Tcl Reference Guide: Part 11 — Miscellaneous Commands

What it is: set_app_var configures application variables that control tool behavior, optimization effort, report formatting, and flow customization. get_app_var retrieves the current value of any application variable for inspection or conditional logic.

Where we use it: At the start of scripts to configure tool behavior. Each variable has a name and a value (string, integer, boolean, or list). Common variables control timing effort, power optimization, routing mode, and log verbosity.

Syntax: set_app_var -name <var_name> -value <value> | get_app_var -name <var_name>

# Set application variables
set_app_var -name physopt_power_effort -value high
set_app_var -name route_drc_prevention -value true

# Get variable value
set effort [get_app_var -name physopt_power_effort]
puts "Current power effort: $effort"

What it is: get_attribute retrieves attributes (metadata) stored on design objects like cells, nets, pins, and ports. set_attribute assigns or modifies these attributes. Attributes store properties such as placement status, timing derate, or user-defined flags.

Where we use it: To query cell properties (is_placed, is_fixed, is_hard_macro) during scripting. Also used to mark cells for special handling like dont_touch, size_only, or keep_placement.

Syntax: get_attribute -name <attr> <object> | set_attribute -name <attr> -value <val> <object>

# Query cell attributes
set is_fixed [get_attribute -name is_fixed [get_cells inst_123]]
set cell_area [get_attribute -name area [get_cells inst_123]]

# Set attributes
set_attribute -name is_fixed -value true [get_cells macro_inst]
set_attribute -name dont_touch -value true [get_cells clock_buffer_*]

What it is: redirect captures the console output of one or more Tcl commands and writes it to a file. It is the primary method for generating reports and logs. Output can be appended to existing files or used to suppress console display.

Where we use it: Every report generation uses redirect. It is also used to create detailed logs of flow execution for debugging and documentation. The -tee option displays output on screen while also writing to file.

Syntax: redirect [-append] [-tee] <file> {<commands>}

# Redirect single command output
redirect -file reports/area.rpt { report_area }

# Redirect with tee (show and save)
redirect -tee -file logs/flow.log { report_qor }

# Append to existing file
redirect -append -file logs/debug.log { check_design }

What it is: source loads and executes a Tcl script file within the current tool session. It is the standard way to organize reusable procedures, setup scripts, and constraint files into modular components.

Where we use it: At the start of every flow to source technology setup, timing constraints, and custom Tcl procedures. Scripts can be nested (a sourced script can source other scripts) for complex design flows.

Syntax: source [-echo] [-verbose] <file_path>

# Source setup scripts
source scripts/tech_setup.tcl
source scripts/constraints/my_chip.sdc

# Source with echo (show commands as they execute)
source -echo scripts/custom_procs.tcl

# Source with verbose messages
source -verbose scripts/flow_control.tcl

What it is: save_session saves the complete tool session state including design data, constraints, application variables, and GUI settings to disk. restore_session loads a previously saved session, allowing you to resume work without re-running the entire flow.

Where we use it: Before major optimization steps as a checkpoint. If optimization degrades the design, you can restore the checkpoint. Also used to share design state between team members or switch between machines.

Syntax: save_session -name <session_name> | restore_session <session_name>

# Save checkpoint
save_session -name before_placement

# Save with specific directory
save_session -name checkpoint/after_cts

# Restore a session
restore_session before_placement