[pprmvgii] 128-bit multiplication in C++

/*

Demonstration of multiplying two 64-bit integers to get a 128-bit result in C++ (gcc).



./a.out 0xdeadbeefdeadbeef 12345678901234567890



Input arguments (parsed by strtoul) may be specified in hexdecimal (with 0x prefix) or decimal or octal (with 0 prefix).



This code is public domain.

*/



// define this to test that the relevant types exist as builtins, without any include files

//#define TYPESONLY



#ifndef TYPESONLY

#include <cstdlib>

#include <cassert>

#include <iostream>

using std::endl;

using std::cout;

#endif



int main(int argc, char**argv){

#ifndef TYPESONLY

assert(argc>2);

assert(sizeof(unsigned long)==8);

#endif

// uint64_t is also available in <cstdint>

unsigned long x;

unsigned long y;

#ifndef TYPESONLY

x= strtoul(argv[1],0,0);

y= strtoul(argv[2],0,0);

cout << x << " * " << y << endl << "=" << endl;

#endif



unsigned __int128 zfull=static_cast<unsigned __int128 >(x) * static_cast<unsigned __int128 >(y);

// __uint128_t is also available as a builtin, though some documentation says <cstdint>



unsigned long zhi = zfull >> 64;

unsigned long zlo = zfull; // get lower bits via truncation



unsigned long test=x*y;

#ifndef TYPESONLY

assert(test==zlo);



// cout does not support 128-bit.

// This output format can be cut and pasted into an arbitrary precision calculator.



cout << zhi << " *2^64+ " << zlo << endl;

#endif

}

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