Index: ModulesPerf/README.md
===================================================================
--- /dev/null
+++ ModulesPerf/README.md
@@ -0,0 +1,20 @@
+A test suite to collect stats on modules' performance.
+
+# How to run the test suite
+
+For example,
+
+    mkdir sandbox
+    ./run-tests.py --workdir ./sandbox --binary path/to/clang
+
+
+# How to view stats
+
+It is not easy to understand the collected data. To view it in a more
+manageable form use
+
+    ./view-stats.py path/to/tests/directory
+
+To compare stats for different compiler builds
+
+    ./view-stats.py path/to/baseline path/to/test/after/making/some/change
Index: ModulesPerf/run-tests.py
===================================================================
--- /dev/null
+++ ModulesPerf/run-tests.py
@@ -0,0 +1,228 @@
+#!/usr/bin/env python
+
+import argparse
+import datetime
+import os
+import shutil
+import subprocess
+
+
+class BaseInputGenerator(object):
+    def __init__(self, inputs_dir, modules_count):
+        assert modules_count >= 0, "Modules count cannot be negative"
+        self._inputs_dir = inputs_dir
+        self._modules_count = modules_count
+        self.frameworks_path = os.path.join(self._inputs_dir, "Frameworks")
+        self.impl_file_path = os.path.join(self._inputs_dir, "test.m")
+
+    def _module_name(self, module_index):
+        return f"Mod{module_index}"
+
+    def _module_import(self, module_name):
+        return f"#import <{module_name}/{module_name}.h>\n"
+
+    def _module_header_path(self, module_name):
+        return os.path.join(self.frameworks_path, f"{module_name}.framework", "Headers", f"{module_name}.h")
+
+    def _write_file(self, file_path, content, is_appending=False):
+        with open(file_path, "at" if is_appending else "wt") as f:
+            f.write(content)
+
+    def _generate_framework(self, framework_name, content):
+        framework_dir = os.path.join(self.frameworks_path, f"{framework_name}.framework")
+        os.mkdir(framework_dir)
+        os.mkdir(os.path.join(framework_dir, "Headers"))
+        self._write_file(os.path.join(framework_dir, "Headers", f"{framework_name}.h"), content)
+        os.mkdir(os.path.join(framework_dir, "Modules"))
+        self._write_file(os.path.join(framework_dir, "Modules", "module.modulemap"), """
+        framework module {name} {{
+          umbrella header "{name}.h"
+          export *
+          module * {{ export * }}
+        }}
+        """.format(name=framework_name))
+
+
+class LinearDependencyInputGenerator(BaseInputGenerator):
+    """Represents a linear dependency topology.
+
+        Mod1 <- Mod2 <- ... <- ModN <- test.m.
+    """
+
+    def generate_files(self):
+        last_module_name = None
+        if self._modules_count > 0:
+            # Generate frameworks.
+            os.mkdir(self.frameworks_path)
+            for i in range(self._modules_count):
+                content = self._module_import(last_module_name) if last_module_name is not None else "// First module\n"
+                module_name = self._module_name(i)
+                self._generate_framework(module_name, content)
+                last_module_name = module_name
+        # Generate implementation file.
+        self._write_file(self.impl_file_path,
+                self._module_import(last_module_name)
+                if last_module_name is not None
+                else "// no dependencies")
+
+    def update_root_module(self):
+        assert self._modules_count > 0
+        module_name = self._module_name(self._modules_count-1)
+        self._write_file(self._module_header_path(module_name), "\n// updated\n", is_appending=True)
+
+    def update_leaf_module(self):
+        assert self._modules_count > 0
+        module_name = self._module_name(0)
+        self._write_file(self._module_header_path(module_name), "\n// updated\n", is_appending=True)
+
+
+class StarDependencyInputGenerator(BaseInputGenerator):
+    """Represents a star dependency topology.
+
+        Mod0 <- Mod1, Mod0 <- Mod2, .., Mod0 <- ModN.
+
+    It might be a horrible decision but in constructor `modules_count` means
+    the amount of edge modules. So even if `modules_count == 0`, we still have
+    one central module.
+    """
+
+    def generate_files(self):
+        # Generate frameworks.
+        os.mkdir(self.frameworks_path)
+        central_module_name = self._module_name(0)
+        central_module_import = self._module_import(central_module_name)
+        self._generate_framework(central_module_name, "// Central module\n")
+        for i in range(1, self._modules_count+1):
+            self._generate_framework(self._module_name(i), central_module_import)
+        # Implementation file is generated by subclasses.
+
+    def update_central_module(self):
+        assert self._modules_count > 0
+        module_name = self._module_name(0)
+        self._write_file(self._module_header_path(module_name), "\n// updated\n", is_appending=True)
+
+    def update_edge_module(self):
+        assert self._modules_count > 0
+        module_name = self._module_name(self._modules_count)
+        self._write_file(self._module_header_path(module_name), "\n// updated\n", is_appending=True)
+
+class StarDependencyOnCenterInputGenerator(StarDependencyInputGenerator):
+    def generate_files(self):
+        super(StarDependencyOnCenterInputGenerator, self).generate_files()
+        self._write_file(self.impl_file_path, self._module_import(self._module_name(0)))
+
+class StarDependencyOnEdgeInputGenerator(StarDependencyInputGenerator):
+    def __init__(self, inputs_dir, modules_count):
+        assert modules_count >= 1, "Need at least 1 edge module to depend on it"
+        super(StarDependencyOnEdgeInputGenerator, self).__init__(inputs_dir, modules_count)
+
+    def generate_files(self):
+        super(StarDependencyOnEdgeInputGenerator, self).generate_files()
+        self._write_file(self.impl_file_path, self._module_import(self._module_name(self._modules_count)))
+
+
+class TestExecutor(object):
+    def __init__(self, tests_root_dir, clang_path):
+        self._tests_root_dir = tests_root_dir
+        self._clang_path = clang_path
+
+    def _compiler_command_fixed_input_size(self, test_dir, inputs_dir, input_generator):
+        return [self._clang_path, "-fsyntax-only",
+            "-fmodules", "-fmodules-cache-path=" + os.path.join(test_dir, "ModuleCache.noindex"),
+            "-fno-integrated-cc1",
+            "-save-stats=cwd",
+            "-F", input_generator.frameworks_path,
+            input_generator.impl_file_path]
+
+    def _compiler_command_variable_input_size(self, test_dir, inputs_dir, input_generator):
+        return [self._clang_path, "-fsyntax-only",
+            "-fmodules", "-fmodules-cache-path=" + os.path.join(inputs_dir, "ModuleCache.noindex"),
+            "-fno-integrated-cc1",
+            "-save-stats=cwd",
+            "-F", input_generator.frameworks_path,
+            input_generator.impl_file_path]
+
+    def _test_fixed_input_size(self, test_dir, input_generator_class, extra_stats_on_input_change):
+        # Generate test data.
+        inputs_dir = os.path.join(test_dir, "Input")
+        os.mkdir(inputs_dir)
+        input_generator = input_generator_class(inputs_dir, modules_count=5)
+        input_generator.generate_files()
+
+        # Run clang and collect stats.
+        command = self._compiler_command_fixed_input_size(test_dir, inputs_dir, input_generator)
+        subprocess.check_call(command)
+        shutil.copy(os.path.join(test_dir, "test.stats"), os.path.join(test_dir, "clean_build.stats"))
+
+        subprocess.check_call(command)
+        shutil.copy(os.path.join(test_dir, "test.stats"), os.path.join(test_dir, "noop_build.stats"))
+
+        for (generator_method_name, stats_file_name) in extra_stats_on_input_change:
+            getattr(input_generator, generator_method_name)()
+            subprocess.check_call(command)
+            shutil.copy(os.path.join(test_dir, "test.stats"), os.path.join(test_dir, f"{stats_file_name}.stats"))
+
+    def _test_variable_input_size(self, test_dir, input_generator_class, extra_stats_on_input_change):
+        for modules_count in range(1, 9):
+            inputs_dir = os.path.join(test_dir, f"InputSize{modules_count}")
+            os.mkdir(inputs_dir)
+            input_generator = input_generator_class(inputs_dir, modules_count)
+            input_generator.generate_files()
+
+            # Run clang and collect stats.
+            command = self._compiler_command_variable_input_size(test_dir, inputs_dir, input_generator)
+            subprocess.check_call(command)
+            shutil.copy(os.path.join(test_dir, "test.stats"), os.path.join(test_dir, f"clean_build_size{modules_count}.stats"))
+
+            subprocess.check_call(command)
+            shutil.copy(os.path.join(test_dir, "test.stats"), os.path.join(test_dir, f"noop_build_size{modules_count}.stats"))
+
+            for (generator_method_name, stats_file_name) in extra_stats_on_input_change:
+                getattr(input_generator, generator_method_name)()
+                subprocess.check_call(command)
+                shutil.copy(os.path.join(test_dir, "test.stats"), os.path.join(test_dir, f"{stats_file_name}_size{modules_count}.stats"))
+
+    def test_module_topology(self, test_name, input_generator_class, extra_stats_on_input_change=tuple()):
+        test_dir = os.path.join(self._tests_root_dir, test_name)
+        os.mkdir(test_dir)
+        os.chdir(test_dir)
+        self._test_fixed_input_size(test_dir, input_generator_class, extra_stats_on_input_change)
+        variable_test_dir = os.path.join(test_dir, "variable")
+        os.mkdir(variable_test_dir)
+        os.chdir(variable_test_dir)
+        self._test_variable_input_size(variable_test_dir, input_generator_class, extra_stats_on_input_change)
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--binary", required=True,
+                        help="path to Clang binary")
+    parser.add_argument("--workdir", required=True,
+                        help="location where to store test results and temporary files")
+    args = parser.parse_args()
+
+    test_run_dir = os.path.join(args.workdir, datetime.datetime.now().strftime("test-%Y-%m-%d_%H-%M-%S"))
+    test_run_dir = os.path.abspath(test_run_dir)
+    os.makedirs(test_run_dir)
+    print(f"Test results can be found in {test_run_dir}")
+    clang_path = os.path.abspath(args.binary)
+
+    test_executor = TestExecutor(test_run_dir, clang_path)
+
+    # Collect perf data for a linear dependency chain of modules.
+    # For incremental rebuilds test updating a root of the chain and a leaf.
+    test_executor.test_module_topology("linear_dependency", LinearDependencyInputGenerator, (
+        ("update_root_module", "incremental_update_root"),
+        ("update_leaf_module", "incremental_update_leaf"),
+    ))
+
+    # Collect perf data for a star dependency topology.
+    test_executor.test_module_topology("star_dependency_on_center", StarDependencyOnCenterInputGenerator, (
+        ("update_central_module", "incremental_update_center"),
+        ("update_edge_module", "incremental_update_edge"),
+    ))
+
+    test_executor.test_module_topology("star_dependency_on_edge", StarDependencyOnEdgeInputGenerator, (
+        ("update_central_module", "incremental_update_center"),
+        ("update_edge_module", "incremental_update_edge"),
+    ))
Index: ModulesPerf/view-stats.py
===================================================================
--- /dev/null
+++ ModulesPerf/view-stats.py
@@ -0,0 +1,483 @@
+#!/usr/bin/env python
+
+import argparse
+import collections
+import glob
+import http.server
+import json
+import os
+
+
+SCRIPT = """
+function collectMetricData(metrics, buildKindNames) {
+  // Collect metric names.
+  var metricNames = new Set()
+  Object.values(metrics.fixed_input_size).forEach(metricsDict => {
+    Object.keys(metricsDict).forEach(metricName => {
+      metricNames.add(metricName)
+    })
+  })
+  metricNames = Array.from(metricNames)
+  metricNames.sort()
+  
+  // Collect build kinds. At first take values from provided `buildKindNames`, then sort the remainders.
+  var allBuildKinds = new Set(Object.keys(metrics.fixed_input_size))
+  var buildKinds = []
+  buildKindNames.forEach(buildKindName => {
+    if (allBuildKinds.has(buildKindName)) {
+      buildKinds.push(buildKindName)
+      allBuildKinds.delete(buildKindName)
+    }
+  })
+  if (allBuildKinds.size > 0) {
+    var remaining = Array.from(allBuildKinds)
+    remaining.sort()
+    buildKinds = buildKinds.concat(remaining)
+  }
+  
+  // Collect data to be displayed in a table. It is a dictionary of dictionaries.
+  // First we index by a metric name, then by build kind.
+  var fixedData = new Object()
+  metricNames.forEach(name => { fixedData[name] = new Object() })
+  Object.entries(metrics.fixed_input_size).forEach(entry => {
+    let buildKind = entry[0]
+    let metricDict = entry[1]
+    Object.entries(metricDict).forEach(metricEntry => {
+      fixedData[metricEntry[0]][buildKind] = metricEntry[1]
+    })
+  })
+  
+  var variableData = new Object()
+  metricNames.forEach(name => {
+    variableData[name] = new Object()
+    buildKinds.forEach(buildKind => { variableData[name][buildKind] = new Array() })
+  })
+  Object.entries(metrics.variable_input_size).forEach(entry => {
+    let buildKind = entry[0]
+    let variableDict = entry[1]
+    Object.entries(variableDict).forEach(variableEntry => {
+      let xValue = variableEntry[0]
+      let metricDict = variableEntry[1]
+      Object.entries(metricDict).forEach(metricEntry => {
+        variableData[metricEntry[0]][buildKind].push({x: xValue, y: metricEntry[1]})
+      })
+    })
+  })
+  metricNames.forEach(metricName => {
+    buildKinds.forEach(buildKind => {
+      variableData[metricName][buildKind].sort((a, b) => d3.ascending(a.x, b.x))
+    })
+  })
+  
+  return {
+    metricNames: metricNames,
+    buildKinds: buildKinds,
+    fixedSizeData: [fixedData],
+    variableSizeData: [variableData],
+  }
+}
+
+function mergeSorted(leftArray, rightArray) {
+  var result = new Array()
+  var left = 0, leftLength = leftArray.length,
+      right = 0, rightLength = rightArray.length
+  while ((left < leftLength) && (right < rightLength)) {
+    if (leftArray[left] < rightArray[right]) {
+      result.push(leftArray[left])
+      ++left
+    } else if (leftArray[left] > rightArray[right]) {
+      result.push(rightArray[right])
+      ++right
+    } else {
+      result.push(leftArray[left])
+      ++left
+      ++right
+    }
+  }
+  if (left < leftLength) {
+    result = result.concat(leftArray.slice(left))
+  }
+  if (right < rightLength) {
+    result = result.concat(rightArray.slice(right))
+  }
+  return result
+}
+
+function mergeMetricData(leftMetricData, rightMetricData) {
+  return {
+    metricNames: mergeSorted(leftMetricData.metricNames, rightMetricData.metricNames),
+    buildKinds: mergeSorted(leftMetricData.buildKinds, rightMetricData.buildKinds),
+    fixedSizeData: leftMetricData.fixedSizeData.concat(rightMetricData.fixedSizeData),
+    variableSizeData: leftMetricData.variableSizeData.concat(rightMetricData.variableSizeData),
+  }
+}
+
+function createFixedSizeMetricsTable(data) {
+  let table = document.createElement("table")
+  table.classList.add("stats-numbers")
+  {
+    let head = document.createElement("thead")
+    let row = document.createElement("tr")
+    row.appendChild(document.createElement("th"))
+    data.buildKinds.forEach(name => {
+      let cell = document.createElement("th")
+      cell.appendChild(document.createTextNode(name))
+      row.appendChild(cell)
+    })
+    head.appendChild(row)
+    table.appendChild(head)
+  }
+  {
+    let body = document.createElement("tbody")
+    data.metricNames.forEach(metricName => {
+      let row = document.createElement("tr")
+      let cell = document.createElement("th")
+      cell.appendChild(document.createTextNode(metricName))
+      row.appendChild(cell)
+      data.buildKinds.forEach(buildKind => {
+        let cell = document.createElement("td")
+        let value = data.fixedSizeData[0][metricName][buildKind]
+        cell.appendChild(document.createTextNode(value == null ? "–" : value))
+        row.appendChild(cell)
+      })
+      body.appendChild(row)
+    })
+    table.appendChild(body)
+  }
+  return table
+}
+
+function createFixedSizeComparisonMetrics(data) {
+  let result = document.createElement("div")
+  data.buildKinds.forEach(buildKind => {
+    let table = document.createElement("table")
+    table.classList.add("stats-numbers")
+    let caption = document.createElement("caption")
+    caption.appendChild(document.createTextNode(buildKind))
+    table.appendChild(caption)
+    let body = document.createElement("tbody")
+    data.metricNames.forEach(metricName => {
+      let row = document.createElement("tr")
+      let cell = document.createElement("th")
+      cell.appendChild(document.createTextNode(metricName))
+      row.appendChild(cell)
+      data.fixedSizeData.forEach((dataDict, i) => {
+        let cell = document.createElement("td")
+        let value = dataDict[metricName][buildKind]
+        cell.appendChild(document.createTextNode(value == null ? "–" : value))
+        row.appendChild(cell)
+        if (i !== 0) {
+          let diffCell = document.createElement("td")
+          let baselineValue = data.fixedSizeData[0][metricName][buildKind]
+          // For difference purposes treat missing values as 0.
+          let difference = (value || 0) - (baselineValue || 0)
+          if (difference != 0) {
+            diffCell.appendChild(document.createTextNode(difference))
+          }
+          row.appendChild(diffCell)
+        }
+      })
+      body.appendChild(row)
+    })
+    table.appendChild(body)
+    result.appendChild(table)
+  })
+  return result
+}
+
+function createMetricPlot(dataSets, xExtent, yExtent) {
+  // Collect plot settings together to simplify appearance tweaking.
+  const plotSettings = {
+    width: 250,
+    height: 150,
+    margin: {top: 10, right: 10, bottom: 30, left: 40},
+  }
+
+  const x = d3.scaleLinear()
+    .domain(xExtent).nice()
+    .range([plotSettings.margin.left, plotSettings.width - plotSettings.margin.right])
+  
+  const y = d3.scaleLinear()
+    .domain(yExtent).nice()
+    .range([plotSettings.height - plotSettings.margin.bottom, plotSettings.margin.top])
+  
+  const svg = d3.create("svg")
+    .attr("width", plotSettings.width)
+    .attr("height", plotSettings.height)
+  
+  svg.append("g")
+    .attr("transform", `translate(0,${plotSettings.height - plotSettings.margin.bottom})`)
+    .call(d3.axisBottom(x).ticks(3))
+  
+  svg.append("g")
+    .attr("transform", `translate(${plotSettings.margin.left},0)`)
+    .call(d3.axisLeft(y).ticks(3))
+  
+  const line = d3.line()
+    .x(d => x(d.x))
+    .y(d => y(d.y))
+
+  let colors = d3.scaleOrdinal(d3.schemeCategory10)
+  dataSets.forEach((dataSet, i) => {
+    if (dataSet.length == 0) {
+      return
+    }
+    svg.append("path")
+      .datum(dataSet)
+      .attr("fill", "none")
+      .attr("stroke", colors(i))
+      .attr("stroke-width", 1)
+      .attr("stroke-linejoin", "round")
+      .attr("stroke-linecap", "round")
+      .attr("d", line)
+  })
+  return svg.node()
+}
+
+function createVariableSizeMetricPlots(data) {
+  let table = document.createElement("table")
+  table.classList.add("stats-plots")
+  // Table head DOM manipulation.
+  {
+    let head = document.createElement("thead")
+    let row = document.createElement("tr")
+    row.appendChild(document.createElement("th"))
+    data.buildKinds.forEach(name => {
+      let cell = document.createElement("th")
+      cell.appendChild(document.createTextNode(name))
+      row.appendChild(cell)
+    })
+    head.appendChild(row)
+    table.appendChild(head)
+  }
+  // Share x-domain across all plots.
+  let xMin = d3.min(data.variableSizeData, completeDict => {
+    return d3.min(Object.values(completeDict), dataSeriesDict => {
+      return d3.min(Object.values(dataSeriesDict), data => d3.min(data, d => d.x))
+    })
+  })
+  let xMax = d3.max(data.variableSizeData, completeDict => {
+    return d3.max(Object.values(completeDict), dataSeriesDict => {
+      return d3.max(Object.values(dataSeriesDict), data => d3.max(data, d => d.x))
+    })
+  })
+  // Table body DOM manipulation.
+  {
+    let body = document.createElement("tbody")
+    data.metricNames.forEach(metricName => {
+      // Share y-domain across plots in the same row.
+      let yMin = d3.min(data.variableSizeData, completeDict => {
+        return d3.min(Object.values(completeDict[metricName]), data => {
+          return d3.min(data, d => d.y)
+        })
+      })
+      let yMax = d3.max(data.variableSizeData, completeDict => {
+        return d3.max(Object.values(completeDict[metricName]), data => {
+          return d3.max(data, d => d.y)
+        })
+      })
+
+      let row = document.createElement("tr")
+      let cell = document.createElement("th")
+      cell.appendChild(document.createTextNode(metricName))
+      row.appendChild(cell)
+      data.buildKinds.forEach(buildKind => {
+        let cell = document.createElement("td")
+        let dataSeries = data.variableSizeData.map(completeDict => completeDict[metricName][buildKind])
+        if (dataSeries.some(data => data.length > 0)) {
+          cell.appendChild(createMetricPlot(dataSeries, [d3.min([xMin, 0]), xMax], [d3.min([yMin, 0]), yMax]))
+        }
+        row.appendChild(cell)
+      })
+      body.appendChild(row)
+    })
+    table.appendChild(body)
+  }
+  return table
+}
+
+function displayStats(statsDataRuns, testName, placeholderNodeId) {
+  const STABLE_BUILD_KINDS = ["clean_build", "noop_build"]
+  var metricData = collectMetricData(statsDataRuns[0][testName], STABLE_BUILD_KINDS)
+  for (let i = 1; i < statsDataRuns.length; i++) {
+    metricData = mergeMetricData(metricData,
+                                 collectMetricData(statsDataRuns[i][testName], STABLE_BUILD_KINDS))
+  }
+  let fixedSizeMetrics = metricData.fixedSizeData.length > 1 ?
+      createFixedSizeComparisonMetrics(metricData) :
+      createFixedSizeMetricsTable(metricData)
+  let variableSizePlots = createVariableSizeMetricPlots(metricData)
+
+  let placeholderNode = document.getElementById(placeholderNodeId)
+  placeholderNode.appendChild(fixedSizeMetrics)
+  placeholderNode.appendChild(variableSizePlots)
+}
+
+document.addEventListener("DOMContentLoaded", event => {
+  displayStats(stats, "linear_dependency", "linear-dependency-placeholder")
+  displayStats(stats, "star_dependency_on_center", "star-dependency-on-center-placeholder")
+  displayStats(stats, "star_dependency_on_edge", "star-dependency-on-edge-placeholder")
+})
+"""
+
+CSS = """
+body {
+  font-family: Helvetica, Arial, sans-serif;
+  margin: 0;
+  padding: 0;
+}
+
+.content {
+  margin: 0 20px;
+}
+
+table {
+  border-spacing: 0;
+}
+
+.stats-numbers tbody {
+  line-height: 1.3;
+}
+.stats-numbers td {
+  text-align: right; /* numbers should be aligned to the right */
+  padding-right: 10px;
+}
+.stats-numbers thead th {
+  padding-right: 5px;
+}
+.stats-numbers tbody th {
+  text-align: left;
+  font-weight: normal;
+}
+/*
+.stats-numbers tbody tr:hover {
+  background-color: #dee5ff;
+}
+*/
+
+.stats-numbers + .stats-plots, .stats-numbers + .stats-numbers {
+  margin-top: 10px;
+}
+.stats-plots tbody th {
+  text-align: left;
+  font-weight: normal;
+}
+"""
+
+PAGE = """
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Module stats</title>
+  <script type="text/javascript">
+    let stats = {stats_json}
+  </script>
+  <script type="text/javascript" src="https://d3js.org/d3.v6.min.js"></script>
+  <script type="text/javascript">
+    {script}
+  </script>
+  <style>
+    {css}
+  </style>
+</head>
+<body>
+ <div class="content">
+  <div>
+    <h2>Linear dependency chain</h2>
+    <p>The dependency graph between modules looks like</p>
+    <pre>Mod0 <- Mod1 <- Mod2 <- ... <- ModN <- test.m</pre>
+    <p>We measure the cost of an incremental build when updating a leaf module (Mod0) and a root module (ModN).</p>
+    <div id="linear-dependency-placeholder"></div>
+  </div>
+
+  <div>
+    <h2>Star dependency chain. Implementation depends on center</h2>
+    <p>The dependency graph between modules looks like</p>
+    <pre>Mod0 <- Mod1
+Mod0 <- Mod2
+...
+Mod0 <- ModN
+Mod0 <- test.m</pre>
+    <p>We measure the cost of an incremental build when updating a central module (Mod0) and an edge module (ModN).</p>
+    <div id="star-dependency-on-center-placeholder"></div>
+  </div>
+
+  <div>
+    <h2>Star dependency chain. Implementation depends on edge</h2>
+    <p>The dependency graph between modules looks like</p>
+    <pre>Mod0 <- Mod1
+Mod0 <- Mod2
+...
+Mod0 <- ModN
+        ModN <- test.m</pre>
+    <p>We measure the cost of an incremental build when updating a central module (Mod0) and an edge module (ModN).</p>
+    <div id="star-dependency-on-edge-placeholder"></div>
+  </div>
+ </div>
+</body>
+</html>
+"""
+
+class RequestHandler(http.server.BaseHTTPRequestHandler):
+    def do_GET(self):
+        self.send_response(200)
+        self.send_header("Content-type", "text/html")
+        self.end_headers()
+        self.wfile.write(PAGE.format(
+            script=SCRIPT, css=CSS,
+            stats_json=json.dumps(self.server.stats),
+        ).encode())
+
+
+def read_stats_files(files_root_dir):
+    fixed_stats = {}
+    fixed_test_files = glob.glob(os.path.join(files_root_dir, "*.stats"))
+    assert len(fixed_test_files) > 0, "Expect some test files"
+    for file_path in fixed_test_files:
+        basename = os.path.basename(file_path)
+        if basename == "test.stats":
+            continue
+        mode_name, _ = os.path.splitext(basename)
+        with open(file_path, "rt") as f:
+            fixed_stats[mode_name] = json.load(f)
+
+    variable_stats = collections.defaultdict(dict)
+    SIZE_LABEL = "_size"
+    variable_test_files = glob.glob(os.path.join(files_root_dir, "variable", f"*{SIZE_LABEL}*.stats"))
+    assert len(variable_test_files) > 0, "Expect some test files"
+    for file_path in variable_test_files:
+        basename = os.path.basename(file_path)
+        name, _ = os.path.splitext(basename)
+        pos = name.rfind(SIZE_LABEL)
+        mode_name = name[:pos]
+        size = int(name[pos+len(SIZE_LABEL):])
+        with open(file_path, "rt") as f:
+            variable_stats[mode_name][size] = json.load(f)
+    return {"fixed_input_size": fixed_stats, "variable_input_size": variable_stats}
+
+def load_stats(test_run_locations):
+    assert len(test_run_locations) > 0, "Need at least one test run location"
+    result = []
+    for test_run_location in test_run_locations:
+        stats = {}
+        with os.scandir(test_run_location) as dir_iter:
+            for test_dir in dir_iter:
+                if not test_dir.is_dir():
+                    continue
+                stats[test_dir.name] = read_stats_files(test_dir.path)
+        result.append(stats)
+    return result
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument("stats_directories", metavar="stats-directory", nargs="+",
+                        help="path to the directory with different stats")
+    args = parser.parse_args()
+
+    server_address = ('', 8000)
+    httpd = http.server.HTTPServer(server_address, RequestHandler)
+    httpd.stats = load_stats(args.stats_directories)
+    print(f"Running server on 127.0.0.1:{server_address[1]}...")
+    httpd.serve_forever()