How to Load 8 Characters from Memory into an __m256 Variable as Packed Single Precision Floats Using AVX2?

Loading 8 Characters from Memory into an __m256 Variable as Packed Single Precision Floats

In Gaussian blur algorithms, optimizing for faster execution can be achieved by efficiently loading data into vector registers. One such optimization involves replacing an array of floats with an __m256 variable. This article provides an optimal solution for this task, leveraging the power of AVX2 instructions.

Solution Using AVX2 Instructions

To effectively load 8 characters from memory into an __m256 variable using AVX2, the following instructions are recommended:

VPMOVZXBD  ymm0,  [rsi]  ; or SX to sign-extend  (Byte to DWord)
VCVTDQ2PS   ymm0, ymm0     ; convert to packed foat

Specifics of the Instructions

• VPMOVZXBD: Zero-extends the 8-bit characters into 32-bit integers within the ymm0 register.
• VCVTDQ2PS: Converts the 32-bit integers into packed single-precision floats, directly storing them in ymm0.

Additional Optimization

To further optimize this process, consider using a broadcast load to feed the VPMOVZXBD instruction and a Vpshufb instruction for the high 64 bits. This strategy reduces the overall uop count, improving efficiency:

<code class="pseudocode">__m256 b = [float(new_image[x+7]), float(new_image[x+6]), ... , float(new_image[x])];
__m256 b = _mm256_broadcast_ss(&new_image[x])
_mm256_shuffle_epi8(b, _mm256_set1_epi8(0)); // fills upper 64 bits with zeroes
_mm256_cvtps_epu32(b); // convert to integers
_mm256_cvtepu32_ps(b); // convert back to floats</code>

Avoid Suboptimal Techniques

• Avoid using multiple 128-bit or 256-bit loads and subsequent shuffles, as it may introduce unnecessary bottlenecks.
• Do not use a VPMOVZXD instruction followed by separate memory operands for VPMOVZX, as it leads to suboptimal code generation.

Additional Considerations

• Consider using a safe intrinsic, if available, to avoid potential issues with memory alignment or accessing uninitialized memory.
• Use appropriate _mm_loadl_epi64 or _mm_loadu_si64 intrinsics to avoid loading more data than necessary or causing potential segmentation faults.

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