Detailed Placement#

The detailed placement module in OpenROAD (dpl) is based on OpenDP, or Open-Source Detailed Placement Engine. Its key features are:

Fence region support
Fragmented row support
Mixed-cell-height (1x–4x) legalization
Two placement engines selectable at runtime

Placement Engines#

Diamond Search#

The default engine performs a BFS-style diamond search from each cell’s global placement position, expanding outward in Manhattan order until a legal site is found.

NegotiationLegalizer#

An optional two-pass legalizer based on the NBLG paper. Enabled with -use_negotiation on the detailed_placement command.

Global Placement result
        │
        ▼
┌───────────────────┐
│   Abacus Pass     │  Fast DP sweep, row-by-row.
│   (Skipped)       │  Near-optimal for uncongested cells.
│                   │  Mixed-cell-height via row assignment.
│  Handles:         │  Power-rail alignment enforced.
│  - 1x/2x/3x/4x    │  Fence violations → skipped (→ negotiation).
│  - Fence regions  │
└────────┬──────────┘
         │ illegal cells (overlap / fence violated)
         ▼
┌───────────────────┐
│ Negotiation Pass  │  Iterative rip-up & replace (from NBLG paper).
│                   │  Illegal cells + spatial neighbors compete for
│  NBLG components: │  grid resources.  History cost penalises
│  - Adaptive pf    │  persistent congestion.  Isolation point skips
│  - Isolation pt   │  already-legal cells in phase 2.
│  - History cost   │
└────────┬──────────┘
         │
         ▼
┌───────────────────┐
│ Post-optimisation │  Greedy displacement improvement (5 passes).
│     (Skipped)     │  Cell swap via bipartite matching within groups.
│                   │  
└────────┬──────────┘
         │
         ▼
   Legal placement written back to OpenDB

Commands#

Note

Parameters in square brackets [-param param] are optional.
Parameters without square brackets -param2 param2 are required.

Detailed Placement#

The detailed_placement command performs detailed placement of instances to legal locations after global placement.

detailed_placement
    [-max_displacement disp|{disp_x disp_y}]
    [-disallow_one_site_gaps]
    [-report_file_name filename]
    [-use_negotiation]
    [-abacus]

Options#

Switch Name	Description
`-max_displacement`	Max distance that an instance can be moved (in microns) when finding a site where it can be placed. Either set one value for both directions or set `{disp_x disp_y}` for individual directions. The default values are `{0, 0}`, and the allowed values within are integers `[0, MAX_INT]`.
`-disallow_one_site_gaps`	Option is deprecated.
`-report_file_name`	File name for saving the report to (e.g. `report.json`.)
`-incremental`	By default DPL initiates with all instances unplaced. With this flag DPL will check for already legalized instances and set them as placed.
`-report_file_name`	File name for saving the report to (e.g. `report.json`.)
`-use_negotiation`	Use the NegotiationLegalizer instead of the default diamond search engine.
`-abacus`	Enable the Abacus pre-pass within the NegotiationLegalizer. Only effective when `-use_negotiation` is set.

Set Placement Padding#

The set_placement_padding command sets left and right padding in multiples of the row site width. Use the set_placement_padding command before legalizing placement to leave room for routing. Use the -global flag for padding that applies to all instances. Use -instances for instance-specific padding. The instances insts can be a list of instance names, or an instance object returned by the SDC get_cells command. To specify padding for all instances of a common master, use the -filter “ref_name == ” option to get_cells.

set_placement_padding
    -global|-masters masters|-instances insts
    [-right site_count]
    [-left site_count]

Options#

Warning

Either one of these flags must be set: -global | -masters | -instances. The order of preference is global > masters > instances

Switch Name	Description
`-global`	Set padding globally using `left` and `right` values.
`-masters`	Set padding only for these masters using `left` and `right` values.
`-instances`	For `-instances`, you will set padding only for these insts using `left` and `right` values.
`-left`	Left padding (in site count).
`-right`	Right padding (in site count).
`instances`	Set padding for these list of instances. Not to be confused with the `-instances` switch above.

Filler Placement#

The filler_placement command fills gaps between detail-placed instances to connect the power and ground rails in the rows. filler_masters is a list of master/macro names to use for filling the gaps. Wildcard matching is supported, so FILL* will match, e.g., FILLCELL_X1 FILLCELL_X16 FILLCELL_X2 FILLCELL_X32 FILLCELL_X4 FILLCELL_X8. To specify a different naming prefix from FILLER_ use -prefix <new prefix>.

filler_placement
    [-prefix prefix]
    [-verbose]
    filler_masters

Options#

Switch Name	Description
`-prefix`	Prefix to name the filler cells. The default value is `FILLER_`.
`-verbose`	Print the filler cell usage.
`filler_masters`	Filler master cells.

Remove Fillers#

This command removes all filler cells.

remove_fillers

Check Placement#

The check_placement command checks the placement legality. It returns 0 if the placement is legal.

check_placement
    [-verbose]
    [-disallow_one_site_gaps]
    [-report_file_name filename]

Options#

Switch Name	Description
`-verbose`	Enable verbose logging.
`-disallow_one_site_gaps`	Option is deprecated.
`-report_file_name`	File name for saving the report to (e.g. `report.json`.)

Optimize Mirroring#

The optimize_mirroring command mirrors instances about the Y axis in a weak attempt to reduce the total half-perimeter wirelength (HPWL).

optimize_mirroring

Improve Placement#

The improve_placement command optimizes a given placed design.

improve_placement
    [-random_seed seed]
    [-max_displacement disp|{disp_x disp_y}]
    [-disallow_one_site_gaps]

Useful Developer Commands#

If you are a developer, you might find these useful. More details can be found in the source file or the swig file.

Command Name	Description
`detailed_placement_debug`	Debug detailed placement.
`get_masters_arg`	Get masters from a design.
`get_inst_bbox`	Get bounding box of an instance.
`get_inst_grid_bbox`	Get grid bounding box of an instance.
`format_grid`	Format grid (takes in length `x` and site width `w` as inputs).
`get_row_site`	Get row site name.

Example scripts#

Examples scripts demonstrating how to run dpl on a sample design of aes as follows:

./test/aes.tcl

Regression tests#

There are a set of regression tests in ./test. Refer to this section for more information.

Simply run the following script:

./test/regression

Limitations#

The following limitations apply when using the NegotiationLegalizer (-use_negotiation):

Abacus cluster chain: The current Abacus implementation uses a simplified cluster structure. A production version should maintain an explicit doubly-linked list of cells within each cluster, as in the original Spindler et al. paper.
Multithreading: The negotiation pass is single-threaded. Extend with the inter-region parallelism from NBLG (Algorithm 2, dynamic region adjustment) using OpenMP or std::thread.
Fence region R-tree: Replace linear scan in FenceRegion::nearestRect() with a spatial index (Boost.Geometry rtree or OpenROAD’s existing RTree) for large designs with many fence sub-rectangles.
Row rail inference: Currently uses row-index parity as a proxy for VDD/VSS. Replace with actual LEF pg_pin parsing once available in the build context.

FAQs#

Check out GitHub discussion about this tool.

Authors#

SangGi Do and Mingyu Woo (respective Ph. D. advisors: Seokhyeong Kang, Andrew B. Kahng).
Rewrite and port to OpenDB/OpenROAD by James Cherry, Parallax Software

References#

Do, S., Woo, M., & Kang, S. (2019, May). Fence-region-aware mixed-height standard cell legalization. In Proceedings of the 2019 on Great Lakes Symposium on VLSI (pp. 259-262). (.pdf)
P. Spindler et al., “Abacus: Fast legalization of standard cell circuits with minimal movement,” ISPD 2008.
J. Chen et al., “NBLG: A Robust Legalizer for Mixed-Cell-Height Modern Design,” IEEE TCAD, vol. 41, no. 11, 2022.
L. McMurchie and C. Ebeling, “PathFinder: A negotiation-based performance-driven router for FPGAs,” 1995.

License#

BSD 3-Clause License. See LICENSE file.

Detailed Placement

Contents

Detailed Placement#

Placement Engines#

Diamond Search#

NegotiationLegalizer#

Commands#

Detailed Placement#

Options#

Set Placement Padding#

Options#

Filler Placement#

Options#

Remove Fillers#

Check Placement#

Options#

Optimize Mirroring#

Improve Placement#

Useful Developer Commands#

Example scripts#

Regression tests#

Limitations#

FAQs#

Authors#

References#

License#