Calculate which functions require predicates in SSA CFG construction

[michaeljklein]

Problem

Currently, we don't explicitly identify side-effect-free functions. This can make it difficult to identify all of the sources of side effects individually and is related to multiple bugs where side effects run in non-executed branches, for example #5462

• The branch condition is a simple function applied to a dynamic/unknown input
• There are side effects inside the branch
• The side effects erroneously run when the condition is false

if foo(x) {
  // side effects in `bar`
  bar();
}

Happy Case

Identify side-effects in functions when calculating the control-flow graph (CFG) during SSA.

• Add side-effect metadata to the CFG during construction
• Add a method to check whether a function has side effects from the completed CFG
• Use this check to ensure predicates (i.e. EnableSideEffectsIf conditions) are handled properly
• Evaluate whether this allows us to remove the no_predicates attribute

See @jfecher's comment for a partial list of side effects to handle.

Project Impact

Nice-to-have

Impact Context

• It can be challenging to identify whether functions have side effects without a graph.
• This also affects Brillig, e.g. because Brillig calls can be considered side effects
• Using no_predicates properly is hard, because currently you need to check all dependent functions for side effects manually.
  • no_predicates is useful for performance.
  • The list of builtins or features that are allowed with no_predicates could change.

Workaround

Yes

Workaround Description

For Noir users:
Manually ensure there are no predicates and use the no_predicates attribute.

For developing Noir:

1. Test control flow (see chore: test blackbox binary op instructions #5484, which found [T]::pop_front() panics when inside non-executed if #5462)
2. Fix side-effect-in-branch bugs individually

Additional Context

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response