Optimizing __mm_add_epi32_inplace_purego Using Assembly

This question seeks to optimize the inner loop of the __mm_add_epi32_inplace_purego function, which performs a positional population count on an array of bytes. The goal is to improve performance by utilizing assembly instructions.

The original Go implementation of the inner loop:

    __mm_add_epi32_inplace_purego(&counts[i], expand)

The use of '&counts[i]' to pass the address of an array element can be inefficient. To optimize this, we can pass the pointer to the entire array instead:

__mm_add_epi32_inplace_inplace_purego(counts, expand)

This modification reduces the overhead associated with passing arrays as arguments.

Additionally, the inner loop can be further optimized using assembly instructions. The following assembly code is a version of __mm_add_epi32_inplace_purego implemented in assembly:

// func __mm_add_epi32_inplace_asm(counts *[8]int32, expand *[8]int32)
TEXT ·__mm_add_epi32_inplace_asm(SB),NOSPLIT,$0-16
    MOVQ counts 0(FP), DI
    MOVQ expand 8(FP), SI
    MOVL 8*0(DI), AX        // load counts[0]
    ADDL 8*0(SI), AX        // add expand[0]
    MOVL AX, 8*0(DI)        // store result in counts[0]
    MOVL 8*1(DI), AX        // load counts[1]
    ADDL 8*1(SI), AX        // add expand[1]
    MOVL AX, 8*1(DI)        // store result in counts[1]
    MOVL 8*2(DI), AX        // load counts[2]
    ADDL 8*2(SI), AX        // add expand[2]
    MOVL AX, 8*2(DI)        // store result in counts[2]
    MOVL 8*3(DI), AX        // load counts[3]
    ADDL 8*3(SI), AX        // add expand[3]
    MOVL AX, 8*3(DI)        // store result in counts[3]
    MOVL 8*4(DI), AX        // load counts[4]
    ADDL 8*4(SI), AX        // add expand[4]
    MOVL AX, 8*4(DI)        // store result in counts[4]
    MOVL 8*5(DI), AX        // load counts[5]
    ADDL 8*5(SI), AX        // add expand[5]
    MOVL AX, 8*5(DI)        // store result in counts[5]
    MOVL 8*6(DI), AX        // load counts[6]
    ADDL 8*6(SI), AX        // add expand[6]
    MOVL AX, 8*6(DI)        // store result in counts[6]
    MOVL 8*7(DI), AX        // load counts[7]
    ADDL 8*7(SI), AX        // add expand[7]
    MOVL AX, 8*7(DI)        // store result in counts[7]
    RET

This assembly code loads the elements of 'counts' and 'expand' into registers, performs the addition, and stores the result back into 'counts'. By avoiding the need to pass arrays as arguments and by using efficient assembly instructions, this code significantly improves the performance of the inner loop.

In summary, by passing the pointer to the array instead of the address of an element and by implementing the inner loop in assembly, the __mm_add_epi32_inplace_purego function can be optimized to achieve improved performance in positional population counting operations.