Why CPUs Do Not Accept Pre-Decoded Instructions from Compilers: Tradeoffs and Design Challenges

In many cases, it may seem counterintuitive why CPUs are not designed to accept pre-decoded instructions from compilers. Wouldn't this save both silicon space and processing time? The answer is no, and this article will explore why.

Tradeoffs in Processor Design

The design of CPUs involves numerous tradeoffs, one of which is the instruction size length. While it might seem logical that compacting instructions would save significant processing time, there are often more important considerations.

The Nature of High-End x86-64 Machines

A typical high-end x86-64 machine uses a mechanism called micro-ops (microoperations). The decode unit fetches instructions and generates these micro-ops, which are then stored in a local cache. These micro-ops are designed to be very compact and are used for scheduling and executing operations. The external instructions fetched by the machine are indeed compact and do not require much memory to encode, making them efficient to handle.

Compact vs. Compactibility

Micro-ops, however, are not compact for the purpose of saving silicon space. Instead, they are designed to simplify the internal architecture of the processor. Micro-ops can access more registers than the external instruction set, which helps address instruction dependency issues. These micro-ops are also "renamed," meaning they can be associated with different external registers, to facilitate efficient execution.

Loop Optimization and Decoding

When a high-level code segment describes a tight loop, micro-ops stored in the internal cache can represent the entire loop, eliminating the need for the decode unit to fetch instructions from memory. This is crucial for avoiding memory fetches, even from the L1 cache, which significantly speeds up the processor. Internal code can also issue micro-ops in parallel, out-of-order, or in any sequence that is convenient for the processor's architecture.

Flexibility and Patchability

The non-regular format of micro-ops allows the hardware designer to maximize flexibility and optimize the internal structure without being constrained by compatibility issues. If an encoding is found that would speed things up or simplify the design, the hardware designer can easily implement it without the external software needing to be aware of the change. This makes it possible to update the hardware without updating the software, which is a significant advantage.

Potential for Dynamic Updates

Theoretically, if something is found to be broken in the hardware, internal tables could be rewritten to patch the issue. While this is not as simple as it sounds, it does offer the potential for dynamic updates, further enhancing the flexibility of the processor design.

ARM Machines: Similar Tradeoffs

While the concept of pre-decoded instructions might seem beneficial, the current design of both ARM and x86-64 machines is efficient enough. For ARM machines, the current scheme is adequate, especially since most devices are not generally “compute-bound.” Even if ARM microservers were a potential application, the optimization for simplicity and efficiency already in place means that increasing complexity for pre-decoding would not be a wise move.

Memory Bandwidth Considerations

CPUs are now much faster than memory, and reducing memory bandwidth is a key strategy to improving performance. Pre-decoded instructions would increase memory usage and bandwidth consumption, which would not be a direction to head. Even if simpler micro-ops could be designed, the increased bandwidth would negate the intended savings.

Historical and Projected Perspectives

In the late 1970s, the concept of multi-flow instruction sets was considered, but that era has passed. While it is possible that some variant of this idea might be useful in the future, no such proposal has been made as of yet. The emphasis remains on optimizing existing instruction sets to balance performance, complexity, and compatibility.

Therefore, the current design of CPUs that accept and decode instructions on-the-fly is well-suited for modern computing needs, and attempting to implement pre-decoded instructions would likely introduce unnecessary complexity and negate the benefits of reduced memory bandwidth usage.