ACIR Optimization Loop

This workflow targets ACIR circuit size for constrained Noir programs. It does not apply to unconstrained (Brillig) functions — Brillig runs on a conventional VM where standard profiling and algorithmic improvements apply instead, and bb gates won't reflect Brillig performance.

Measuring Circuit Size

Binary projects

Compile the program and measure gate count with:

nargo compile && bb gates -b ./target/<package>.json

Library projects

Libraries cannot be compiled with nargo compile . Instead, mark the functions you want to measure with #[export] and use nargo export :

nargo export && bb gates -b ./export/<function_name>.json

Artifacts are written to the export/ directory and named after the exported function (not the package).

If bb is not available, ask the user for their backend's equivalent command. Other backends should have a similar CLI interface.

The output contains two fields:

• circuit_size : the actual gate count after backend compilation. This determines proving time, which is generally the bottleneck.

• acir_opcodes : number of ACIR operations. This affects execution time (witness generation). A change can reduce opcodes without affecting circuit size or vice versa — both matter, but prioritize circuit_size when they conflict.

Always record a baseline of both metrics before making changes.

Optimization Loop

• Baseline: compile and record circuit_size .

• Apply one change at a time.

• Recompile and measure: compare circuit_size to the baseline.

• Revert if worse: if circuit_size increased or stayed the same, undo the change. Not every "optimization" helps — the compiler may already handle it, or the overhead of the new approach may outweigh the savings.

• Repeat from step 2 with the next candidate change.

What to Try

Candidate optimizations roughly ordered by impact:

• Hint and verify: replace expensive in-circuit computation with an unconstrained hint and constrained verification. This is the highest-impact optimization for most programs.

• Reduce what you hint: if you're hinting intermediate values (selectors, masks, indices), see if you can hint only the final result and verify it directly.

• Hoist assertions out of branches: replace if c { assert_eq(x, a) } else { assert_eq(x, b) } with assert_eq(x, if c { a } else { b }) .

• Simplify comparisons: inequality checks (< , <= ) cost more than equality (== ). But don't introduce extra state to avoid them — measure first.

What Not to Try

• Don't hint division or modular arithmetic: the compiler already injects unconstrained helpers for these.

• Don't hand-roll conditional selects: if/else expressions compile to the same circuit as c * (a - b) + b .

• Don't replace <= with flag tracking without measuring: adding mutable state across loop iterations can produce more gates than a simple comparison.