在不使用指令MUL，IMUL，SHL，SHR，LOOP的情况下将数字相乘 [英] Multiply numbers without using instructions MUL, IMUL, SHL, SHR, LOOP

问题描述

是否可以在不使用x86汇编语言中的指令MUL，IMUL，SHL，SHR，LOOP，JMP的情况下计算乘法结果?

Is it possible to calculate result of multiplication without using instructions MUL, IMUL, SHL, SHR, LOOP, JMP in x86 assembly language?

推荐答案

是否可以在不使用x86汇编语言中的指令MUL，IMUL，SHL，SHR，LOOP，JMP的情况下计算乘法结果?

Is it possible to calculate result of multiplication without using instructions MUL, IMUL, SHL, SHR, LOOP, JMP in x86 assembly language?

是的

没有MUL，通常的方法是循环循环"SHIFT LEFT，TEST和ADD"，如下所示:

Without MUL the normal approach is "SHIFT LEFT and TEST and ADD" in a loop, like this:

    result = 0;
    while(a > 0) {
        result = result << 1;
        if( a & 0x80000000 != 0) {
            result = result + b;
        }
        a = a << 1;
    }

请注意，像这样的32位整数循环最多(最多)进行32次迭代.

Note that a loop like this for 32-bit integers will have (at most) 32 iterations.

您可以用附加值替换这些班次(例如，用add eax, eax替换shl eax, 1)；您可以将LOOP替换为显式循环(例如dec ecx，jne next)或展开该循环(重复执行32次代码).这些替代品可能会提高性能.

You can replace these shifts with additions (e.g. shl eax, 1 replaced with add eax, eax); and you can replace LOOP with an explicit loop (e.g. dec ecx, jne next) or unroll the loop (repeat the code 32 times). These replacements will probably improve performance.

一旦您执行了无符号乘法运算，就可以将IMUL替换为将值转换为正数并使用无符号乘法运算的分支.例如.像:

Once you have unsigned multiplication, IMUL can be replaced with branches that convert the values to positive and uses unsigned multiplication. E.g. like:

    if(a < 0) {
        a = -a;
        if(b < 0) {
            b = -b
            return a*b;    // Unsigned multiplication
        } else {
            return -a*b;   // Unsigned multiplication
        }
    } else {
        if(b < 0) {
            b = -b
            return -a*b;   // Unsigned multiplication
        } else {
            return a*b;    // Unsigned multiplication
    }
}

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