I am using ARMv8 GCC compiler and I would like to optimize Neon Intrinsics code for better execution time performance. I have already tried loop unrolling and I am using look up table for the computation of log10. Any ideas?
Here is the code:
static inline void func(float32x4x4_t inputData_real, float * outputs ) { float32x4x4_t outputData; float32x4x4_t outputData1; for(unsigned short i =0; i < 4; i++) { outputData.val[i] = vmulq_f32 (inputData_real.val[i], inputData_real.val[i]); outputData1.val[i] = vmlaq_f32 (outputData.val[i], inputData_imag.val[i], inputData_imag.val[i]); outputs[i] = 10.0F * log10f_c(vaddvq_f32(outputData1.val[i]) + (a_imagvalue[i] * b_imagvalue[i]) + (a_realvalue[i] * b_realvalue[i])) - 7.89865767467723; } }
I checked the timing of my code yesterday and it was improved from 2.18ms to 2.11ms so I guess I am doing right.
Good. A memory hint might further improve it. (BTW the "F" is missing on the 7.8....).
One more note: Using short for loop variables is no good idea. In general, if not specific size is needed it is better to use int .
Hi @khan77,
Measuring sub-millisecond durations can be tricky, depending on the timer used.
You might want to measure several times in a row to be sure, and/or use a cycle counter.
Thanks for your suggestion and correction. Could you please elaborate more on memory hint?
Yeah, I am measuring time of my code like this :
#define TIMEDIFF(t1,t2) (t2 - t1) #define MILLISECONDS(t) (1000.0 * t / COUNTS_PER_SECOND) // Start a test void startTest() { XTime_GetTime(&start); } void endTest() { XTime_GetTime(&end); double time_curr = TIMEDIFF(start, end); double msec = MILLISECONDS(time_curr); printf("Run-time = %.2f msec...\n", msec); // Achieved Bandwidth = (total bytes transferred) / (msec) // Average Latency = (msec) / (total memory accesses) } ...... startTest(); for(i= 0; i<4500; i++) { //Func call } endTest();
There are "memory hint" instruction, which tell the memory subsystem to preload some memory you will access soon. So while calculating the first chunk, the next will be loaded into cache.
I do not know the instruction by heart.
If you run the test with a warm cache, you get false results. Depending on the size of your test data, it might end up in the caches and from then on you measure only the cache bandwidth plus calculation time. If that's what you want, ok. If not you need to invalidate the cache prio each call to the function under test.