From 27635785c8f3d730fd3e954521aed90b3c0665b7 Mon Sep 17 00:00:00 2001
From: myrtle <gatecat@ds0.me>
Date: Tue, 1 Jul 2025 15:32:28 +0200
Subject: [PATCH] heap: Allow customising legalisation ordering (#1507)

Signed-off-by: gatecat <gatecat@ds0.me>
---
 common/place/placer_heap.cc       | 16 +++++++++++-----
 common/place/placer_heap.h        |  5 +++++
 himbaechel/uarch/xilinx/xilinx.cc |  7 +++++++
 3 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/common/place/placer_heap.cc b/common/place/placer_heap.cc
index 940d1d71..be8a670d 100644
--- a/common/place/placer_heap.cc
+++ b/common/place/placer_heap.cc
@@ -862,7 +862,7 @@ class HeAPPlacer
         // the simple greedy largest-macro-first approach.
         std::priority_queue<std::pair<int, IdString>> remaining;
         for (auto cell : solve_cells) {
-            remaining.emplace(chain_size[cell->name], cell->name);
+            remaining.emplace(chain_size[cell->name] * cfg.get_cell_legalisation_weight(ctx, cell), cell->name);
         }
         int ripup_radius = 2;
         int total_iters = 0;
@@ -875,7 +875,8 @@ class HeAPPlacer
             // Was now placed, ignore
             if (ci->bel != BelId())
                 continue;
-            // log_info("   Legalising %s (%s) %d\n", top.second.c_str(ctx), ci->type.c_str(ctx), top.first);
+            if (ctx->debug)
+                log_info("   Legalising %s (%s) priority=%d\n", top.second.c_str(ctx), ci->type.c_str(ctx), top.first);
             FastBels::FastBelsData *fb;
             fast_bels.getBelsForCellType(ci->type, &fb);
             int radius = 0;
@@ -973,7 +974,8 @@ class HeAPPlacer
                     CellInfo *bound = ctx->getBoundBelCell(bestBel);
                     if (bound != nullptr) {
                         ctx->unbindBel(bound->bel);
-                        remaining.emplace(chain_size[bound->name], bound->name);
+                        remaining.emplace(chain_size[bound->name] * cfg.get_cell_legalisation_weight(ctx, bound),
+                                          bound->name);
                     }
                     ctx->bindBel(bestBel, ci, STRENGTH_WEAK);
                     placed = true;
@@ -1035,7 +1037,9 @@ class HeAPPlacer
                             } else {
                                 // It's legal, and we've tried enough. Finish.
                                 if (bound != nullptr)
-                                    remaining.emplace(chain_size[bound->name], bound->name);
+                                    remaining.emplace(chain_size[bound->name] *
+                                                              cfg.get_cell_legalisation_weight(ctx, bound),
+                                                      bound->name);
                                 Loc loc = ctx->getBelLocation(sz);
                                 cell_locs[ci->name].x = loc.x;
                                 cell_locs[ci->name].y = loc.y;
@@ -1098,7 +1102,9 @@ class HeAPPlacer
                         for (auto &swap : swaps_made) {
                             // Where we have ripped up cells; add them to the queue
                             if (swap.second != nullptr)
-                                remaining.emplace(chain_size[swap.second->name], swap.second->name);
+                                remaining.emplace(chain_size[swap.second->name] *
+                                                          cfg.get_cell_legalisation_weight(ctx, swap.second),
+                                                  swap.second->name);
                         }
 
                         placed = true;
diff --git a/common/place/placer_heap.h b/common/place/placer_heap.h
index c79e3dfc..704eb157 100644
--- a/common/place/placer_heap.h
+++ b/common/place/placer_heap.h
@@ -29,6 +29,8 @@
 #include "log.h"
 #include "nextpnr.h"
 
+#include <functional>
+
 NEXTPNR_NAMESPACE_BEGIN
 
 struct PlacerHeapCfg
@@ -53,6 +55,9 @@ struct PlacerHeapCfg
     // These cell types are part of the same unit (e.g. slices split into
     // components) so will always be spread together
     std::vector<pool<BelBucketId>> cellGroups;
+
+    // this is an optional callback to prioritise certain cells/clusters for legalisation
+    std::function<float(Context *, CellInfo *)> get_cell_legalisation_weight = [](Context *, CellInfo *) { return 1; };
 };
 
 extern bool placer_heap(Context *ctx, PlacerHeapCfg cfg);
diff --git a/himbaechel/uarch/xilinx/xilinx.cc b/himbaechel/uarch/xilinx/xilinx.cc
index a1aa3575..ef059f01 100644
--- a/himbaechel/uarch/xilinx/xilinx.cc
+++ b/himbaechel/uarch/xilinx/xilinx.cc
@@ -278,6 +278,13 @@ void XilinxImpl::configurePlacerHeap(PlacerHeapCfg &cfg)
     cfg.hpwl_scale_y = 1;
     cfg.beta = 0.5;
     cfg.placeAllAtOnce = true;
+    cfg.get_cell_legalisation_weight = [this](Context *, CellInfo *ci) {
+        if (ci->type != id_SLICE_LUTX)
+            return 1;
+        auto tags = get_tags(ci);
+        // Place memory first, because they require entire SLICEMs
+        return tags->lut.is_memory ? 100 : 1;
+    };
 }
 
 void XilinxImpl::preRoute()