The AI Engine contains a scalar processor that can be used to implement scalar math operations, non-linear functions, and other general purpose operations. Sometimes it can be helpful to have a golden scalar reference version of code. But note that the scalar version of code takes much more time to run in simulation and hardware compared to the vectorized version.
The following provides example code for the scalar version of a 32-tap
filter:
static cint16 eq_coef[32]={{1,2},{3,4},...};
//keep margin data between different invocations of the graph
static cint16 delay_line[32];
__attribute__((noinline)) void fir_32tap_scalar(input_stream<cint16> * sig_in,
output_stream<cint16> * sig_out){
//For profiling only
unsigned cycle_num[2];
aie::tile tile=aie::tile::current();
for(int i=0;i<SAMPLES;i++){
cycle_num[0]=tile.cycles();//cycle counter of the AI Engine tile
cint64 sum={0,0};//larger data to mimic accumulator
for(int j=0;j<32;j++){
//auto integer promotion to prevent overflow
sum.real+=delay_line[j].real*eq_coef[j].real-delay_line[j].imag*eq_coef[j].imag;
sum.imag+=delay_line[j].real*eq_coef[j].imag+delay_line[j].imag*eq_coef[j].real;
}
sum=sum>>SHIFT;
//produce one sample per loop iteration
writeincr(sig_out,{(int16)sum.real,(int16)sum.imag});
for(int j=0;j<32;j++){
if(j==31){
delay_line[j]=readincr(sig_in);
}else{
delay_line[j]=delay_line[j+1];
}
}
//For profiling only
cycle_num[1]=tile.cycles();//cycle counter of the AI Engine tile
printf("start=%d,end=%d,total=%d\n",cycle_num[0],cycle_num[1],cycle_num[1]-cycle_num[0]);
}
}
void fir_32tap_scalar_init()
{
//initialize data
for (int i=0;i<32;i++){
int tmp=get_ss(0);
delay_line[i]=*(cint16*)&tmp;
}
};
Note:
- Function
fir_32tap_scalar_init
is used as an initialization function for the kernel, which will only be called once aftergraph.run()
. - Rounding and saturation modes are not supported in the scalar
processor. They can be implemented via standard C operations, like
shift
. - Tile counter is used for profiling the main loop of code.
From the profiling result, you can see that each sample takes 3068 cycles.
You can also view similar information under the Profile section in the Vitis IDE if you
enable the option
--profile
during AI Engine
simulation.Figure 1. Profiling Details
For more information about graph construction and different kinds of profiling techniques, see AI Engine Simulation-Based Performance Analysis and Performance Analysis of AI Engine Graph Application on Hardware in AI Engine Tools and Flows User Guide (UG1076).