High Level Synthesis Tools: A Designer’s Perspective

In Designs: A HLS Perspective, Design requirements are presented and categorized based on High level Synthesis tool perspective. How those requirements are met, was not discussed in the article.

To meet the requirements outlined in Designs: A HLS Perspective, high level synthesis tools have to employ few techniques. These techniques may be employed in a manner that these  are transparent to designers or may create usability issues and add to learning curve.

For example, A command or extension need to be added to existing language like ANSI-C to determine the width of data path. Now it can be achieved in following different ways

  • Method A: A ‘special’ type keyword added to ANSI C. Existing ANSI-C Code is modified for each data type.
  • Method B: A ‘special’ type keyword added to ANSI C. Existing ANSI-C Code modification is required for only input & output data types. All other internal data types are automatically extracted.
  • Method C: A constraint command is defined which can be added to ANSI C Code ‘inline style’ or can be specified in constraints file. All other internal data types are automatically extracted.

Method ‘C’ doesn’t necessiate need of two separate files, one as golden reference and other as ‘hardware’ version and according to me, Method C is the best option. What do you think? Please vote.

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Similar to data path synthesis, special timing constructs are needed to implement bus interface in ANSI C. Issue is that once these constructs are available, design style becomes like RTL coding style. Even if tool may not require usage of these constructs to implement algorithmic portion of application, designers will use it anyway. To force designer to think at higher level of abstraction, these option should not be part of language at all. This creates further sub catagory in High Level Synthesis tools, called Algorithm Synthesis tools. Such high level synthesis tool will not be able to handle interface/bus protocol effectively but is adept at handling algorithms. I think it is okay as long as bus protocols are standards driven.

In Ranking System, pure ANSI C gets highest mark and added constructs/limitations will cost. On other hand, pure ANSI C based tool will not be able to handle bus protocols. Reality is that no tool is going to achieve perfect score. But any tool with ease of use will win eventually and remember QoR is the key!

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