VIP: compile-time parametrizable constants

[charles-cooper]

Simple Summary

allow constants which can be set at compilation time, instead of current status quo (either in the source code or via immutables). this is similar to the -D flag in C/C++.

Motivation

currently, runtime constants can only be set in the source code, or via the immutables mechanism (set at deploy time). however, this blocks a whole class of optimizations which could be done if we had access to the immutables values earlier. here is an example which must currently be done with immutables (but cannot be optimized because the immutable value is only known at deploy-time):

N: immutable(uint256)

@deploy
def __init__(n: uint256):
    assert n < 3
    N = n

@external
def foo(x: uint256):
    if N == 0:
        return x
    if N == 1:
        return x + 5
    if N == 2:
        return x * 9

Specification

allow constant() values to be declared without specification, e.g.

N: constant(uint256)

if run from the CLI, the constant value must be included with vyper -D N=<value> (otherwise, the compiler should complain with something like "ConstantNotSpecified").

Backwards Compatibility

backwards compatible

References

https://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Preprocessor-Options.html

Copyright

Copyright and related rights waived via CC0