c++ - CMPXCHG16B 正确吗？

Question

虽然我不确定为什么，但这似乎并不完全正确。建议很好，因为 CMPXCHG16B 的文档非常少(我没有任何英特尔手册...)

template<>
inline bool cas(volatile types::uint128_t *src, types::uint128_t cmp, types::uint128_t with)
{
    /*
    Description:
     The CMPXCHG16B instruction compares the 128-bit value in the RDX:RAX and RCX:RBX registers 
     with a 128-bit memory location. If the values are equal, the zero flag (ZF) is set, 
     and the RCX:RBX value is copied to the memory location. 
     Otherwise, the ZF flag is cleared, and the memory value is copied to RDX:RAX.
     */
    uint64_t * cmpP = (uint64_t*)&cmp;
    uint64_t * withP = (uint64_t*)&with;
    unsigned char result = 0;
    __asm__ __volatile__ (
    "LOCK; CMPXCHG16B %1\n\t"
    "SETZ %b0\n\t"
    : "=q"(result)  /* output */ 
    : "m"(*src), /* input */
      //what to compare against
      "rax"( ((uint64_t) (cmpP[1])) ), //lower bits
      "rdx"( ((uint64_t) (cmpP[0])) ),//upper bits
      //what to replace it with if it was equal
      "rbx"( ((uint64_t) (withP[1])) ), //lower bits
      "rcx"( ((uint64_t) (withP[0]) ) )//upper bits
    : "memory", "cc", "rax", "rdx", "rbx","rcx" /* clobbered items */
    );
    return result;
}

运行示例时，我得到的是 0，而它应该是 1。有什么想法吗？

Answer 1

注意到一些问题，

(1) 主要问题是约束，“rax”并没有像它看起来那样做，而是第一个字符“r”让 gcc 使用任何寄存器。

(2) 不确定您的存储类型::uint128_t，但假设 x86 平台采用标准的小尾数法，那么高位和低位双字也会交换。

(3) 获取某物的地址并将其转换为其他物可能会破坏别名规则。取决于你的 types::uint128_t 是如何定义的，这是否是一个问题(如果它是两个 uint64_t 的结构则很好)。假设不违反别名规则，带有 -O2 的 GCC 将进行优化。

(4) *src 确实应该被标记为输出，而不是指定内存破坏。但这实际上更多的是性能问题而不是正确性问题。类似地，rbx 和 rcx 不需要指定为 clobbered。

这是一个有效的版本，

#include <stdint.h>

namespace types
{
    // alternative: union with  unsigned __int128
    struct uint128_t
    {
        uint64_t lo;
        uint64_t hi;
    }
    __attribute__ (( __aligned__( 16 ) ));
}

template< class T > inline bool cas( volatile T * src, T cmp, T with );

template<> inline bool cas( volatile types::uint128_t * src, types::uint128_t cmp, types::uint128_t with )
{
    // cmp can be by reference so the caller's value is updated on failure.

    // suggestion: use __sync_bool_compare_and_swap and compile with -mcx16 instead of inline asm
    bool result;
    __asm__ __volatile__
    (
        "lock cmpxchg16b %1\n\t"
        "setz %0"       // on gcc6 and later, use a flag output constraint instead
        : "=q" ( result )
        , "+m" ( *src )
        , "+d" ( cmp.hi )
        , "+a" ( cmp.lo )
        : "c" ( with.hi )
        , "b" ( with.lo )
        : "cc", "memory" // compile-time memory barrier.  Omit if you want memory_order_relaxed compile-time ordering.
    );
    return result;
}

int main()
{
    using namespace types;
    uint128_t test = { 0xdecafbad, 0xfeedbeef };
    uint128_t cmp = test;
    uint128_t with = { 0x55555555, 0xaaaaaaaa };
    return ! cas( & test, cmp, with );
}