I'm analyzing Uniswap V2 core contracts, and have noticed a comment
// overflow is desired
Why overflow is desired?
Because, from my point of view when overflow happens the next line
if (timeElapsed > 0 && _reserve0 != 0 && _reserve1 != 0) {
never will be true due to wrong timeElapsed.
In:
uint32 timeElapsed = blockTimestamp - blockTimestampLast; // overflow is desired
Since timeElapsed is unsigned, if an overflow occurs then its value will necessarily be positive, hence the expression timeElapsed > 0 will necessarily evaluate to true.
If you're planning to dive into Solidity code, then you probably want to learn the basic concepts of unsigned integers and Twos Complement.
That contract is written in pragma solidity =0.5.16. In this version in order to prevent overflow-underflow errors, it has to have SafeMath library checks which is extra operation so it is an extra cost (solidity automatically checks for overflow underflow after v8.0.0)
uint32 timeElapsed = blockTimestamp - blockTimestampLast; // overflow is desired
blockTimestampLast is when those 2 variables are updated
uint public price0CumulativeLast;
uint public price1CumulativeLast;
timeElapsed type is uint32 which represents unsigned integers with values ranging from 0 to 4,294,967,295. To simplify let's say our range is 0-32 and let's say we have those variables
blockTimestamp=30
blockTimestampLast=20
Therefore timeElapsed=10
Assume that 10 seconds passed and we did not update price0CumulativeLast and price1CumulativeLast so blockTimestampLast=20. 10 seconds passed, blockTimestampLast did not change we expect that timeElapsed=20
After 10 seconds blockTimestamp will be 30+10=40. since we assume that our range is 0-32, blockTimestamp will be 40-32=8. Now calculate the timeElapsed
timeElapsed = blockTimestamp - blockTimestampLas
= 8-20 =-12
since we are in 0-32 -12 means 20 seconds. So timeElapsed=20. Since the time difference did not change, overflow is desired in this case.
But imagine a case where you are adding total costs and our range is still 0-32. Now when your total cost is 25 and if you add 10 dollars cost, it will be 35 dollars, in our range it is 3 dollars. So this is not desirable.