common/math, core/vm: implement fast EXP (#3214)

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

Courtesy @chfast & @karalabe

* common/math: fix go vet issues on exp calculation

common/math/exp.go

+package math
+
+import (
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+)
+
+// wordSize is the size number of bits in a big.Int Word.
+const wordSize = 32 << (uint64(^big.Word(0))>>63)
+
+// 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 < wordSize; i++ {
+			if word&1 == 1 {
+				common.U256(result.Mul(result, base))
+			}
+			common.U256(base.Mul(base, base))
+			word >>= 1
+		}
+	}
+	return result
+}

core/vm/instructions.go

@@ -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) {