math/big: mulAddVWW too slow #22643
Description
What version of Go are you using?
go1.9.2 darwin/amd64
Does this issue reproduce with the latest release?
yes
What operating system and processor architecture are you using?
darwin/amd64
What did you do?
The following Go program multiplies big.Int with one Word constant:
package main
import (
"fmt"
"math/big"
)
func main() {
a := big.NewInt(1)
b := big.NewInt(1)
a.Lsh(a, 8096)
for i := uint64(1); i <= 10000000; i++ {
a.Mul(a, b)
}
fmt.Println(a.BitLen())
}
What did you expect to see?
The following C program uses GMP 6.1.2:
#include <stdio.h>
#include <stdint.h>
#include "gmp.h"
int main(int argc, char *argv[]) {
mpz_t a, b;
mpz_init_set_ui(a, 1);
mpz_init_set_ui(b, 1);
mpz_mul_2exp(a, a, 8096);
for (uint64_t i = 1; i <= 10000000ULL; i++) {
mpz_mul(a, a, b);
}
printf("%lu\n", mpz_sizeinbase(a, 2));
return 0;
}
and terminates under around 1s.
What did you see instead?
The Go program completes on my machine in 1.87s, i.e. almost twice as slow.
mulAddVWW is building block for multiplication (by one word, or more) so a better performance would benefit many applications.