C and Matlab: Why does this one line in Matlab become so many lines in C++ code generated by Matlab Coder?

Question

I have some Matlab code that is run many millions of times as mentioned in this question: Matlab: Does calling the same mex function repeatedly from a loop incur too much overhead?

I'm trying to mex-ify it to see whether that helps. Now, when I generate code from the Matlab code using Matlab Coder tool, the code is generally reasonable, but this one line of Matlab code (in a C++ comment in the first line below) begets this monstrosity, and I have no idea why. Any help in understanding and reducing its complexity would be appreciated.

For context, d is a two dimensional matrix, and s1 is a row vector. s1_idx is assigned to be length(s1) + 1 in preceding C++ code,

/* d(:, 1) = 0:length(s1); */
cdiff = s1_idx_0 - 1;
for (nm1d2 = 0; nm1d2 <= cdiff; nm1d2++) {
   tmp_data[nm1d2] = nm1d2;
}
ndbl = (int32_T)muDoubleScalarFloor((real_T)s1_sizes[1] + 0.5);
apnd = ndbl;
cdiff = ndbl - s1_sizes[1];
if (muDoubleScalarAbs((real_T)cdiff) < 4.4408920985006262E-16 * (real_T)s1_sizes[1]) {
   ndbl++;
   apnd = s1_sizes[1];
} else if (cdiff > 0) {
   apnd = ndbl - 1;
} else {
   ndbl++;
}
if (ndbl > 0) {
   b_tmp_data[0] = 0.0;
   if (ndbl > 1) {
       b_tmp_data[ndbl - 1] = (real_T)apnd;
       nm1 = ndbl - 1;
       nm1d2 = nm1;
       nm1d2 = (int32_T)((uint32_T)nm1d2 >> 1);
       for (cdiff = 1; cdiff <= nm1d2 - 1; cdiff++) {
           b_tmp_data[cdiff] = (real_T)cdiff;
           b_tmp_data[(ndbl - cdiff) - 1] = (real_T)(apnd - cdiff);
       }
       if (nm1d2 << 1 == nm1) {
           b_tmp_data[nm1d2] = (real_T)apnd / 2.0;
       } else {
           b_tmp_data[nm1d2] = (real_T)nm1d2;
           b_tmp_data[nm1d2 + 1] = (real_T)(apnd - nm1d2);
       }
   }
}
cdiff = s1_idx_0 - 1;
for (nm1d2 = 0; nm1d2 <= cdiff; nm1d2++) {
   SD->f0.d_data[tmp_data[nm1d2]] = b_tmp_data[nm1d2];
}

Answer 1

That's pretty funny generated code for what you actually want to accomplish. You just want to stuff the integers 0 to k into an array. But the code generator is built to deal with the general case. So there are three parts to the generated code:

1. Create an array of indices specifying where, on the left-hand side, the elements from the right-hand side will go. You used the : expression, but you could have used something else. The code generator has to be prepared for you doing something like d(length(s1):0, 1)=0:length(s1).
2. Create an array of values for the right-hand side. Your just doing sequential integers, but the code generator is prepared to deal with doubles and when creating values from a range of doubles you might have funny rounding issues. It's checking for all kinds of edge cases.
3. Finally, there's the loop that actually assigns the values on the right-hand side to memory slots on the left-hand side as indexed by the array created in step 1.

In the end, it might be that all you need is:

cdiff = s1_idx_0 - 1;
for (nm1d2 = 0; nm1d2 <= cdiff; nm1d2++) {
   SD->f0.d_data[nm1d2] = nm1d2;
}