EIP-7939: Count Leading Zeros (CLZ) Opcode

For almost a decade, the Ethereum Virtual Machine could do everything from 256-bit math to Keccak hashing… but it couldn’t perform one of the simplest operations every modern CPU has supported since the early 90s:

Count Leading Zeros.

Yes, seriously.

If you ever had to compute log2(x), normalize a big integer, or find the highest set bit in a bitmap, you already know how painful it was. Engineers hacked together loops, binary searches, lookup tables, or micro-optimized assembly—and still paid 30–200+ gas for something that hardware can do in one cycle.

EIP-7939 (CLZ) fixes that.

With the upcoming Fusaka (Osaka) hard fork, Ethereum finally gets a native, hardware-style, cheap CLZ opcode. This is one of the simplest EIPs in years—and one of the most impactful for math, crypto, compilers, ZK proving, and even DeFi.

This article is the deep dive I wish existed earlier: what CLZ does, the math behind it, how it fits into the EVM architecture, how compilers will use it, and the real benefits you’ll see across the ecosystem.

What EIP-7939 Actually Proposes

EIP-7939 introduces a single new opcode:

• Opcode: 0x1e
• Mnemonic: CLZ
• Input: 1 word (x, 256-bit)
• Output: Number of leading zeros in x (0–256)
• Gas Cost: 5
• Special Case: CLZ(0) returns 256

Examples:

• CLZ(0x01) → 255
• CLZ(0x80…00) → 0
• CLZ(0) → 256

Once Fusaka activates (expected Dec 2025), this opcode becomes universally available on upgraded nodes.

Why Ethereum Never Had CLZ (And Why It Finally Does)

To understand why this upgrade feels overdue, it helps to look back.

Early EVM Philosophy (2014–2019)

The original EVM avoided “CPU-style” instructions. The philosophy was:

• Keep the opcode set minimal
• Avoid hardware-driven operations
• Rely on high-level constructs
• Keep everything uniform and slow-but-safe Bit operations were considered not essential.

Then Reality Hit

As Ethereum matured:

• Smart contracts needed efficient math (AMMs, oracles, compression)
• ZK-rollups exploded and needed deterministic bit operations
• Bytecode size limits made lookup-table tricks expensive
• Compilers started hitting optimization ceilings

The missing CLZ became more painful each year.

Why Now?

Three forces pushed CLZ over the finish line:

1. ZK proving — CLZ is cheaper to prove than even ADD.
2. Client maturity — Geth, Nethermind, Besu can now adopt low-level ops safely.
3. Compiler demand — Solidity / Yul optimizers benefit massively from CLZ.

CLZ isn’t just convenience—it’s a fundamental primitive Ethereum should have had from day one.

How CLZ Fits Inside the EVM Pipeline

When executed:

STACK BEFORE:
  [..., x]

EXECUTE:
  CLZ(0x1e)

STACK AFTER:
  [..., CLZ(x)]

Before Fusaka:
→ 0x1e throws INVALID.
After Fusaka:
→ 0x1e is fully supported and deterministic.

The Math Behind CLZ (Explained Simply)

For any positive 256-bit integer x:

msb = floor(log2(x))

CLZ gives:

CLZ(x) = 256 - msb - 1

Or reversed:

msb = 255 - CLZ(x)

Implications:

• log2(x) becomes trivial
• Integer sqrt becomes faster
• Cube-root normalization becomes simpler
• Big-integer normalization becomes cheap
• Bitmap scanning becomes a one-step operation

CLZ vs CTZ — Why CLZ Is the Right Choice

A common debate was: Should Ethereum have CTZ instead?
The answer: no.

Because:

• CLZ can emulate CTZ, but
• CTZ cannot emulate CLZ

Example CTZ using CLZ:

y = x & (-x)
ctz = 255 - CLZ(y)

CLZ is the more fundamental primitive.

Gas Benchmark Comparison (Before vs After CLZ)

Here’s what CLZ changes in real numbers:

Operation	Before CLZ	After CLZ	Improvement
log2(x) (bit-length)	50–120 gas	5 gas	10×–24×
sqrt normalization	200–350 gas	~60 gas	4×–6×
bitmap scan	80–150 gas	5 gas	15×+
DeBruijn lookup	+1–2 KB bytecode	0 KB	huge
ZK proof constraints	80–120	<10	massive

This isn't a “nice-to-have.”
This is foundational performance improvement.

Compiler Impact (Solidity, Yul, Vyper, Huff)

This part was missing in most explanations, but it matters.
Solidity

Expected changes:

• clz(uint256 x) becomes a built-in
• Optimizer auto-rewrites patterns into CLZ
• Libraries like Solady, PRBMath, Uniswap v4 will drop DeBruijn tables

Yul

Yul already has:

let z := clz(x)

The optimizer will soon detect:

• normalization loops
• binary search bit-scans
• expensive log2 patterns

…and replace them with CLZ.

Vyper

The Vyper team already signaled support:

• CLZ will appear as a built-in
• Optimization passes become simpler

Huff

Expect immediate use by low-level protocol engineers.
CLZ is a dream instruction for Huff developers doing crypto or AMM math.

Security Considerations (And Why CLZ Is Safe)

Even simple opcodes need vetting.

Here’s the quick security review:

Deterministic Across Clients
All nodes compute CLZ identically.
No floating point, no platform differences.

No Gas Griefing
CLZ uses constant 5 gas.
No dynamic cost = no DoS vector.

Edge Cases

• CLZ(0) returns 256 — safe and documented
• Input always fits in 256 bits — enforced by the EVM
• No state access — side-effect free

Static Analysis Tools
Mythril, Slither, and Foundry already handle custom opcodes; CLZ requires minimal rule updates.

All good.

Upgrade Workflow: How CLZ Reaches Mainnet

1. EIP Draft → Magicians Discussion (2012–2024)
2. Client Implementation (2025)

• Geth (merged)
• Nethermind (released)
• Besu (in progress)
  1. Devnet Testing (2025)
  2. Fusaka Fork Activation (expected Dec 2025)
  3. CLZ becomes standard opcode

No controversy, no objections—just universal support.

Usage Flow (Developer Mental Model)

1. Compiler emits opcode 0x1e.
2. PC reaches 0x1e.
3. Stack pops x.
4. Node computes leading zeros.
5. Pushes result (0–256).
6. Continues execution.

Real-World Use Cases (Where CLZ Actually Matters)

1. AMMs & DeFi Math
Uniswap v4 math for square roots, tick spacing, normalization → major gas drop.

2. ZK Rollups
CLZ is cheaper to prove than ADD.
This is a huge deal for zkEVMs.

3. Compression
Solady’s LibZip + calldata compression become more efficient.

4. Priority Queues & Bitmaps
Finding highest priority index → 1 opcode.

5. Cryptography
Big-int normalization, prefix encodings, and PQC algorithms benefit directly.

Common Pitfalls to Avoid

• Don’t forget: CLZ(0) = 256.
• Avoid using CLZ on pre-Fusaka chains — it will revert.
• Be careful when using CLZ to compute array indices — sanitize inputs.

Timeline: CLZ’s Long Road to Ethereum

2012 — First requests for bit-scan opcodes
2018 — DeBruijn tricks become widespread in Solidity
2022 — ZK rollups demand deterministic bit operations
2024 — EIP-7939 drafted
2025 — Implementations across all major clients
Dec 2025 — Fusaka fork activates CLZ

A 13-year journey for a single instruction.

Community Consensus

Final Thoughts: Why CLZ Matters More Than It Looks

Most people won't celebrate an opcode.
But the engineers who build Ethereum’s foundations absolutely will.

CLZ:

• makes math cheaper
• reduces bytecode size
• simplifies compilers
• accelerates ZK proofs
• unlocks new optimizations
• modernizes the EVM instruction set

This is one of those upgrades that quietly improves everything—DeFi, ZK, libraries, protocols, and gas efficiency—without breaking anything.

Fusaka isn’t just another fork.
It’s the moment Ethereum finally gets a core primitive it should have had from day one.