Add clarifying comment on modulo optimization

Author: jarnesino

cuda/src/poly/eval_at_point.cu

@@ -175,8 +175,9 @@ __global__ void eval_many_at_point_first_pass(m31 **g_coeffs, qm31 *temp, qm31 *
 
     int poly_index = blockIdx.x / blocks_in_poly;
 
-    s_coeffs[idx] = g_coeffs[poly_index][(blockIdx.x % blocks_in_poly) * coeffs_size + idx];
-    s_coeffs[idx + blockDim.x] = g_coeffs[poly_index][(blockIdx.x % blocks_in_poly) * coeffs_size + idx + blockDim.x];
+    // A % X == A & (X-1) when X is a power of two
+    s_coeffs[idx] = g_coeffs[poly_index][(blockIdx.x & (blocks_in_poly - 1)) * coeffs_size + idx];
+    s_coeffs[idx + blockDim.x] = g_coeffs[poly_index][(blockIdx.x & (blocks_in_poly - 1)) * coeffs_size + idx + blockDim.x];
     __syncthreads();
 
     int level_size = coeffs_size >> 1;