Why would a smart contract checker look very closely at parts written in 'assembly code' even if the normal code looks fine?

A smart contract checker meticulously examines parts written in assembly code because this low-level language represents the direct instructions that the blockchain's virtual machine, such as the Ethereum Virtual Machine (EVM), actually executes. While smart contracts are typically authored in high-level languages like Solidity, these languages are translated through a process called compilation into assembly code, also known as bytecode, before they are deployed to the blockchain. The critical point is that the bytecode, not the original source code, is the authoritative version of the program that runs and interacts with the blockchain state. Even if the higher-level code appears correct, the compilation process itself can introduce subtle bugs or unexpected optimizations that alter the contract's behavior in ways not visible at the source code level. For instance, a compiler might reorder operations or allocate memory differently, inadvertently creating a security loophole that was not present in the developer's original intent. Developers sometimes use inline assembly, which means embedding segments of assembly code directly within a high-level language contract. This is often done for advanced gas optimizations, which refer to minimizing the transaction execution cost, or to achieve precise control over low-level operations that are not exposed by the high-level language abstractions. However, using inline assembly bypasses many of the safety checks and guardrails provided by the high-level language, increasing the risk of introducing critical vulnerabilities such as reentrancy, where an external malicious contract can repeatedly call back into a vulnerable contract before the initial transaction is finalized, or storage collisions, where data is unintentionally overwritten due to mismanaged memory addresses. Therefore, analyzing the assembly code allows a checker to confirm the exact operational logic and potential side effects at the machine level, identifying discrepancies or malicious constructs that would be obscured by the apparent correctness of the high-level source code. This ensures the deployed contract's behavior precisely matches its intended secure design.