From 238da79e524ba47e28aac7b30df692f55be3f2d2 Mon Sep 17 00:00:00 2001
From: Maciej Kurc <mkurc@antmicro.com>
Date: Mon, 22 Nov 2021 13:13:28 +0100
Subject: Fixed potential issues with carry-chain cluster expansion, added a
 parameter controlling the ratio of FFs that got glued to carry-chain
 clusters.

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>
---
 nexus/main.cc |  8 ++++++++
 nexus/pack.cc | 29 +++++++++++++++--------------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/nexus/main.cc b/nexus/main.cc
index 83bb02d8..667b6d80 100644
--- a/nexus/main.cc
+++ b/nexus/main.cc
@@ -52,6 +52,7 @@ po::options_description NexusCommandHandler::getArchOptions()
     specific.add_options()("pdc", po::value<std::string>(), "physical constraints file");
     specific.add_options()("no-post-place-opt", "disable post-place repacking (debugging use only)");
     specific.add_options()("no-pack-lutff", "disable packing (clustering) LUTs and FFs together");
+    specific.add_options()("carry-lutff-ratio", po::value<float>(), "ratio of FFs to be added to carry-chain LUT clusters");
 
     return specific;
 }
@@ -79,6 +80,13 @@ std::unique_ptr<Context> NexusCommandHandler::createContext(dict<std::string, Pr
         ctx->settings[ctx->id("no_post_place_opt")] = Property::State::S1;
     if (vm.count("no-pack-lutff"))
         ctx->settings[ctx->id("no_pack_lutff")] = Property::State::S1;
+    if (vm.count("carry-lutff-ratio")) {
+        float ratio = vm["carry-lutff-ratio"].as<float>();
+        if (ratio < 0.0f || ratio > 1.0f) {
+            log_error("Carry LUT+FF packing ration must be between 0.0 and 1.0");
+        }
+        ctx->settings[ctx->id("carry_lutff_ratio")] = ratio;
+    }
     return ctx;
 }
 
diff --git a/nexus/pack.cc b/nexus/pack.cc
index be298ef0..8200f845 100644
--- a/nexus/pack.cc
+++ b/nexus/pack.cc
@@ -2307,6 +2307,11 @@ struct NexusPacker
     void pack_lutffs () {
         log_info("Inferring LUT+FF pairs...\n");
 
+        float carry_ratio = 1.0f;
+        if (ctx->settings.find(ctx->id("carry_lutff_ratio")) != ctx->settings.end()) {
+            carry_ratio = ctx->setting<float>("carry_lutff_ratio");
+        }
+
         size_t num_comb = 0;
         size_t num_ff   = 0;
         size_t num_pair = 0;
@@ -2338,7 +2343,6 @@ struct NexusPacker
                 continue;
             }
             if (di->driver.port != id_F  &&
-                di->driver.port != id_F1 &&
                 di->driver.port != id_OFX)
             {
                 continue;
@@ -2377,27 +2381,24 @@ struct NexusPacker
             // Attach the FF to the existing cluster of the LUT
             else {
 
-                // Find the cluster root
-                CellInfo* root  = nullptr;
-                if (!lut->constr_children.empty()) {
-                    root  = lut;
-                }
-                else {
-                    for (auto &it : ctx->cells) {
-                        if (it.second->cluster == lut->cluster && !it.second->constr_children.empty()) {
-                            root  = it.second.get();
-                            break;
-                        }
+                // No order not to make too large carry clusters pack only the
+                // given fraction of FFs there.
+                if(str_or_default(lut->params, id_MODE, "LOGIC") == "CCU2") {
+                    float r = (float)(ctx->rng() % 1000) * 1e-3f;
+                    if (r > carry_ratio) {
+                        continue;
                     }
                 }
-                NPNR_ASSERT(root  != nullptr);
+
+                // Get the cluster root
+                CellInfo* root = ctx->cells.at(lut->cluster).get();
 
                 // Constrain the FF relative to the LUT
                 ff->cluster = root->cluster;
                 ff->constr_x = lut->constr_x;
                 ff->constr_y = lut->constr_y;
                 ff->constr_z = lut->constr_z + 2;
-                ff->constr_abs_z = false;
+                ff->constr_abs_z = lut->constr_abs_z;
                 root->constr_children.push_back(ff);
 
                 num_glue++;
-- 
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