How to Efficiently Load 8 Single-Precision Floats into an __m256 Variable on the Fly?

Loading 8 Single-Precision Floats into an __m256 Variable on the Fly

In optimizing your Gaussian blur algorithm, you encounter the challenge of replacing a float array with an intrinsic __m256 variable for improved performance. To achieve this effectively, consider the following instructions:

Using AVX2:

Employ the PMOVZX instruction to extend your bytes to 32-bit integers in a 256-bit register. This process allows for in-place conversion to floats using the VCVTDQ2PS instruction. This strategy proves efficient even when dealing with multiple vectors.

Alternative Approach (for Non-AVX2)

If working with AVX1 or earlier, utilize the VPMOVZXBD instruction to extend the byte elements directly into a 256-bit register, followed by VCVTDQ2PS for float conversion.

Avoiding Shuffle Bottlenecks:

To minimize the number of shuffle operations, consider loading high 64-bit values via a broadcast operation and then shuffling them using VPMOVZX and VPSHUFB.

Compiling Woes:

Certain compilers, such as GCC and MSVC, may exhibit suboptimal code generation for VPMOVZXBD with memory operands. To mitigate this, manually implement a version that safely combines a load instruction with VPMOVZXBD.

Intrinsics Conundrum:

Unfortunately, there's a gap in the intrinsics repertoire for accessing VPMOVZXBD with memory operands. As such, you'll need to resort to clever coding techniques to avoid compromising code safety.

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