0x04 Identity

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Overview

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Gas Cost

• Base cost: 15 gas
• Per-word cost: 3 gas per 32-byte word
• Partial words round up

• 0 bytes: 15 gas
• 32 bytes: 18 gas (15 + 3*1)
• 33 bytes: 21 gas (15 + 3*2)
• 64 bytes: 21 gas (15 + 3*2)

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Input Format

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Output Format

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Usage Example

import { execute, PrecompileAddress } from '@tevm/voltaire/precompiles';
import { Hardfork } from '@tevm/voltaire/primitives/Hardfork';
import * as Hex from '@tevm/voltaire/Hex';

// Copy some data
const input = Hex('0x0102030405');

// Calculate required gas
const words = Math.ceil(input.length / 32);
const gasNeeded = 15n + 3n * BigInt(words);

// Execute precompile
const result = execute(
  PrecompileAddress.IDENTITY,
  input,
  gasNeeded,
  Hardfork.CANCUN
);

if (result.success) {
  // Output === input
  console.log('Output:', result.output);
  console.log('Gas used:', result.gasUsed);
} else {
  console.error('Error:', result.error);
}

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Error Conditions

• Out of gas (insufficient for 15 + 3 * ceil(len/32))

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Use Cases

• Data copying: Efficient way to copy data between memory locations
• Gas metering: Test gas consumption for data operations
• Calldata to memory: Copy calldata to memory efficiently
• Proxy contracts: Forward data without modification
• Testing: Validate precompile execution mechanics

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Implementation Details

• Zig: Simple allocate + copy operation using allocator.dupe
• TypeScript: Creates new Uint8Array copy of input
• Integration: Standalone, no dependencies
• Optimization: Most efficient data copy operation in EVM

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Performance Characteristics

• Base cost: 15 gas
• Per-byte cost: ~0.09375 gas/byte
• More efficient than CODECOPY (3 gas/word + memory expansion)
• Cheaper than manual byte-by-byte copying

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Why Does Identity Exist?

1. Efficient memory operations: Copying data via precompile avoids expensive EVM opcodes
2. Gas accounting: Provides predictable gas cost for data operations
3. Testing: Validates precompile calling mechanism
4. Historical: Part of original Ethereum design, maintained for compatibility

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Comparison with Other Copy Operations

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Test Vectors

import * as Hex from '@tevm/voltaire/Hex';

// Test 1: Empty input
const input1 = new Uint8Array(0);
const result1 = executeIdentity(input1);
// Expected: empty array (0 bytes)
// Gas: 15 + 3 * 0 = 15

// Test 2: 5 bytes
const input2 = Hex('0x0102030405');
const result2 = executeIdentity(input2);
// Expected: [1, 2, 3, 4, 5]
// Gas: 15 + 3 * ceil(5/32) = 15 + 3*1 = 18

// Test 3: Exact 32-byte boundary
const input3 = Hex('0x' + 'ff'.repeat(32));
const result3 = executeIdentity(input3);
// Expected: 32 bytes of 0xFF
// Gas: 15 + 3 * 1 = 18

// Test 4: Partial word (33 bytes)
const input4 = Hex('0x' + '00'.repeat(33));
const result4 = executeIdentity(input4);
// Expected: 33 bytes (unchanged)
// Gas: 15 + 3 * ceil(33/32) = 15 + 3*2 = 21

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References

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Specifications

• Yellow Paper - Appendix E (Precompiled Contracts)
• EIP-5656: MCOPY Opcode

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Related

• Precompiles Overview