Exploring the Possibility of a Superscalar Stack-Based CPU: The Case of a Superscalar FORTH Processor

The concept of a superscalar stack-based CPU, particularly a superscalar FORTH processor, is intriguing yet challenging. This article delves into the feasibility of such a design, addressing the unique nuances and advantages associated with stack-based architectures, specifically in the context of FORTH. We'll explore why this is possible, why it is difficult, and what modifications are needed to make it a reality.

The Challenges of Stack-Based Machines in Superscalar Design

Stack-based machines are inherently serial in nature due to their design. Most operations on a stack machine fetch operands implicitly from the top of the stack and place results back on the stack. This contrasts significantly with register-based machines, where operands are explicitly specified. To understand the difficulties, let’s break down the key operations in a stack-based machine:

Stack operations: Any operation that fetches an operand to place it on the stack (e.g., immediate values from the code stream, values in memory). Stack manipulations: Operations that rearrange the stack (e.g., stack rotation, duplicating or dropping entries from the stack).

Register renaming, a common technique in superscalar architectures, can be adapted to work with stack machines. The implicit operands in stack operations can be mapped to stack slots, allowing for straightforward register renaming. However, the main issue lies in the need to minimize stack depth and stack manipulations, which are critical for efficient computation.

Code Generation and Stack Depth Optimization

Code generators for stack-based machines aim to minimize stack depth and reduce the number of stack manipulations. This approach aligns with depth-first search (DFS) algorithms, which favor deepening the computation tree before expanding it. In contrast, breadth-first search (BFS) algorithms, used by register-based machines, seek to maximize parallelism by expanding the computation tree.

Let’s consider a simple example of adding four integer values together:

e  a   b   c   d

A register-based superscalar machine might prefer:

a b c d (a b) (c d)

This strategy maximizes parallelism, enabling the first two additions to occur concurrently. A stack-based machine might prefer:

PUSH a PUSH b ADD PUSH c PUSH d ADD ADD POP e

While this strategy minimizes stack depth, it misses the opportunity for parallelism.

Superscalar Design for Stack Machines

For a stack-based machine to be effective in a superscalar design, it must be able to examine a large enough window of operations to discover instruction-level parallelism among multiple serial computation chains. An out-of-order (OoO) execution pipeline is essential to allow these serial chains to execute concurrently.

In a strict in-order pipeline, the main parallelism is limited to operand fetches from memory and result storage. However, with an OoO pipeline, you can start parallelizing largely-independent computations, provided the window of operations is large enough.

Modern OoO processors have deep reordering buffers, making it likely to find these opportunities. However, the performance gain may not be as significant as a purely register-based architecture on a similar microarchitecture, especially for workloads where parallelism is critical.

The Case of FORTH

FORTH, being a high-level language with powerful facilities for defining and redefining verbs, offers a unique challenge but also an opportunity. While FORTH itself can be implemented in various ways, the core concept remains the same: a stack-based, out-of-order processor.

The specific details of FORTH machines and their bytecode execution are not well-documented. However, from a general perspective, it is possible to implement a superscalar stack-based FORTH processor. The key is to tweak the code generator to prefer parallelism despite the additional stack depth and manipulations.

Conclusion

Building a superscalar stack-based CPU, particularly a superscalar FORTH processor, is theoretically possible but challenging. The main issues lie in minimizing stack depth and stack manipulations while still maximizing instruction-level parallelism. By adapting techniques such as register renaming and using an OoO pipeline, it is feasible to overcome these challenges. However, the performance gains may be limited compared to register-based architectures, especially for workloads that rely heavily on parallelism.