From 2ae2a78f38f9cd972eb48fa7e69df7003cd68cea Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pawe=C5=82=20Bylica?= <chfast@gmail.com>
Date: Wed, 19 Oct 2016 17:16:06 +0200
Subject: [PATCH] vm: optimize EXP

Optimize EXP performance by using simple algorithm that replaces division with
a bit mask.
---
 core/vm/instructions.go | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/core/vm/instructions.go b/core/vm/instructions.go
index 3352877c1f..fcd6274b98 100644
--- a/core/vm/instructions.go
+++ b/core/vm/instructions.go
@@ -192,8 +192,16 @@ 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()
+	result := big.NewInt(1)
+	for exponent.Cmp(common.Big0) != 0 {
+		if exponent.Bit(0) == 1 {
+			U256(result.Mul(result, base))
+		}
+		U256(base.Mul(base, base))
+		exponent.Rsh(exponent, 1)
+	}
+	stack.push(result)
 }
 
 func opSignExtend(instr instruction, pc *uint64, env Environment, contract *Contract, memory *Memory, stack *Stack) {