safe gas add (ethereum#98)

* safe gas add

* fix build

core/vm/instructions.go

@@ -18,6 +18,7 @@ package vm
 
 import (
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/params"
@@ -410,8 +411,12 @@ func touchEachChunksOnReadAndChargeGas(offset, size uint64, address []byte, code
 	for i := offset / 31; i <= (endOffset-1)/31; i++ {
 		index := trieUtils.GetTreeKeyCodeChunk(address, uint256.NewInt(i))
 
-		// TODO safe-add here to catch overflow
-		statelessGasCharged += accesses.TouchAddressOnReadAndComputeGas(index)
+		var overflow bool
+		statelessGasCharged, overflow = math.SafeAdd(statelessGasCharged, accesses.TouchAddressOnReadAndComputeGas(index))
+		if overflow {
+			panic("overflow when adding gas")
+		}
+
 		if code != nil && len(code) > 0 {
 			if deployment {
 				accesses.SetLeafValue(index[:], nil)