# spectral-norm C clang #5 program

## source code

```/* The Computer Language Benchmarks Game
* https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
*
* contributed by Ledrug
* algorithm is a straight copy from Steve Decker et al's Fortran code
* with GCC SSE2 intrinsics
*/

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <malloc.h>
#include <emmintrin.h>

inline double A(int i, int j) {
return ((i+j) * (i+j+1) / 2 + i + 1);
}

double dot(double * v, double * u, int n) {
int i;
double sum = 0;
for (i = 0; i < n; i++)
sum += v[i] * u[i];
return sum;
}

void mult_Av(double * v, double * out, const int n) {
int i;

#pragma omp parallel for
for (i = 0; i < n; i++) {
__m128d sum = _mm_setzero_pd();

int j;
for (j = 0; j < n; j += 2) {
__m128d b = _mm_set_pd(v[j],v[j+1]);
__m128d a = _mm_set_pd(A(i,j), A(i,j+1));
}
out[i] = sum[0] + sum[1];
}
}

void mult_Atv(double * v, double * out, const int n) {
int i;

#pragma omp parallel for
for (i = 0; i < n; i++) {
__m128d sum = _mm_setzero_pd();

int j;
for (j = 0; j < n; j += 2) {
__m128d b = _mm_set_pd(v[j], v[j+1]);
__m128d a = _mm_set_pd(A(j,i), A(j+1,i));
}
out[i] = sum[0] + sum[1];
}
}

double *tmp;
void mult_AtAv(double *v, double *out, const int n) {
mult_Av(v, tmp, n);
mult_Atv(tmp, out, n);
}

int main(int argc, char**argv) {
int n = atoi(argv[1]);
if (n <= 0) n = 2000;
if (n & 1) n++;   // make it multiple of two

double *u, *v;
u = memalign(16, n * sizeof(double));
v = memalign(16, n * sizeof(double));
tmp = memalign(16, n * sizeof(double));

int i;
for (i = 0; i < n; i++) u[i] = 1;
for (i = 0; i < 10; i++) {
mult_AtAv(u, v, n);
mult_AtAv(v, u, n);
}

printf("%.9f\n", sqrt(dot(u,v, n) / dot(v,v,n)));

return 0;
}
```

## notes, command-line, and program output

```NOTES:
Ubuntu clang version 18.1.3

Thu, 06 Jun 2024 22:41:12 GMT

MAKE:
/usr/bin/clang -pipe -Wall -O3 -fomit-frame-pointer -march=ivybridge -fopenmp spectralnorm.clang-5.c -o spectralnorm.clang-5.clang_run -lm
rm spectralnorm.clang-5.c

6.34s to complete and log all make actions

COMMAND LINE:
./spectralnorm.clang-5.clang_run 5500

PROGRAM OUTPUT:
1.274224153
```