Visual C++ x64 加进位 [英] Visual C++ x64 add with carry

问题描述

由于似乎没有 ADC 的内在函数，而且我不能使用 Visual C++ 对 x64 体系结构使用内联汇编器，如果我想使用带进位的加法编写函数但将其包含在一个C++ 命名空间?

Since there doesn't seem to be an intrinsic for ADC and I can't use inline assembler for x64 architecture with Visual C++, what should I do if I want to write a function using add with carry but include it in a C++ namespace?

(不能使用比较运算符进行模拟.这个 256 兆位的增加对性能至关重要.)

(Emulating with comparison operators is not an option. This 256 megabit add is performance critical.)

推荐答案

有现在是 MSVC 中 ADC 的内在:_addcarry_u64.以下代码

#include <inttypes.h>
#include <intrin.h>
#include <stdio.h>

typedef struct {
    uint64_t x1;
    uint64_t x2;
    uint64_t x3;
    uint64_t x4;
} uint256;

void add256(uint256 *x, uint256 *y) {
    unsigned char c = 0;
    c = _addcarry_u64(c, x->x1, y->x1, &x->x1);
    c = _addcarry_u64(c, x->x2, y->x2, &x->x2);
    c = _addcarry_u64(c, x->x3, y->x3, &x->x3);
    _addcarry_u64(c, x->x4, y->x4, &x->x4);
}

int main() {
    //uint64_t x1, x2, x3, x4;
    //uint64_t y1, y2, y3, y4;
    uint256 x, y;
    x.x1 = x.x2 = x.x3 = -1; x.x4 = 0;
    y.x1 = 2; y.x2 = y.x3 = y.x4 = 0;

    printf(" %016" PRIx64 "%016" PRIx64 "%016" PRIx64 "%016" PRIx64 "
", x.x4, x.x3, x.x2, x.x1);
    printf("+");
    printf("%016" PRIx64 "%016" PRIx64 "%016" PRIx64 "%016" PRIx64 "
", y.x4, y.x3, y.x2, y.x1);
    add256(&x, &y);
    printf("=");
    printf("%016" PRIx64 "%016" PRIx64 "%016" PRIx64 "%016" PRIx64 "
", x.x4, x.x3, x.x2, x.x1);
}

从 Visual Studio Express 2013 生成以下程序集输出

produces the following assembly output from Visual Studio Express 2013

mov rdx, QWORD PTR x$[rsp]
mov r8, QWORD PTR x$[rsp+8] 
mov r9, QWORD PTR x$[rsp+16]
mov rax, QWORD PTR x$[rsp+24]
add rdx, QWORD PTR y$[rsp]
adc r8, QWORD PTR y$[rsp+8]
adc r9, QWORD PTR y$[rsp+16]
adc rax, QWORD PTR y$[rsp+24]

按预期有一个 add 和三个 adc.

which has one add and three adc as expected.

对于 _addcarry_u64 的作用似乎有些混乱.如果您查看我在本答案开头链接的 Microsoft 文档，它表明它不需要任何特殊硬件.这会生成 adc 并且它可以在所有 x86-64 处理器上运行(并且 _addcarry_u32 可以在更旧的处理器上运行).它在我测试过的 Ivy Bridge 系统上运行良好.

There seems to be some confusion as to what _addcarry_u64 does. If you look at Microsoft's documentation for this which I linked to at the start of this answer it shows that it does not require any special hardware. This produces adc and it will work on all x86-64 processors (and _addcarry_u32 would work on even older processors). It works fine on the Ivy Bridge system I tested it on.

但是，_addcarryx_u64 确实需要 adx(如 MSFT 的文档中所示)并且确实无法在我的 Ivy Bridge 系统上运行.

However, _addcarryx_u64 does require adx (as shown in MSFT's documentation) and indeed it fails to run on my Ivy Bridge System.

这篇关于Visual C++ x64 加进位的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！