How Can AVX2 and BMI2 Be Used for Efficient Left Packing Based on a Dynamic Mask?

Efficiently Packing Left Elements Based on a Mask with AVX2 and BMI2

In AVX2, achieving efficient left packing requires utilizing specific instructions and techniques. One approach is to leverage both AVX2's vpermps (_mm256_permutevar8x32_ps) for lane-crossing variable shuffling and BMI2's pext (Parallel Bits Extract) for bitwise operations.

Leveraging BMI2 for Mask Generation

BMI2's pext instruction enables the extraction of specific bits from a bitmask, providing a mechanism for dynamically generating lane-crossing shuffle control data on the fly. This eliminates the need for a large pre-computed look-up table (LUT).

The Algorithm

The algorithm involves:

1. Extracting Compressed Indices: Using pext, a compressed bitmask is generated, containing the desired lane indices in the lower bits of an integer register.
2. Unpacking Packed Indices: To unpack the compressed indices, a sequence of shifts and multiplications is employed. This step effectively replicates each bit to fill its corresponding byte, creating a per-byte index mask.
3. Generating the Shuffle Mask: A shuffle mask is computed using the per-byte index mask. This mask is then used to control the lane-crossing variable shuffle operation using vpermps.

Performance Considerations

The advantage of this approach lies in its ability to generate the lane-crossing shuffle mask on the fly, avoiding the creation and storage of a large LUT. This approach could be advantageous in situations where the mask input is dynamic. However, it's important to note that pdep/pext operations can be relatively slow on AMD CPUs prior to Zen 3, so alternative methods like 128-bit vectors orLUT-based approaches may be more suitable for such architectures.

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