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Use yosys stat -json for metrics extraction #4019

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2a92fe9
Use yosys stat -json for metrics extraction
rohithsiddi Mar 20, 2026
547d496
genMetrics: skip non-JSON prefix in synth_stat.json
rohithsiddi Mar 20, 2026
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4 changes: 2 additions & 2 deletions docs/tutorials/FlowTutorial.md
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Original file line number Diff line number Diff line change
Expand Up @@ -311,7 +311,7 @@ abc.constr klayout.lyt klayout_tech.lef lib
| `congestion.rpt` | `VDD.rpt` | `VSS.rpt` |
| `5_route_drc.rpt` | `final_clocks.webp` | `final_placement.webp` |
| `antenna.log` | `final_clocks.webp` | `final.webp` |
| `synth_stat.txt` | `synth_check.txt` | `final_resizer.webp` |
| `synth_stat.json` | `synth_check.txt` | `final_resizer.webp` |

The table below briefly describes the reports directory files.

Expand All @@ -325,7 +325,7 @@ The table below briefly describes the reports directory files.
| `antenna.log` | Antenna check log report. |
| `final_placement.webp` | Extracted image after final placement. |
| `final.webp` | Extracted image after routing. |
| `synth_stat.txt` | Post synthesis design statistics log saved here. |
| `synth_stat.json` | Post synthesis design statistics in JSON format. |

The flow completes with the message below by creating a merged final GDS file.

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2 changes: 1 addition & 1 deletion flow/scripts/synth.tcl
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Expand Up @@ -226,7 +226,7 @@ if { $::env(SYNTH_INSBUF) } {
# Reports
tee -o $::env(REPORTS_DIR)/synth_check.txt check

tee -o $::env(REPORTS_DIR)/synth_stat.txt stat {*}$lib_args
tee -o $::env(REPORTS_DIR)/synth_stat.json stat -json {*}$lib_args
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@vvbandeira vvbandeira Mar 24, 2026

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We should tee to a log file and use the -o option to output a valid JSON:
https://yosyshq.readthedocs.io/projects/yosys/en/latest/cmd/index_backends.html#json-write-design-in-json-format

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@rohithsiddi rohithsiddi Mar 29, 2026

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The json backend (write_json -o) outputs the design netlist rather than synthesis statistics, it doesn't include aggregated cell count or area values that we need for metrics. stat -json provides these but doesn't support -o; its docs recommend using tee for file output. Our current approach uses tee -oto capture stat -json and skips any log prefix (e.g. "Found gzip magic") in genMetrics.py before parsing. Would this be acceptable?


# check the design is composed exclusively of target cells, and
# check for other problems
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56 changes: 43 additions & 13 deletions flow/util/genMetrics.py
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Expand Up @@ -231,20 +231,50 @@ def extract_metrics(
# Synthesis
# =========================================================================

# The new format (>= 0.57) is: <count> <area> cells
extractTagFromFile(
"synth__design__instance__count__stdcell",
metrics_dict,
"^\\s+(\\d+)\\s+[-0-9.]+\\s+cells$",
rptPath + "/synth_stat.txt",
)
synth_stat_file = rptPath + "/synth_stat.json"
try:
with open(synth_stat_file) as f:
content = f.read()
# yosys may emit log messages (e.g. "Found gzip magic in file...")
# before the JSON when reading compressed liberty files. Skip them.
Comment on lines +235 to +239
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@vvbandeira vvbandeira Mar 24, 2026

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If the above is implemented we likely will not need this post-processing.

synth_stat = json.loads(content[content.index("{") :])
modules = synth_stat.get("modules", {})
# Prefer lookup by design name; yosys may prefix module names with a
# backslash in JSON output, so try both forms. Fall back to the last
# module in the output, which yosys emits last for the top-level module.
top_module = (
modules.get(design)
or modules.get("\\" + design)
or (list(modules.values())[-1] if modules else None)
)

num_cells = top_module.get("num_cells") if top_module else None
if num_cells is None:
print(
"[WARN] Tag synth__design__instance__count__stdcell not found in {}.".format(
synth_stat_file
),
"Will use N/A.",
)
metrics_dict["synth__design__instance__count__stdcell"] = "N/A"
else:
metrics_dict["synth__design__instance__count__stdcell"] = int(num_cells)

extractTagFromFile(
"synth__design__instance__area__stdcell",
metrics_dict,
"Chip area for (?:top )?module.*: +(\\S+)",
rptPath + "/synth_stat.txt",
)
area = top_module.get("area") if top_module else None
if area is None:
print(
"[WARN] Tag synth__design__instance__area__stdcell not found in {}.".format(
synth_stat_file
),
"Will use N/A.",
)
metrics_dict["synth__design__instance__area__stdcell"] = "N/A"
else:
metrics_dict["synth__design__instance__area__stdcell"] = float(area)
except (IOError, json.JSONDecodeError, ValueError):
print("[ERROR] Failed to open file:", synth_stat_file)
metrics_dict["synth__design__instance__count__stdcell"] = "ERR"
metrics_dict["synth__design__instance__area__stdcell"] = "ERR"

# Clocks
# =========================================================================
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