The war is over: C++ and SystemC coexist in a single flow

Thomas Bollaert and Mike Fingeroff, Mentor Graphics Corp.
12/15/2010 10:38 AM EST

Since its debut in 2004, the current generation of high-level synthesis (HLS) tools has made tremendous progress in terms of both quality of results (QoR) and wider applicability. The success of this technology cannot be denied: HLS is here to stay. However, as in other arenas of electronic design automation, a language war threatens to divide the user community, pitting C/C++ against SystemC. While some rallied around a more abstract form of modeling in pure untimed C++, others argued for more detailed models; such as the explicit timing, structure, and parallelism of SystemC.

These two standard languages do serve different design needs; yet for this very reason they complement each other to great advantage in a mixed-language HLS flow. For example, while the complex algorithms of next-generation broadband modem ASICs are most effectively expressed in pure C++, the intricate control logic found in other parts of the same device benefit from SystemC cycle-accurate models.

Today, a new era of peace has already begun with the introduction of dual-language HLS tools. Designers can now express complex interface protocols using a timed SystemC source while keeping the rest of the design functionality in pure untimed ANSI C++. And they can express structure and hierarchy either by using SystemC modules or by inferring them from natural C++ boundaries. Now that HLS tools can deliver full-chip synthesis, both pure C++ and SystemC are needed to provide the most efficient and productive way to handle all the various parts of the system.

It is important to understand where and how to use these modeling options in order to increase productivity and QoR. For this discussion, we can divide a design into four modeling domains: processing, control, interfaces, and hierarchy. The guiding principle is to keep the models as simple as possible as long as possible. Simplicity is bound by what constitutes sufficient detail — models must have enough detail to be meaningful; yet you need sufficient detail and no more. A corollary principle is that when modeling untimed aspects of a design, use purely functional C++ models. When timing and concurrency are involved, add the timing detail that SystemC classes provide.

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