Porting Process

Using SIMD.info to find NEON Equivalents

Now that you have a clear view of the example, you can start the process of porting the code to Arm Neon/ASIMD.

This is where SIMD.info comes in.

In SIMD programming, the primary focus is the integrity and accuracy of the calculations. Ensuring that these calculations are done correctly is crucial. Performance is almost always a secondary concern.

For the operations in your SSE4.2 example, you have the following intrinsics:

• _mm_cmpgt_ps
• _mm_add_ps
• _mm_mul_ps
• _mm_sqrt_ps

To gain a deeper understanding of how these intrinsics work and to surface detailed descriptions, you can use the search feature on SIMD.info. Simply enter the name of the intrinsic in the search bar. You can either select from the suggested results or perform a direct search to retrieve information about each intrinsic.

1. By searching for _mm_add_ps you will retrieve information about its purpose, the result type, assembly instructions, prototypes, and an example demonstration. By clicking the engine option “NEON” you can find its equivalents for this engine. The equivalents are: vaddq_f32, vadd_f32. Intrinsics comparison helps you find the right one. Based on the prototype provided, you can choose vaddq_f32 as it works with 128-bit vectors which is the same as SSE4.2.

2. Moving to the next intrinsic, _mm_mul_ps, you can use the Intrinsics Tree on SIMD.info to find the equivalent.

Start by expanding the Arithmetic branch and then navigate to the branch Vector Multiply. As you are working with 32-bit floats, open the Vector Multiply 32-bit floats branch, where you will find several options. The recommended choice is vmulq_f32 , following the same reasoning as before; it operates on 128-bit vectors.

3. For the third intrinsic, _mm_sqrt_ps, the easiest way to find the corresponding NEON intrinsic is by typing “Square Root” in the search bar on SIMD.info. From the search results , look for the float-specific version and select vsqrtq_f32 , which, like the others, works with 128-bit vectors. In the equivalents section about SSE4.2, you can see that _mm_sqrt_ps has its place as a direct match for this operation.

4. For the last intrinsic, _mm_cmpgt_ps, follow a similar approach as before. Inside the intrinsics tree, start by expanding the Comparison folder. Navigate to the subfolder Vector Compare Greater Than, and as you are working with 32-bit floats, proceed to Vector Compare Greater Than 32-bit floats. The recommended choice is again the 128-bit variant vcgtq_f32 .

Now that you have found the NEON equivalents for each SSE4.2 intrinsic, you’re ready to begin porting the code. Understanding these equivalents is key to ensuring that the code produces the correct results in the calculations as you switch between SIMD engines.