Abstract:
A method is provided for updating an existing netlist to reflect a design change. A design incorporating the design change and the existing netlist are provided to a synthesis tool. The design and the existing netlist are processed with the synthesis tool reusing logic structures from the existing netlist. A result is generated by the synthesis tool including the existing netlist and a new portion of a netlist reflecting the design change.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to integrated circuits and, more particularly, to methods for synthesizing an integrated circuit design. 
         [0002]    Logic synthesis is a process by which an abstract form of desired circuit behavior (typically register transfer level (RTL)) is turned into a design implementation in terms of logic gates. Typical practical implementations of a logic function utilize a multilevel network of logic elements. Starting from an RTL description of a design, a synthesis tool constructs a corresponding multilevel Boolean network. Next, this network is optimized using several technology-independent techniques before technology-dependent optimizations are performed. 
         [0003]    Finally, technology-dependent optimization transforms the technology-independent circuit into a network of gates in a given technology. Mapping is constrained by factors such as the available gates (logic functions) in the technology library, the drive sizes for each gate, and the delay, power, and area characteristics of each gate. The result of this synthesis is generally a netlist, which describes the connectivity of an electronic design. 
         [0004]    After synthesis, the resulting netlist is used for placement and routing of the gates and other components, as well as used for additional simulations such as timing. Because the synthesis and optimization are highly numerical processes, the results generated by these processes may be very path dependent. Because of the numerical aspects of the path dependency, a side effect of synthesis tools is that generated netlists may be very different based on small changes in inputs, causing extensive efforts and repetitive work for placement, routing and timing of any updated designs. 
         [0005]    In an ASIC chip development process, for example, once the logic synthesis and initial place and route stage have taken place, any engineering change due to RTL modification is a very complicated and time consuming task, potentially adding delay to the development process. The development process is taxed due to last minute RTL changes, as a complete new netlist is generated every time such a change occurs, forcing a time consuming manual processes to find minimal changes in the new netlist (when the new RTL has induce some additional change) as compared to the previous design. 
         [0006]    What is needed therefore is a process that minimizes the impact of design changes late in the product development process. 
       BRIEF SUMMARY 
       [0007]    Embodiments of the invention provide a method of updating an existing netlist to reflect a change in an integrated circuit design change. A design incorporating the design change and the existing netlist are provided to a synthesis tool. The design and the existing netlist are synthesized in a combined manner with the synthesis tool, which reuses logic structures from the existing netlist. A result generated by the synthesis tool includes both the existing netlist and a new portion of a netlist reflecting the design change. In some embodiments, the synthesis tool may perform an optimization of the existing netlist and the new design with an objective to minimize the design space. The existing netlist may act as a constraint on the optimization and may prevent changes to the existing netlist. 
         [0008]    After the generation of the new netlist, the existing netlist may then be updated with the new portion of the netlist reflecting the design change. Modifying the existing netlist generally entails matching inputs and outputs of a portion of the existing netlist with the new portion of the netlist generated by the synthesis tool. Then a portion of the existing netlist may be replaced with the new portion of the netlist to reflect the design change. In some embodiments, modifying the existing netlist may be performed by the synthesis tool. Once the netlist has been updated, other post-processing functions such as updating placement, routing, and timing may be performed using the modified netlist. In some embodiments the new design may be provided in the form of a netlist. In other embodiments, the new design may be provided in the form of an RTL. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
           [0010]      FIG. 1A  is a diagrammatic representation of a design space for a combined synthesis of an existing netlist and a new design. 
           [0011]      FIG. 1B  is a diagrammatic representation of a result of the combined synthesis in  FIG. 1A . 
           [0012]      FIG. 1C  is a diagrammatic representation of an addition of a new netlist to an existing netlist from the result of  FIG. 1B . 
           [0013]      FIG. 2  is a diagrammatic representation of a design space for a combined synthesis of another existing netlist and new design and the resulting combination. 
           [0014]      FIG. 3  is a flowchart of a process for performing a combined synthesis. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Embodiments of the invention assist in overcoming issues related to design changes after placement, routing, and timing analysis has been performed using an exiting netlist. The method presented in the embodiments of the invention provides an efficient way to perform minimal engineering changes to an existing netlist required by design changes. The method lends itself to automation thereby reducing manual efforts at later stages in the design cycle, as well as aiding the overall chip development process by reducing efforts for timing, layout, etc. for engineering changes, which may occur late in the design cycle. 
         [0016]    As disclosed above, ASIC designs are generally delivered in an RTL format, which is synthesized with known synthesis tools, generating a netlist. This netlist is then used for downstream design applications such as gate placement, routing between the components, and analysis such as timing analysis, confirming that the design meets the specifications. When changes are made to an ASIC design, a new RTL is provided and synthesized, resulting in a new netlist. Because the synthesis process may be path dependent, the netlist resulting from the changes RTL may be vastly different from the original netlist, even for small changes made to the ASIC design. The design from the new netlist may then require replacement and rerouting of the gates on the ASIC, resulting in long redesign times. Additionally, a preservation of the existing design becomes a tedious manual task, prone to errors and also adding to delays in the design time. 
         [0017]    With reference now to  FIG. 1A , the methodology presented for an exemplary embodiment of the invention uses a combined synthesis approach to preserve structure from earlier designs, thus minimizing engineering changes with design changes late in the process. As shown diagrammatically in  FIG. 1A , a design space  10  contains both an existing netlist  12  and the new design in the form of an RTL  14 , or in other embodiments, in the form of another netlist. The primary inputs  16  of the new RTL  14  and existing netlist  12  are combined. The contents of the existing netlist  12  are then given read-only type properties, ensuring that the primary outputs  18  of the existing netlist  12 , as well as the netlist  12  itself, do not change their logic structure and logic functions. The new RTL  14  may also have a set of primary outputs  20  associated with the updated design. Depending on the magnitude of the design change in the new RTL  14 , it is likely that there will be an overlap between the primary outputs  18  of the existing netlist  12  and the primary outputs  20  of the new RTL  14 . 
         [0018]    A synthesis is then performed on both the existing netlist  12  and the new RTL  14 . An objective of the synthesis performed on the combined structures is to minimize the overall design space. The read-only existing netlist  12  acts as constraint on the objective resulting in the reuse of structure already defined in the existing netlist  12 . The minimized design space is bounded by the two extremes. The first boundary exists where the new RTL  14  being synthesized is exactly the same as the design generating the existing netlist  12 . A result of the synthesis in this case would be the existing netlist  12 . At the other extreme, the new RTL  14  is a design completely different from the design generating the existing netlist  12 . A result of the synthesis in this case would be two netlists, one, unchanged, for the existing netlist  12 , and a new netlist  22  with no overlap, for the new RTL  14 . In typical operation, the design space will generally be between these two extremes resulting in an unchanged netlist  12  and a smaller new netlist  22  representing the change to the design as seen in the diagrammatic representation in  FIG. 1B . 
         [0019]    The new netlist corresponds to an old netlist  24  of the existing netlist  12 . The new netlist  22  connects at points at the boundary of the old netlist  24  connecting the new netlist  22  to structure from the existing netlist  12  that has not changed as seen in  FIG. 1B . Additionally, the primary outputs  18  of the existing netlist  12  are divided into outputs  26  corresponding to the unchanged portions of the netlist  12  and outputs  28  corresponding to the old netlist  24  of the netlist. Moreover, the new netlist  22  also has associated with it a set of new outputs  30 . The old netlist  24  may now be removed from netlist  12  and the new netlist  22  may be connected. The result is shown diagrammatically in  FIG. 1C . Because much of the existing netlist  12  has been preserved, placement and rerouting need only be performed for the new netlist  22 . Similarly, timing may need only be performed on the new portions of the design, rather than the entire design. 
         [0020]    Removal of the old netlist  24  of the existing netlist  12  and the addition of the new netlist  22  may be accomplished manually using existing editing tools known in the art. The manual process of removing the old netlist  24  and replacing it with the new netlist  22  may be greatly simplified due to the reuse of the majority of the existing netlist  12  in the new design. Generally the process includes matching inputs and outputs of a portion of the existing netlist  12  with the new netlist  22 . Once matched, the old netlist  24  portion of the existing netlist  12  may be replaced with the new netlist  22 . In some embodiments, the replacement of the old netlist  24  of the existing netlist  12  with the new netlist  22  may be automated. This automated process may be incorporated into the synthesis such that the after the optimization to reduce the design space  10  has completed, the synthesis tool may then create a new netlist  22  that is representative of the combination of parts of the existing netlist  12  and new netlist  22  as illustrated in  FIG. 1C . Alternatively, the automated process may be a post-processing function performed by a separate module after the synthesis has completed. This post-processing step may be performed prior to steps for placement, routing, and/or timing. 
         [0021]    In an alternate embodiment, the new design may affect the inputs rather than the outputs. In this exemplary embodiment diagrammatically illustrated in  FIG. 2 , a design space  50  again contains an existing netlist  52  and a new RTL  54  representing the design change. In this embodiment, primary outputs  56  are connected to both the existing netlist  52  and the new RTL  54 . Also in the design space are the inputs  58  for the existing netlist  52  and the inputs  60  for the new RTL. Similar to the embodiment above, the existing netlist  52  is set to a read-only state so that the structures in the netlist remain unchanged after the synthesis process. Again part of the synthesis process includes an objective function to minimize the design space, resulting in a new netlist  62  that represents the changes from the updated design. After the synthesis process has completed, as above with the previous embodiment, an old netlist may be removed and replaced with the new netlist  62 . Inputs  64  belonging to the remainder of the existing netlist  52  and inputs  66  specific to the new netlist are also provided. Similar to the outputs of the embodiments above, the original inputs  58  for the existing netlist contain portions that are common to the existing and updated design. Those inputs  64  are preserved through the synthesis process by keeping the existing netlist  52  in a read-only condition. Also similar to embodiments disclosed above, the replacement of the old netlist with the new netlist  62  may be a manual process performed with existing editing tools, or the process may be an automated process performed during the synthesis process or as a post-processing step to the synthesis process. 
         [0022]    The process may be summarized by flowchart  100  in  FIG. 3 . An existing netlist and a new design change, in the form of a new RTL or a new netlist, may be provided to an existing synthesis tool (block  102 ). The existing netlist provided is marked as read-only (block  104 ) to prevent changes from being made to the existing netlist. The existing netlist and the new design are then subjected to a combined synthesis, optimizing the design space to a minimum (block  106 ). The read-only existing netlist acts as a constraint on the optimization, preserving and reusing as much of the existing structure as possible. As disclosed above, the bounds for the design space are the return of the original netlist if the new design is the same as the design that originally created the existing netlist as none of the structures would have changed. The other extreme is the creation of a complete second netlist if the new design is not related at all to the original design that created the existing netlist. In practice, the more common result will likely be the return of the existing netlist, and a new netlist that represents a portion of the existing netlist that has changed due to the design change. 
         [0023]    Once the new netlist portion has been generated from the synthesis process, the new portion replaces part of the structure in the existing netlist (block  108 ), which generally preserves the majority of the original design. After the netlist has been updated, other post-processing steps such as placement, routing, and timing, related to the updated netlist may be performed (block  110 ). Because much of the existing structures may have been preserved from the previous designs, engineering changes occurring late in the design cycle may not have as large of an impact on production as the smaller changes may be incorporated more readily. 
         [0024]    While the present invention has been illustrated by a description of one or more embodiments thereof and while these embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, changes to the design may involve changes that affect both inputs and outputs simultaneously. Additionally, this methodology may be applied directly to new designs, using portions of existing netlists to impose certain structures in the design. Moreover, because the read-only netlists act as design constraints, multiple netlists may be used in a single design space for the combined synthesis. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.