worker
12-06 23:30:58.701 32133 32314 E AndroidRuntime: Process: org.mlperf.inference, PID: 32133
12-06 23:30:58.701 32133 32314 E AndroidRuntime: java.lang.UnsatisfiedLinkError: No implementation found for void org.mlperf.inference.MLPerfDriverWrapper.nativeDeleteBackend(long) (tried Java_org_mlperf_inference_MLPerfDriverWrapper_nativeDeleteBackend and Java_org_mlperf_inference_MLPerfDriverWrapper_nativeDeleteBackend__J)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at org.mlperf.inference.MLPerfDriverWrapper.nativeDeleteBackend(Native Method)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at org.mlperf.inference.MLPerfDriverWrapper.access$000(MLPerfDriverWrapper.java:23)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at org.mlperf.inference.MLPerfDriverWrapper$Builder.useMLBackend(MLPerfDriverWrapper.java:205)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at org.mlperf.inference.RunMLPerfWorker.handleMessage(RunMLPerfWorker.java:102)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at android.os.Looper.loop(Looper.java:193)
12-06 23:30:58.701 32133 32314 E AndroidRuntime: at android.os.HandlerThread.run(HandlerThread.java:65)
12-06 23:30:58.703 1439 1467 W ActivityManager: Force finishing activity org.mlperf.inference/.MLPerfEvaluation
12-06 23:30:58.707 1439 1585 I ActivityManager: Showing crash dialog for package org.mlperf.inference u0
12-06 23:30:58.715 1439 1584 W BroadcastQueue: Background execution not allowed: receiving Intent { act=android.intent.action.DROPBOX_ENTRY_ADDED flg=0x10 (has extras) } to com.google.android.gms/.stats.service.DropBoxEntryAddedReceiver

Answer 1

I encountered the exact same issue, no error in compilation but the app crashes immediately on startup.
It turns out you need to use the "-static-openmp -fopenmp" flags. Your build gradle has to look like this basically :
externalNativeBuild {
cmake {
arguments "-DANDROID_ARM_NEON=ON"
cppFlags "-static-openmp -fopenmp"
}