type-optimization

Purpose

Identify variable declarations and arithmetic expressions where type choices add masking overhead or consume unnecessary storage slots. The EVM operates natively on 256-bit words; types smaller than 256 bits add AND masking opcodes in computation contexts and waste storage slots when used as standalone state variables.

When to Use

• Writing or reviewing state variable declarations
• Reviewing function signatures with small integer types
• Reviewing arithmetic-heavy functions or loop bodies
• Pre-audit cleanup of variable declarations

When NOT to Use

• Variables inside packed storage structs: smaller types ARE the optimization in that context (they reduce slot count). TY-001 and SL-001 apply in opposite contexts.
• When a smaller type is required for interface compatibility (e.g., ERC-20 decimals() returns uint8, ERC-2981 royaltyInfo returns uint96)
• When semantic range enforcement is the intent (a value that must wrap at 255)

Platform Detection

Trigger on any .sol file containing:

• uint8, uint16, uint32, or uint64 local variables, parameters, or return types used in computation outside of a packed struct (TY-001)
• string state variable assigned a short literal or fixed value (TY-002)
• Three or more standalone bool state variable declarations (TY-003)
• An explicit downcast immediately followed by widening in arithmetic (TY-004)

Quick Reference

Context	Pattern	Fix
Local variable / return / param NOT in packed struct	uint8/uint16/uint32/uint64 in arithmetic	uint256 (no masking)
Short fixed string (≤31 bytes) as state var	string public symbol	bytes32 (one slot, no length)
3+ standalone bool state vars	bool a; bool b; bool c;	uint256 bitmap
Downcast then widen in same expression	uint128(x) * uint256(y)	uint256(x) * uint256(y)
Bool in packed struct alongside address	OK — keeps slot full	No change needed

Workflow

1. Scan state variables: standalone bools (TY-003) and string-as-bytes32 (TY-002). List all state variable declarations at the contract level. Count consecutive or standalone bool declarations not inside a struct. If three or more exist, flag for TY-003 bitmap conversion. For each string state variable, check its assigned value: is it a short identifier (symbol, name, version string) of ≤31 bytes? If so, flag for TY-002 replacement with bytes32.