Hoist constants shared across functions into the global space

[vezenovm]

Problem

Post #7021 we will have globals from SSA fully implemented in Brillig.

However, we do not account for shared local constants across functions. For example let's take this simplified version of global_var_regression_simple

global EXPONENTIATE: [[Field; 2]; 2] = [[1, 1], [0, 0]];

fn main(x: Field, y: pub Field) {
    let mut acc: Field = 0;
    for i in 0..2 {
        for j in 0..2 {
            acc += EXPONENTIATE[i][j];
        }
    }
    assert(x != y);
    dummy_again(x, y);
}
fn dummy_again(x: Field, y: Field) {
    let mut acc: Field = 0;
    for i in 0..2 {
        for j in 0..2 {
            acc += EXPONENTIATE[i][j];
        }
    }
    assert(x != y);
}

Running nargo info --force-brillig --inliner-aggressiveness -9223372036854775808 --profile-execution will give you 178 opcodes executed.

If we made a global ZERO: Field = 0 and used that to initialize acc in both functions the opcodes executed would go down to 176. This is a pretty small example, but for larger programs with lots of constants performing this hoisting could be quite beneficial.

Happy Case

We should be able to automatically hoist constants shared across functions into the global space. We should not have any repeated initialization of constants.

Workaround

Yes

Workaround Description

Manually writing globals that are shared across functions

Additional Context

No response

Project Impact

Nice-to-have

Blocker Context

This provides execution benefits, so even though it is not a true blocker, it is high priority.

Would you like to submit a PR for this Issue?

None

Support Needs

No response