# Get started

There are several ways to use SIMD in your C/C++ program. The most direct way is to embed SIMD assembly in your code, which is apparently cumbersome and error-prone. You can also let the compiler automatically optimize your code with SIMD by using openMP (we will cover openMP later in this course). For example:

```c
#pragma omp simd
for(i=0;i<N;i++)
{
    C[i] = A[i] + B[i];
}
```

However, the code patterns that can be automatically detected and optimized by Compiler are limited.&#x20;

SIMD intrinsics are convenient if you want to manually optimize your code using SIMD without touching assembly. It is basically a library that wraps SIMD assembly as C functions. Let's re-write our vector addition example by SIMD intrinsics.

{% code title="add\_simd.c" %}

```c
#include <xmmintrin.h> //header file for sse
#include <stdio.h>

void add_simd(float a[4], float b[4], float result[4])
{
	__m128 v1 = _mm_load_ps(a); // load float[4] as __m128 vector
	__m128 v2 = _mm_load_ps(b);
	__m128 v3 = _mm_add_ps(v1, v2); // add them up
	_mm_store_ps(result, v3); // store the result to float[4]
}

int main()
{
	float a[4] = {1, 2, 3, 4};
	float b[4] = {2, 3, 4, 5};
	float r[4];
	add_simd(a, b, r);
	printf("%f %f %f %f\n", r[0], r[1], r[2], r[3]);
}
```

{% endcode %}

***Notice** that before using SIMD intrinsics, you have to include a header file that contains the intrinsic you want. To find out which header file is required, you can refer to* [*Intel Intrinsic Guide*](https://software.intel.com/sites/landingpage/IntrinsicsGuide/)*. For example, `xmmintrin.h` is required for `__mm_add_ps`:*

![](https://2267274569-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LoF4fSjDhFLH6o9i9dX%2F-LoGT_IXLwdBSWMAjaTP%2F-LoGqNMgdRM7farT0sfZ%2Fimage.png?alt=media\&token=296f08ad-39f7-44b4-8d92-254cae33c531)

Before we explain the details of the code, let's compile and run the program first. To compile the above code with SSE enabled, run the following code:

```bash
gcc add_simd.c -o add_simd -msse
```

***Notice** that we should add an additional flag "-m\[CPUID Flag]"("-msse" in the above code snippet) when compiling your code with GCC Compiler. This flag is used to enable SSE instructions. Different instructions need different flags, you can refer to* [*Intel Intrinsic Guide*](https://software.intel.com/sites/landingpage/IntrinsicsGuide/) *to find the corresponding flag. For example:*

![](https://2267274569-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LoF4fSjDhFLH6o9i9dX%2F-LoGT_IXLwdBSWMAjaTP%2F-LoGv1d8X7jqpiJYEzPf%2Fimage.png?alt=media\&token=f0014e64-831f-4879-afb4-9aaf7f6d9d21)

Run the compiled executable, you can see the result is as expected:

![](https://2267274569-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LoF4fSjDhFLH6o9i9dX%2F-LoGT_IXLwdBSWMAjaTP%2F-LoGwJxH55zh6s_lBt-d%2Fimage.png?alt=media\&token=11a20aee-0633-4836-a9da-567e2cb5a670)


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