drt: limit detailed-route grid graph to the routing-layer range

FlexGridGraph::initTracks() added every routing layer in the tech to the grid,
sizing nodes_, the per-node arrays (prevDirs_/srcs_/dsts_/guides_), and the
maze search bounds across the full layer stack. Routing edges and access vias
are already capped at TOP_ROUTING_LAYER, so layers above it were dead weight.
Skip routing layers above TOP_ROUTING_LAYER so the grid graph spans only the
routing range. Default TOP_ROUTING_LAYER is INT_MAX, so this is a no-op unless
the top routing layer is explicitly lowered.

Once the grid is trimmed it no longer spans the full layer stack, so maze-cost
init must not assume every tech layer maps to a grid node. initMazeCost_fixedObj
and initMazeCost_terms looped over the whole stack and called
getMazeZIdx(layerNum) for routing layers above TOP_ROUTING_LAYER, returning an
out-of-range z that the cost path dereferenced (getLayerNum -> zCoords_[z]),
crashing on platforms with fixed shapes or instance pins above the top routing
layer (e.g. ihp-sg13g2, with PDN and sg13g2_io pins on TopMetal above Metal5).
Guard both so fixed-shape costing skips layers outside the grid's
[getMinLayerNum, getMaxLayerNum] span. The skip is before any cost call, and is
a no-op when the grid spans every routing layer. (The setBlocked/setGuide paths
in initMazeCost_planarTerm, modBlockedEdgesForMacroPin and the guide helper are
already bounds-checked, so they need no change.)

Verified:
- gt2n gcd: at the tech-top layer the routed DB is bit-identical to before
  (true no-op); with the top routing layer lowered to M5, detailed-route wall
  time dropped ~10% and peak memory ~14%, 0 DRC.
- ihp-sg13g2 gcd (fixedObj) and i2c-gpio-expander (terms) crashed before the
  guard and route DRC-clean after it.

Signed-off-by: mrg <mrg@ucsc.edu>

Author: mguthaus

src/drt/src/dr/FlexDR_init.cpp

@@ -2722,13 +2722,25 @@ void FlexDRWorker::initMazeCost_fixedObj(const frDesign* design)
     result.clear();
     if (getTech()->getLayer(layerNum)->getType() == dbTechLayerType::ROUTING) {
       isRoutingLayer = true;
+      // The grid graph need not span the full layer stack: TOP_ROUTING_LAYER
+      // can trim routing layers above it out of the grid. Skip fixed-shape
+      // costing on layers absent from the grid so getMazeZIdx / getLayerNum
+      // stay in range. No-op when the grid spans every routing layer.
+      if (layerNum < gridGraph_.getMinLayerNum()
+          || layerNum > gridGraph_.getMaxLayerNum()) {
+        continue;
+      }
       zIdx = gridGraph_.getMazeZIdx(layerNum);
     } else if (getTech()->getLayer(layerNum)->getType()
                == dbTechLayerType::CUT) {
       isRoutingLayer = false;
       if (getTech()->getBottomLayerNum() <= layerNum - 1
           && getTech()->getLayer(layerNum - 1)->getType()
                  == dbTechLayerType::ROUTING) {
+        if (layerNum - 1 < gridGraph_.getMinLayerNum()
+            || layerNum - 1 > gridGraph_.getMaxLayerNum()) {
+          continue;
+        }
         zIdx = gridGraph_.getMazeZIdx(layerNum - 1);
       } else {
         continue;
@@ -2894,13 +2906,23 @@ void FlexDRWorker::initMazeCost_terms(
             if (getTech()->getLayer(layerNum)->getType()
                 == dbTechLayerType::ROUTING) {
               isRoutingLayer = true;
+              // Grid may not span the full stack (TOP_ROUTING_LAYER trim);
+              // skip layers absent from the grid. No-op when fully spanned.
+              if (layerNum < gridGraph_.getMinLayerNum()
+                  || layerNum > gridGraph_.getMaxLayerNum()) {
+                continue;
+              }
               zIdx = gridGraph_.getMazeZIdx(layerNum);
             } else if (getTech()->getLayer(layerNum)->getType()
                        == dbTechLayerType::CUT) {
               isRoutingLayer = false;
               if (getTech()->getBottomLayerNum() <= layerNum - 1
                   && getTech()->getLayer(layerNum - 1)->getType()
                          == dbTechLayerType::ROUTING) {
+                if (layerNum - 1 < gridGraph_.getMinLayerNum()
+                    || layerNum - 1 > gridGraph_.getMaxLayerNum()) {
+                  continue;
+                }
                 zIdx = gridGraph_.getMazeZIdx(layerNum - 1);
               } else {
                 continue;
@@ -2971,13 +2993,23 @@ void FlexDRWorker::initMazeCost_terms(
             if (getTech()->getLayer(layerNum)->getType()
                 == dbTechLayerType::ROUTING) {
               isRoutingLayer = true;
+              // Grid may not span the full stack (TOP_ROUTING_LAYER trim);
+              // skip layers absent from the grid. No-op when fully spanned.
+              if (layerNum < gridGraph_.getMinLayerNum()
+                  || layerNum > gridGraph_.getMaxLayerNum()) {
+                continue;
+              }
               zIdx = gridGraph_.getMazeZIdx(layerNum);
             } else if (getTech()->getLayer(layerNum)->getType()
                        == dbTechLayerType::CUT) {
               isRoutingLayer = false;
               if (getTech()->getBottomLayerNum() <= layerNum - 1
                   && getTech()->getLayer(layerNum - 1)->getType()
                          == dbTechLayerType::ROUTING) {
+                if (layerNum - 1 < gridGraph_.getMinLayerNum()
+                    || layerNum - 1 > gridGraph_.getMaxLayerNum()) {
+                  continue;
+                }
                 zIdx = gridGraph_.getMazeZIdx(layerNum - 1);
               } else {
                 continue;

src/drt/src/dr/FlexGridGraph.cpp

@@ -471,6 +471,17 @@ void FlexGridGraph::initTracks(
       continue;
     }
     frLayerNum currLayerNum = layer->getLayerNum();
+    // Layers above TOP_ROUTING_LAYER carry no routing edges or access vias
+    // (initEdges and the special-access via loop already cap work at
+    // TOP_ROUTING_LAYER), yet they were still added to the grid here, sizing
+    // nodes_, the per-node arrays, and the maze search bounds across the full
+    // layer stack. Skip them so the grid graph spans only the routing range,
+    // cutting detailed-route memory and runtime when the design's top routing
+    // layer is below the technology top. Default TOP_ROUTING_LAYER is INT_MAX,
+    // so this is a no-op unless the top routing layer is explicitly lowered.
+    if (currLayerNum > router_cfg_->TOP_ROUTING_LAYER) {
+      continue;
+    }
     dbTechLayerDir currPrefRouteDir = layer->getDir();
     for (auto& tp : design->getTopBlock()->getTrackPatterns(currLayerNum)) {
       // allow wrongway if global variable and design rule allow