2

Does the EVM always allocate new memory or can it reuse already allocated, but unused, memory?

Reading the docs for delete (http://solidity.readthedocs.io/en/develop/types.html), it is mentioned that:

delete a assigns the initial value for the type to a. I.e. for integers it is equivalent to a = 0, but it can also be used on arrays, where it assigns a dynamic array of length zero or a static array of the same length with all elements reset. For structs, it assigns a struct with all members reset.

Does this mean that if a is a dynamic array (uint[]) of, say, length n, then delete a assigns to a an empty array:a = new uint[](0)? Not the original array, of length n, with its contents set to 0?

So, if new and delete are used many times in a Solidity contract, will the EVM keep allocating new memory (expanding memory), or will it reuse already allocated memory when possible?

Consider the loop:

uint[] a;
for (uint i = 0; i < 1000; ++i) {
   a = new uint[](100);
   delete a;
}

Will this allocate 100 or 100000 memory cells?

2

This is an interesting question, so I wrote some quick code to test it out.

contract Memsize {
    function foo(uint _its) pure public returns (uint) {
        uint ms;
        uint[] memory a;
        for (uint i = 0; i < _its; ++i) {
            a = new uint[](100);
            delete a;
        }
        assembly{
            ms := msize()
        }
        return(ms);
    }
}

Results;

foo(1) 3392
foo(2) 6656
foo(3) 9920

Etc. So, no the memory is not re-used by Solidity and is allocated fresh each time.

This is not a limitation of the EVM. There's no reason why the memory couldn't be re-used. It is a property of the Solidity compiler.

If you really care about this stuff (efficiency optimisation of smart contracts), then you may be interested in LLL as a contract language. Some docs here.

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  • Thanks, once again for an excellent answer. I didn't knew how to test it, so thanks for your code. – Shuzheng Nov 30 '17 at 11:04

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