HOG-07C: Removal Upward Shift Optimization

[MerlinEgalite]

HOG-07C: Removal Upward Shift Optimization

Type	Severity	Location
Gas Optimization		HeapOrdering.sol:L258

Description:

The shiftUp operation is performed unconditionally whilst a child and parent's value can indeed match, thus executing an upwards shift inefficiently if the element removed was next-to-last in the tree and its children had an equal value to it.

Example:

// If the swapped account is in the heap, restore the invariant: its value can be smaller or larger than the removed value.
if (rank <= _heap.size) {
    if (_removedValue > getAccount(_heap, rank).value) shiftDown(_heap, rank);
    else shiftUp(_heap, rank);
}

Recommendation:

We advise the else statement to be adjusted to an else if (_removedValue < getAccount(_heap, rank).value) one, with the getAccount(_heap, rank).value value being cached to a local variable within the upper-most if clause to avoid the duplicate read gas cost between the if and else if conditional evaluations.

[QGarchery]

These changes seems unlikely to save gas in our case: the amounts will all be different because of the interests earn. So even if 2 person supply exactly the same amount, if one does so one block after the other, the amounts will be different.