Go中的地址对齐

Question

So I am using c2go to link C code with Go. The C code requires certain arguments of a function called from Go to be 256 bit aligned (the function arguments are all pointers to Go variables). Is there a way to achieve this is Go (i.e. to specify 256 alignment for a variable in Go)?

In Go, "unsafe.Alignof(f)" shows as 8 bytes aligned (for "var f [8]float32") i.e. the f is guaranteed by Go to be only 8 bytes aligned. I need it to be 32 bytes aligned somehow.

For the curious: The C code is using SIMD instructions (AVX to be specific). I was using "vmovaps" instruction which requires 256 bit alignment of operands. I can get away with using "vmovups" which doesn't require alignment, but I suspect that has a performance penalty.

Answer 1

For example, trading more memory for less CPU time,

package main

import (
    "fmt"
    "unsafe"
)

// Float32Align32 returns make([]float32, n) 32-byte aligned.
func Float32Align32(n int) []float32 {
    // align >= size && align%size == 0
    const align = 32 // SIMD AVX byte alignment
    const size = unsafe.Sizeof(float32(0))
    const pad = int(align/size - 1)
    if n <= 0 {
        return nil
    }
    s := make([]float32, n+pad)
    p := uintptr(unsafe.Pointer(&s[0]))
    i := int(((p+align-1)/align*align - p) / size)
    j := i + n
    return s[i:j:j]
}

func main() {
    f := Float32Align32(8) // SIMD AVX
    fmt.Printf(
        "SIMD AVX: %T %d %d %p %g
",
        f, len(f), cap(f), &f[0], f,
    )
    CFuncArg := &f[0]
    fmt.Println("CFuncArg:", CFuncArg)
}

Playground: https://play.golang.org/p/mmFnHEwGKt

Output:

SIMD AVX: []float32 8 8 0x10436080 [0 0 0 0 0 0 0 0]
CFuncArg: 0x10436080