MIR Optimization: Constant Folding & Propagation

[gakonst]

Summary

Implement constant folding and propagation on MIR to evaluate constant expressions at compile time.

Parent issue: #687

Context

Constant folding evaluates operations with constant operands at compile time:

• add(5, 3) → 8
• mul(x, 0) → 0 (even if x is not constant)
• and(x, 0xFF) where x is uint8 → x (no-op)

This reduces bytecode size and gas costs, and enables further optimizations.

Tasks

Basic constant folding

• For arithmetic ops with constant operands, compute result
• Handle 256-bit arithmetic correctly (wrapping, overflow)
• Support all arithmetic ops: add, sub, mul, div, mod, exp
• Support all bitwise ops: and, or, xor, not, shl, shr, sar

Algebraic simplifications

• add(x, 0) → x
• mul(x, 1) → x
• mul(x, 0) → 0
• div(x, 1) → x
• and(x, 0) → 0
• or(x, 0) → x
• xor(x, 0) → x
• shl(x, 0) → x
• sub(x, x) → 0
• xor(x, x) → 0

Comparison folding

• eq(5, 5) → true
• lt(3, 5) → true
• iszero(0) → true
• Propagate through branches (see SCCP issue)

Integration with lowering

• Fold during HIR→MIR lowering where possible
• Avoid generating constant operations in the first place

Example

; Before constant folding
  v0 = const 10
  v1 = const 5
  v2 = add(v0, v1)     ; → const 15
  v3 = mul(v2, 2)      ; → const 30
  v4 = arg(0)
  v5 = add(v4, 0)      ; → v4
  return v5

; After constant folding
  v4 = arg(0)
  return v4

Acceptance Criteria

• All 256-bit arithmetic handled correctly
• Algebraic identities applied
• Tests verify no incorrect simplifications
• Measurable reduction in instruction counts

Estimated Complexity

Small-Medium - Straightforward but many cases to handle

Dependencies

• MIR structure (done)

[Mike4751]

Hello, can I work on this??

[gakonst]

Sorry I'm on it

[Mike4751]

If there's any available to work on I'd be happy too 🫡