95-Misc.md

Misc

SWAR

• simd with a register
• see: x/swar

Turn a bytes slice to a uint64:

data := *(*uint64)(unsafe.Add(unsafe.Pointer(bytes), offset))

Read out ascii chars from memory (gist):

package main

import (
        "fmt"
        "log"
        "unsafe"
)

func main() {
        defer func() {
                if err := recover(); err != nil {
                        log.Println(err)
                }
        }()
        s := "hi!"
        b := unsafe.StringData(s) // *uint8
        i := 0
        for {
                x := unsafe.Add(unsafe.Pointer(b), i) // pointer arithmetic
                c := *(*uint8)(x)
                if c <= 122 && c >= 65 {
                        fmt.Printf("%c", c) // 0x4ba06b
                }
                i++
        }
        fmt.Println()
        // 00843609
        // unexpected fault address 0x578000
}

Some notions

ASCII occupies the lower 7 bits, so bitwise AND with 0x80 will be greater 0, if value was not ASCII.

const asciiMask = 0x8080808080808080 // 8 bytes

Other:

const BroadcastSemicolon = 0x3B3B3B3B3B3B3B3B
const Broadcast0x01 = 0x0101010101010101
const Broadcast0x80 = 0x8080808080808080

Semi:

In [1]: chr(0x3b)
Out[1]: ';'

• Faster utf8.Valid using multi-byte processing without SIMD.

Readings

• Validating UTF-8 In Less Than One Instruction Per Byte