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golang快速入门[9.1]-深入字符串的存储、编译与运行

2020-04-01  本文已影响0人  唯识相链2

前文

前言

var a string = `hello world`
var b string = "hello world"


func (s *scanner) next() {
    ...
    c := s.getr()
    for c == ' ' || c == '\t' || c == '\n' && !nlsemi || c == '\r' {
        c = s.getr()
    }
    // token start
    s.line, s.col = s.source.line0, s.source.col0
    if isLetter(c) || c >= utf8.RuneSelf && s.isIdentRune(c, true) {
        s.ident()
        return
    }

    switch c {
    case '"':
        s.stdString()

    case '`':
        s.rawString()

    ...
    }


func (s *scanner) rawString() {
    s.startLit()

    for {
        r := s.getr()
        if r == '`' {
            break
        }
        if r < 0 {
            s.errh(s.line, s.col, "string not terminated")
            break
        }
    }
    // We leave CRs in the string since they are part of the
    // literal (even though they are not part of the literal
    // value).

    s.nlsemi = true
    s.lit = string(s.stopLit())
    s.kind = StringLit
    s.tok = _Literal
}


func (s *scanner) stdString() {
    s.startLit()

    for {
        r := s.getr()
        if r == '"' {
            break
        }
        if r == '\\' {
            s.escape('"')
            continue
        }
        if r == '\n' {
            s.ungetr() // assume newline is not part of literal
            s.error("newline in string")
            break
        }
        if r < 0 {
            s.errh(s.line, s.col, "string not terminated")
            break
        }
    }

    s.nlsemi = true
    s.lit = string(s.stopLit())
    s.kind = StringLit
    s.tok = _Literal
}


str := " 微信:
1131052403 "


// go/src/cmd/compile/internal/gc
func (p *noder) basicLit(lit *syntax.BasicLit) Val {
    case syntax.StringLit:
        if len(s) > 0 && s[0] == '`' {
            // strip carriage returns from raw string
            s = strings.Replace(s, "\r", "", -1)
        }
        // Ignore errors because package syntax already reported them.
        u, _ := strconv.Unquote(s)
        return Val{U: u}

    default:
        panic("unhandled BasicLit kind")
    }
}

字符串拼接

/usr/local/go/src/cmd/compile/internal/gc/noder.go
func (p *noder) sum(x syntax.Expr) *Node {
    for i := len(adds) - 1; i >= 0; i-- {
        add := adds[i]

        r := p.expr(add.Y)
        if Isconst(r, CTSTR) && r.Sym == nil {
            if nstr != nil {
                // Collapse r into nstr instead of adding to n.
                chunks = append(chunks, r.Val().U.(string))
                continue
            }

            nstr = r
            chunks = append(chunks, nstr.Val().U.(string))
        } else {
            if len(chunks) > 1 {
                nstr.SetVal(Val{U: strings.Join(chunks, "")})
            }
            nstr = nil
            chunks = chunks[:0]
        }
        n = p.nod(add, OADD, n, r)
    }
    if len(chunks) > 1 {
        nstr.SetVal(Val{U: strings.Join(chunks, "")})
    }

    return n
}


    var a = "hello"
    str :=  a + "xxs"


go/src/cmd/compile/internal/gc/walk.go
func walkexpr(n *Node, init *Nodes) *Node {
    case OADDSTR:
        n = addstr(n, init)
}


func addstr(n *Node, init *Nodes) *Node {
    // build list of string arguments
    args := []*Node{buf}
    for _, n2 := range n.List.Slice() {
        args = append(args, conv(n2, types.Types[TSTRING]))
    }

    var fn string
    if c <= 5 {
        // small numbers of strings use direct runtime helpers.
        // note: orderexpr knows this cutoff too.
        fn = fmt.Sprintf("concatstring%d", c)
    } else {
        // large numbers of strings are passed to the runtime as a slice.
        fn = "concatstrings"

        t := types.NewSlice(types.Types[TSTRING])
        slice := nod(OCOMPLIT, nil, typenod(t))
        if prealloc[n] != nil {
            prealloc[slice] = prealloc[n]
        }
        slice.List.Set(args[1:]) // skip buf arg
        args = []*Node{buf, slice}
        slice.Esc = EscNone
    }

    cat := syslook(fn)
    r := nod(OCALL, cat, nil)
    r.List.Set(args)
    r = typecheck(r, ctxExpr)
    r = walkexpr(r, init)
    r.Type = n.Type

    return r
}


type StringHeader struct {
    Data uintptr
    Len  int
}


/usr/local/go/src/runtime/string.go
func concatstrings(buf *tmpBuf, a []string) string {
    idx := 0
    l := 0
    count := 0
    for i, x := range a {
        n := len(x)
        if n == 0 {
            continue
        }
        if l+n < l {
            throw("string concatenation too long")
        }
        l += n
        count++
        idx = i
    }
    if count == 0 {
        return ""
    }

    // If there is just one string and either it is not on the stack
    // or our result does not escape the calling frame (buf != nil),
    // then we can return that string directly.
    if count == 1 && (buf != nil || !stringDataOnStack(a[idx])) {
        return a[idx]
    }
    s, b := rawstringtmp(buf, l)
    for _, x := range a {
        copy(b, x)
        b = b[len(x):]
    }
    return s
}

func concatstring2(buf *tmpBuf, a [2]string) string {
    return concatstrings(buf, a[:])
}

func concatstring3(buf *tmpBuf, a [3]string) string {
    return concatstrings(buf, a[:])
}

func concatstring4(buf *tmpBuf, a [4]string) string {
    return concatstrings(buf, a[:])
}

func concatstring5(buf *tmpBuf, a [5]string) string {
    return concatstrings(buf, a[:])
}


func rawstringtmp(buf *tmpBuf, l int) (s string, b []byte) {
    if buf != nil && l <= len(buf) {
        b = buf[:l]
        s = slicebytetostringtmp(b)
    } else {
        s, b = rawstring(l)
    }
    return
}

字符串与字节数组的转换

    a := "微信:1131052403"
    b := []byte(a)
    c := string(b)


func slicebytetostring(buf *tmpBuf, b []byte) (str string) {
    ...
    var p unsafe.Pointer
    if buf != nil && len(b) <= len(buf) {
        p = unsafe.Pointer(buf)
    } else {
        p = mallocgc(uintptr(len(b)), nil, false)
    }
    stringStructOf(&str).str = p
    stringStructOf(&str).len = len(b)
    memmove(p, (*(*slice)(unsafe.Pointer(&b))).array, uintptr(len(b)))
    return
}

如下为字符串转换为字节数组

func stringtoslicebyte(buf *tmpBuf, s string) []byte {
    var b []byte
    if buf != nil && len(s) <= len(buf) {
        *buf = tmpBuf{}
        b = buf[:len(s)]
    } else {
        b = rawbyteslice(len(s))
    }
    copy(b, s)
    return b
}

总结

参考资料

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