Merge pull request #10642 from calewis/gpl-placement-perturbation

[GPL] Add randomized initial placement perturbation in GPL

Author: maliberty

src/gpl/BUILD

@@ -81,6 +81,7 @@ cc_library(
         "//src/utl",
         "@abseil-cpp//absl/container:inlined_vector",
         "@boost.polygon",
+        "@boost.random",
         "@coin-or-lemon//:lemon",
         "@eigen",
         "@openmp",

src/gpl/README.md

@@ -104,9 +104,11 @@ global_placement
     [-pad_right pad_right]\
     [-disable_revert_if_diverge]\
     [-disable_pin_density_adjust]\
-    [-enable_routing_congestion]
-    [-virtual_cts]
-    [-virtual_cts_max_skew_fraction virtual_cts_max_skew_fraction]
+    [-enable_routing_congestion]\
+    [-virtual_cts]\
+    [-virtual_cts_max_skew_fraction virtual_cts_max_skew_fraction]\
+    [-random_seed random_seed]\
+    [-perturb_dist perturb_dist]
 ```
 
 #### Options
@@ -135,6 +137,8 @@ global_placement
 | `-enable_routing_congestion` | Flag to run global routing after global placement, enabling the Routing Congestion Heatmap.|
 | `-virtual_cts` | Flag to build a lightweight virtual clock tree during global placement. Clock tree is used to compute clock network latency per clock sink to model clock skew during timing-driven placement. Virtual CTS runs before each timing-driven iteration. |
 | `-virtual_cts_max_skew_fraction` | Set max insertion delay as fraction of clock period. Valid range is [0, 1]; out-of-range values are clamped. Default is 0.10 (10%). |
+| `-random_seed` | Set random seed for placement perturbation. Perturbation shifts standard cells by random offsets drawn from a 2D circular Gaussian distribution at the start of global placement to explore layout variations. Default is `1`. |
+| `-perturb_dist` | Set the perturbation distance (radius) in nanometers. Under a 2D circular Gaussian distribution, 99.5% of the cell perturbations will fall within this radius. If not provided (or <= 0), it defaults to 0.5 microns or one standard cell height (row height), whichever is smaller. |
 
 #### Initial-Placement Arguments
 

src/gpl/doc/TclCommands.md

@@ -14,7 +14,8 @@ global_placement
     [-initial_place_max_iter max_iter]
     [-initial_place_max_fanout max_fanout]
     [-verbose_level verbose_level]
-
+    [-random_seed random_seed]
+    [-perturb_dist perturb_dist]
 ```
 
 ## Flow Control
@@ -31,6 +32,8 @@ global_placement
 
 ## Other Options
 * __verbose_level__ [0-10, int] : Set verbose level for RePlAce. Default: 1
+* __random_seed__ [int] : Set random seed for placement perturbation. Perturbation shifts standard cells by random offsets drawn from a 2D circular Gaussian distribution at the start of global placement. Default is 1.
+* __perturb_dist__ [float] : Set the perturbation distance (radius) in nanometers. Under a 2D circular Gaussian distribution, 99.5% of the cell perturbations will fall within this radius. If not provided (or <= 0), it defaults to 0.5 microns or one standard cell height (row height), whichever is smaller.
 
 Note that all of the TCL commands are defined in the
 [../src/replace.tcl](https://github.com/The-OpenROAD-Project/OpenROAD/blob/master/src/gpl/src/replace.tcl)

src/gpl/include/gpl/Replace.h

@@ -84,6 +84,8 @@ struct PlaceOptions
   int binGridCntX = 0;
   int binGridCntY = 0;
   float density = 0.7;
+  int initialPlacePerturbationSeed = 1;
+  float initialPlacePerturbationDist = -1.0f;
 
   float routabilityCheckOverflow = 0.3;
   float routabilitySnapshotOverflow = 0.6;

src/gpl/src/nesterovBase.cpp

@@ -15,14 +15,14 @@
 #include <iterator>
 #include <memory>
 #include <optional>
-#include <random>
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>
 #include <vector>
 
 #include "boost/polygon/polygon.hpp"
+#include "boost/random/normal_distribution.hpp"
 #include "fft.h"
 #include "gpl/Replace.h"
 #include "nesterovPlace.h"
@@ -1080,7 +1080,9 @@ NesterovBaseVars::NesterovBaseVars(const PlaceOptions& options)
       binCntX(isSetBinCnt ? options.binGridCntX : 0),
       binCntY(isSetBinCnt ? options.binGridCntY : 0),
       minPhiCoef(options.minPhiCoef),
-      maxPhiCoef(options.maxPhiCoef)
+      maxPhiCoef(options.maxPhiCoef),
+      initialPlacePerturbationSeed(options.initialPlacePerturbationSeed),
+      initialPlacePerturbationDist(options.initialPlacePerturbationDist)
 {
 }
 
@@ -1941,6 +1943,58 @@ void NesterovBaseCommon::reportInstanceExtensionByPinDensity() const
   }
 }
 
+// Calculate a random initial placement perturbation offset.
+// Standard cells are shifted by random offsets drawn from a 2D circular
+// Gaussian distribution.
+//
+// The displacement is generated by sampling independent normal distributions
+// in X and Y with standard deviation sigma:
+//   sigma = r_dbu * sqrt(-1 / (2 * ln(1 - P)))
+//
+// where r_dbu is the target perturbation radius in DBUs, and P is the target
+// probability (99.5%) of a cell falling within the circle of radius r_dbu.
+// This Rayleigh-distributed radius has:
+//   sigma \approx 0.3071963263271184 * r_dbu.
+//
+// The target perturbation radius defaults to min(site_y, 0.5 micron) if not
+// explicitly set by -perturb_dist.
+std::pair<int, int> NesterovBase::calculatePlacementPerturbationOffset(
+    int dbu_per_micron) const
+{
+  int site_y = pb_->getSiteSizeY();
+  if (site_y <= 0) {
+    return {0, 0};
+  }
+
+  // If perturbation distance is explicitly set to 0, skip perturbation.
+  if (nbVars_.initialPlacePerturbationDist == 0.0f) {
+    return {0, 0};
+  }
+
+  double r_dbu = 0.0;
+  if (nbVars_.initialPlacePerturbationDist > 0.0f) {
+    r_dbu = nbVars_.initialPlacePerturbationDist * (dbu_per_micron / 1000.0);
+  } else {
+    r_dbu = std::min(0.5 * dbu_per_micron, static_cast<double>(site_y));
+  }
+
+  // To ensure that 99.5% of the perturbations fall within a circle of radius
+  // r_dbu, we use a Rayleigh distribution for the radial distance R = sqrt(X^2
+  // + Y^2). CDF(r) = 1 - exp(-r^2 / (2 * sigma^2)) = 0.995 => sigma = r_dbu *
+  // sqrt(-1 / (2 * ln(0.005))) sigma \approx 0.3071963263271184 * r_dbu.
+  const double sigma = r_dbu * 0.3071963263271184;
+
+  boost::random::normal_distribution<double> dist(0.0, sigma);
+
+  double dx = dist(generator_);
+  double dy = dist(generator_);
+
+  int x_offset = std::round(dx);
+  int y_offset = std::round(dy);
+
+  return {x_offset, y_offset};
+}
+
 ////////////////////////////////////////////////
 // NesterovBase
 
@@ -1951,7 +2005,7 @@ NesterovBase::NesterovBase(
     // NOLINTNEXTLINE(performance-unnecessary-value-param)
     std::shared_ptr<NesterovBaseCommon> nbc,
     utl::Logger* log)
-    : nbVars_(nbVars)
+    : nbVars_(nbVars), generator_(nbVars.initialPlacePerturbationSeed)
 {
   pb_ = std::move(pb);
   nbc_ = std::move(nbc);
@@ -1961,8 +2015,6 @@ NesterovBase::NesterovBase(
              "---- Initialize Nesterov Region: {}",
              pb_->getGroup() ? pb_->getGroup()->getName() : "Top-level");
 
-  // Set a fixed seed
-  srand(42);
   // area update from pb
   stdInstsArea_ = pb_->stdInstsArea();
   macroInstsArea_ = pb_->macroInstsArea();
@@ -1976,8 +2028,8 @@ NesterovBase::NesterovBase(
 
   // add place instances
   for (auto& pb_inst : pb_->placeInsts()) {
-    int x_offset = rand() % (2 * dbu_per_micron) - dbu_per_micron;
-    int y_offset = rand() % (2 * dbu_per_micron) - dbu_per_micron;
+    auto [x_offset, y_offset]
+        = calculatePlacementPerturbationOffset(dbu_per_micron);
 
     GCell* gCell = nbc_->pbToNb(pb_inst);
     if (pb_inst != gCell->insts()[0]) {
@@ -2244,7 +2296,7 @@ void NesterovBase::initFillerGCells()
   // mt19937 supports huge range of random values.
   // rand()'s RAND_MAX is only 32767.
   //
-  std::mt19937 randVal(0);
+  boost::random::mt19937 randVal(0);
   for (int i = 0; i < fillerCnt; i++) {
     // instability problem between g++ and clang++!
     auto randX = randVal();

src/gpl/src/nesterovBase.h

@@ -19,6 +19,7 @@
 #include <variant>
 #include <vector>
 
+#include "boost/random/mersenne_twister.hpp"
 #include "boost/unordered/unordered_flat_map.hpp"
 #include "gpl/Replace.h"
 #include "odb/db.h"
@@ -755,6 +756,8 @@ struct NesterovBaseVars
 
   const float minPhiCoef;
   float maxPhiCoef;  // may be updated after initialization
+  const int initialPlacePerturbationSeed;
+  const float initialPlacePerturbationDist;
 
   static constexpr float minWireLengthForceBar = -300;
 };
@@ -1149,11 +1152,15 @@ class NesterovBase
 
   odb::dbGroup* getGroup() const { return pb_->getGroup(); }
 
+  std::pair<int, int> calculatePlacementPerturbationOffset(
+      int dbu_per_micron) const;
+
  private:
   NesterovBaseVars nbVars_;
   std::shared_ptr<PlacerBase> pb_;
   std::shared_ptr<NesterovBaseCommon> nbc_;
   utl::Logger* log_ = nullptr;
+  mutable boost::random::mt19937 generator_;
 
   BinGrid bg_;
   std::unique_ptr<FFT> fft_;

src/gpl/src/replace.i

@@ -82,6 +82,8 @@ static gpl::PlaceOptions getOptions(
   checkKey(keys,
            "-timing_driven_repair_tns_end_percent",
            options.timingDrivenRepairTnsEndPercent);
+  checkKey(keys, "-random_seed", options.initialPlacePerturbationSeed);
+  checkKey(keys, "-perturb_dist", options.initialPlacePerturbationDist);
 
   if (auto it = keys.find("-density"); it != keys.end()) {
     if (it->second == "uniform") {

src/gpl/src/replace.tcl

@@ -39,6 +39,8 @@ sta::define_cmd_args "global_placement" {\
     [-pad_right pad_right]\
     [-disable_revert_if_diverge]\
     [-disable_pin_density_adjust]\
+    [-random_seed random_seed]\
+    [-perturb_dist perturb_dist]\
     [-enable_routing_congestion]
 }
 
@@ -61,6 +63,8 @@ proc global_placement { args } {
       -timing_driven_repair_tns_end_percent \
       -keep_resize_below_overflow \
       -virtual_cts_max_skew_fraction \
+      -random_seed \
+      -perturb_dist \
       -pad_left -pad_right} \
     flags {-skip_initial_place \
       -force_center_initial_place \