Restructure

The restructure module in OpenROAD (rmp) is based on an interface to ABC for local resynthesis. The package allows logic restructuring that targets area or timing. It extracts a cloud of logic to ABC using the cut module. Multiple recipes for area or timing are run to obtain multiple structures from ABC; the most desirable among these is used to improve the netlist. The resynthesized logic is then read back to OpenDB. Reading back of constants requires insertion of tie cells which should be provided by the user as per the interface described below.

Commands

Note

• Parameters in square brackets [-param param] are optional.

• Parameters without square brackets -param2 param2 are required.

Restructure

Restructuring can be done in two modes: area or delay.

• Method 1: Area Mode Example: restructure -liberty_file ckt.lib -target area -tielo_pin ABC -tiehi_pin DEF

• Method 2: Timing Mode Example: restructure -liberty_file ckt.lib -target delay -tielo_pin ABC -tiehi_pin DEF -slack_threshold 1 -depth_threshold 2

restructure 
    [-slack_threshold slack_val]
    [-depth_threshold depth_threshold]
    [-target area|delay]
    [-abc_logfile logfile]
    [-liberty_file liberty_file]
    [-tielo_port  tielo_pin_name]
    [-tiehi_port tiehi_pin_name]
    [-work_dir work_dir]

Options

Switch Name	Description
-liberty_file	Liberty file with description of cells used in design. This is passed to ABC.
-target	Either area or delay. In area mode, the focus is area reduction, and timing may degrade. In delay mode, delay is likely reduced, but the area may increase. The default value is area.
-slack_threshold	Specifies a (setup) timing slack value below which timing paths need to be analyzed for restructuring. The default value is 0, and the allowed values are floats [0, MAX_FLOAT].
-depth_threshold	Specifies the path depth above which a timing path would be considered for restructuring. The default value is 16, and the allowed values are [0, MAX_INT].
-tielo_pin	Tie cell pin that can drive constant zero. The format is <cell>/<port>.
-tiehi_pin	Tie cell pin that can drive constant one. The format is <cell>/<port>.
-abc_logfile	Output file to save abc logs to.
-work_dir	Name of the working directory for temporary files. If not provided, run directory would be used.

Resynth

Resynthesize parts of the design in an attempt to fix negative slack.

resynth
    [-corner corner]

Options

Switch Name	Description
-corner	Process corner to use.

Resynth with simulated annealing

Resynthesize parts of the design with an ABC script found via simulated annealing. The script is a series of operations on ABC’s internal AIG data structure. A neighboring solution is a script with one operation added, removed, or two operations swapped. The optimization function is defined as the worst slack.

resynth_annealing
    [-corner corner]
    [-slack_threshold slack_threshold]
    [-seed seed]
    [-temp temp]
    [-iters iters]
    [-revert_after revert_after]
    [-initial_ops initial_ops]

Options

Switch Name	Description
-corner	Process corner to use.
-slack_threshold	Specifies a (setup) timing slack value below which timing paths need to be analyzed for restructuring. The default value is 0.
-seed	Seed to use for randomness in simulated annealing.
-temp	Initial temperature for simulated annealing. The default is the required arrival time on the worst slack endpoint.
-iters	Number of iterations to run simulated annealing for.
-revert_after	After the given number of iterations that worsen slack, revert to best found solution.
-initial_ops	Size of the initial random solution (number of commands in the script for ABC).

Resynth with genetic slack tuning

Resynthesize parts of the design with an ABC script found via genetic algorithm. An individual in a population is a series of operations on ABC’s internal AIG data structure. Each such operation is considered a gene. Genotype can be changed by a mutation with operations such as adding, removing or swapping genes with a mutation_probability probability. Individual can also be changed by crossing two genes together with a crossover_probability probability. The optimization function is defined as the worst slack.

resynth_genetic
    [-corner corner]
    [-slack_threshold slack_threshold]
    [-seed seed]
    [-population_size population_size]
    [-mutation_probability mutation_probability]
    [-crossover_probability crossover_probability]
    [-tournament_probability tournament_probability]
    [-tournament_size tournament_size]
    [-iters iters]
    [-initial_ops initial_ops]

Options

Switch Name	Description
-corner	Process corner to use.
-slack_threshold	Specifies a (setup) timing slack value below which timing paths need to be analyzed for restructuring. The default value is 0.
-seed	Seed to use for randomness.
-population_size	Population size.
-mutation_probability	Probability of applying mutation operator.
-crossover_probability	Probability of applying crossover operator.
-tournament_probability	Tournament probability.
-tournament_size	Tournament size.
-iters	Number of iterations to run genetic algorithm.
-initial_ops	Size of the initial random solution (number of commands in the script for ABC).

Example scripts

Example scripts on running rmp for a sample design of gcd as follows:

./test/gcd_restructure.tcl

Regression tests

There are a set of regression tests in ./test. For more information, refer to this section.

Simply run the following script:

./test/regression

Limitations

FAQs

Check out GitHub discussion about this tool.

Authors

• Sanjiv Mathur

• Ahmad El Rouby

License

BSD 3-Clause License. See LICENSE file.