对 64 位 UInt 右移充当有符号移位感到困惑

Question

为什么右移行为如下？

let a: UInt = 0x6177206d, b: UInt = 0x9e3779b1
let m: UInt = a * b

print("""
    let m = \(String(format:"%x", a)) * \(String(format:"%x", b)) = \(String(format:"%x", m))

    m>>4=\(String(format:"%x", m >> 4))
    m>>8=\(String(format:"%x", m >> 8))
    m>>16=\(String(format:"%x", m >> 16))
    m>>32=\(String(format:"%x", m >> 32))
    """)

生产：

let m = 6177206d * 9e3779b1 = ef1bf05d

m>>4=fef1bf05
m>>8=efef1bf0
m>>16=a4efef1b
m>>32=3c3ca4ef

Answer 1

m≠0xef1bf05d；它是 0x3c3ca4efef1bf05d。

%x 仅处理 32 位值，而您处理的是 64 位值。对 64 位值使用 %lx 或 String(m, radix: 16)：

let a: UInt = 0x6177206d, b: UInt = 0x9e3779b1
let m: UInt = a * b

print("""
    let m = \(String(format:"%lx * %lx = %lx", a, b, m))

    m>>4  = \(String(format:"%lx", m >> 4))
    m>>8  = \(String(format:"%lx", m >> 8))
    m>>16 = \(String(format:"%lx", m >> 16))
    m>>32 = \(String(format:"%lx", m >> 32))
    """)

或者如果你想看到它，请使用 %016lx zero-padded（这样更容易看到正在发生的变化），产生：

let m = 000000006177206d * 000000009e3779b1 = 3c3ca4efef1bf05d

m>>4  = 03c3ca4efef1bf05
m>>8  = 003c3ca4efef1bf0
m>>16 = 00003c3ca4efef1b
m>>32 = 000000003c3ca4ef

对 64 位 UInt 右移充当有符号移位感到困惑

Puzzled with 64-bit UInt right shift acting as signed shift

signed

bit-shift

uint64

swift