Need support for integer division in Vyper

[yzhang90]

What's your issue about?

I noticed that a recent change Vyper where / returns decimal instead of int128. However, if a developer do want to use integer division (int128 // int128), he/she has to use floor to do the conversion. However, this conversion can potentially waste gas because '/' will first multiply DECIMAL_DIVISOR and floor will divide DECIMAL_DIVISOR. On the other hand, EVM opcode DIV performs the integer division.

How can it be fixed?

Introduce operator // for integer division and it can be directly translated to DIV opcode in EVM.

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[fubuloubu]

Potential solution: add optimization that int128 = floor(int128/int128) should use DIV opcode directly instead of more complex math, e.g. catch and remove clamp in that limited circumstance in our optimization module

[jacqueswww]

@fubuloubu yes, was thinking the same, the only problem comes when it's a nested expression.

[fubuloubu]

That's why optimization is the right solution. If it's not simple to figure out, don't do the optimization! Positive confirmation.

[yzhang90]

@fubuloubu why not use // to replace floor(int128/int128) instead of optimizing floor function?

[fubuloubu]

Because from an auditability perspective, it may be easy to miss that extra / and misunderstand what the command is doing. That was our concern with //.

Vyper is ease of audit, not ease of writability.

[dani-corie]

I think decimal numbers in a smart contract are a luxury. Ethereum is integer-based, and 99% of all use cases ever call for integers, not decimals.

I think the best solution would be to disallow integers and decimals to co-mingle.

decimal / decimal = decimal (decimal division)
integer / integer = integer (integer division)
decimal / integer = COMPILE TIME ERROR
integer / decimal = COMPILE TIME ERROR

Alternatively, we could still have separate operators for integer and decimal division!

decimal / decimal = decimal (decimal division)
integer / integer = COMPILE TIME ERROR

integer // integer = integer (integer division)
decimal // decimal = COMPILE TIME ERROR

decimal / integer = COMPILE TIME ERROR
integer / decimal = COMPILE TIME ERROR

decimal // integer = COMPILE TIME ERROR
integer // decimal = COMPILE TIME ERROR

In fact, I'd remove ALL implicit conversions. Operators should only ever operate on operands of the exact same type, and return the same type. If you need to add an int128 to a uint256, cast explicitly. If you need to divide a decimal with an integer, cast explicitly.

[fubuloubu]

You know, this is probably a stronger and simpler approach. It makes auditability the main goal, and it would be simpler to describe. No implict conversions at all.

What difficulties exist with this approach?

[dani-corie]

I don't see any issue beyond code becoming more verbose (which, for Vyper, is more a requirement than a problem). Existing code not compiling after the change should not be an issue since Vyper is still in alpha.

[jacqueswww]

As mentioned in #717 (comment), I think we should put the above suggestion (#716 (comment)) into a VIP.

This means we drop support for integer / integer = decimal, floor and ceil remain decimal operators and any cross-type division is not supported forcing explicit conversion.

[fubuloubu]

Sounds good to me!

[jacqueswww]

Merge into #737 as a VIP, thanks ;)