How to Load 8 Characters from Memory into an __m256 Variable: Three Efficient Approaches

Loading 8 Chars from Memory into an __m256 Variable: An Analysis

Problem:

You want to optimize an algorithm for Gaussian blur on an image by replacing a float buffer[8] with an intrinsic __m256 variable to enhance performance.

Solution 1: Using AVX2's PMOVZX and VCVTDQ2PS

This approach utilizes PMOVZX to extend 8-bit characters into 32-bit integers and then converts them to floating-point values through VCVTDQ2PS. Specifically:

VPMOVZXBD   ymm0,  [rsi]   ; Byte to DWord
VCVTDQ2PS   ymm0, ymm0     ; convert to packed float

Solution 2: Combining Broadcast Load and Shuffling

This strategy involves performing a 128-bit broadcast load to yield a 64-bit shuffle control vector for vpshufb, allowing for zero extension and packed float conversion. It offers a high throughput by eliminating the need for additional shuffle instructions.

VPMOVSXBD   xmm0,  [rsi]   ; Byte to DWord
VPMOVSXBD   xmm1,  [rsi+4] 
VINSERTF128 ymm0, ymm0, xmm1, 1   
VCVTDQ2PS   ymm0, ymm0     ; convert to packed float.

Solution 3: Handling AVX1 Limitations

In the absence of AVX2, the following steps can be employed:

VPMOVZXBD   xmm0,  [rsi]
VPMOVZXBD   xmm1,  [rsi+4]
VINSERTF128 ymm0, ymm0, xmm1, 1   ; put the 2nd load of data into the high128 of ymm0
VCVTDQ2PS   ymm0, ymm0     ; convert to packed float.

Additional Notes:

• Consider using VPADDQ instead of VCVTDQ2PS for further performance enhancement.
• Be cautious of potential compiler optimizations in different languages.
• Refer to the specific resources linked within the solution for additional insights.

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