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common/math, core/vm: implement fast EXP.

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Courtesy @chfast & @karalabe
This commit is contained in:
Jeffrey Wilcke 2016-10-20 15:00:53 +02:00
parent 890ffa05f8
commit b9923b46f6
2 changed files with 37 additions and 2 deletions
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34
common/math/exp.go Normal file
View file

@ -0,0 +1,34 @@
package math
import (
"math/big"
"github.com/ethereum/go-ethereum/common"
)
const (
// Perverted constants straight out of big.Int
// https://golang.org/src/math/big/arith.go
_m = ^big.Word(0)
_logS = _m>>8&1 + _m>>16&1 + _m>>32&1
_S = 1 << _logS
_W = _S << 3 // word size in bits
)
// Exp implement exponentiation by squaring algorithm.
//
// Courtesy @karalabe and @chfast
func Exp(base, exponent *big.Int) *big.Int {
result := big.NewInt(1)
for _, word := range exponent.Bits() {
for i := 0; i < _W; i++ {
if word&1 == 1 {
common.U256(result.Mul(result, base))
}
common.U256(base.Mul(base, base))
word >>= 1
}
}
return result
}

5
core/vm/instructions.go
View file

@ -21,6 +21,7 @@ import (
"math/big"
"github.com/ethereum/go-ethereum/common"
"github.com/ethereum/go-ethereum/common/math"
"github.com/ethereum/go-ethereum/crypto"
"github.com/ethereum/go-ethereum/params"
)
@ -192,8 +193,8 @@ func opSmod(instr instruction, pc *uint64, env Environment, contract *Contract,
}
func opExp(instr instruction, pc *uint64, env Environment, contract *Contract, memory *Memory, stack *Stack) {
x, y := stack.pop(), stack.pop()
stack.push(U256(x.Exp(x, y, Pow256)))
base, exponent := stack.pop(), stack.pop()
stack.push(math.Exp(base, exponent))
}
func opSignExtend(instr instruction, pc *uint64, env Environment, contract *Contract, memory *Memory, stack *Stack) {