More optimal for loops

In test.sol there are 3 functions with the total gas cost in comments above them:

• increment0 : The unchecked style for loop

• increment1 : A cheaper more optimized version of increment0

Total gas cost breakdown

increment0

loop count	gas cost
5	22138
100	27743
1000	80855

increment1

loop count	gas cost
5	22051
100	26326
1000	66838

increment2

loop count	gas cost
5	22019
100	26294
1000	66806

OPCODES

increment0

58 total gas per loop

OPCODES for 1 loop with increment0

OPCODE	GAS
JUMPDEST	1 gas, one time cost. wont count this in total.
DUP4.	3 gas
DUP2	3 gas
LT	3 gas
ISZERO	3 gas
PUSH2 0112	3 gas
JUMPI	10 gas
DUP1	3 gas
PUSH1 01	3 gas
ADD	3 gas
SWAP1	3 gas
POP	2 gas
DUP1	3 gas
SWAP2	3 gas
POP	2 gas
PUSH2 00fc	3 gas
JUMP	8 gas

increment1

47 gas total per loop

OPCODES for 1 loop with increment1

OPCODE	GAS
JUMPDEST	1 gas, one time on cost. wont count this in total
DUP1	3 gas
PUSH1 01	3 gas
ADD	3 gas
SWAP1	3 gas
POP	2 gas
DUP1	3 gas
SWAP2	3 gas
POP	2 gas
DUP4	3 gas
DUP2	3 gas
GT	3 gas
ISZERO	3 gas
PUSH2 0122	3 gas
JUMPI	10 gas

EXPLANATION

So when we declare our loop break condition in the for loop, we have check at the beginning of every loop, followed by a JUMPI which is a conditional jump that basically says if this is false jump out of the loop but if its true jump into the loop. And if its true we continue through the loop and then hit the end and have to hit an additonal jump to get back to the top of the loop. But in the new way we just continue through the loop and hit our condition at the end and then hit only one JUMPI which says either jump back to the top of the loop or just jump out of the loop. All in all saving us 11 gas a loop, plus possibly some additional gas on the for loop setup

Now for this to work your if statement with the loop break has to be the very last thing in your loop. You have to increment i then check the conditional