RR Graph - Cut interposer crossing wires #3342

AmirhosseinPoolad · 2025-11-11T21:49:13Z

This PR adds the last piece of functionality for modeling 2.5D architectures in upstream VPR: Actually cutting down the wires and connections that cross between the dice.

I had an issue with code duplication and there are still some functions that look very similar but do different things. I tried my best to keep the duplication to the lowest level functions and have more general higher level functions that wrap them but there's still some remaining. If you have an idea to do this in a better way please let me know.

AlexandreSinger

Looks good. Some comments.

libs/librrgraph/src/base/rr_graph_builder.h

libs/librrgraph/src/base/rr_graph_storage.cpp

libs/librrgraph/src/base/rr_graph_storage.h

libs/librrgraph/src/base/rr_graph_view.h

vpr/src/route/rr_graph_generation/rr_graph_interposer.cpp

vpr/src/route/rr_graph_generation/interposer_cut.cpp

vpr/src/route/rr_graph_generation/rr_graph.h

vpr/src/route/rr_graph_generation/rr_node_indices.cpp

amin1377

Thanks, Amir!

doc/src/arch/reference.rst

libs/librrgraph/src/base/rr_graph_builder.h

libs/librrgraph/src/base/rr_graph_view.cpp

libs/librrgraph/src/base/rr_graph_view.h

vpr/src/route/rr_graph_generation/rr_graph.cpp

amin1377 · 2025-11-14T20:08:46Z

vpr/src/route/rr_graph_generation/rr_graph_interposer.cpp

+        RRNodeId src_node = rr_graph.edge_src_node(edge_id);
+        RRNodeId sink_node = rr_graph.edge_sink_node(edge_id);
+
+        // TODO: ignoring ChanZ nodes for now


Can you elaborate a bit more on why chanz is ignored? My guess is that since this code is focused on the 2.5D architecture, the assumption is that chanz is not stretched across the 2D plane but along the Z-axis, so the interposer line wouldn’t apply to it. If that’s the case, I’d also add an ASSERT_DEBUG here to ensure that x_low == x_high and y_low == y_high.

I haven't thought about 3D architectures that also have interposer cut lines so I'm honestly not sure what should happen to edges to connect to/from chanz nodes. I think this needs further discussions about what it even means to have an architecture like this.

I'd update the comment to say that we don't allow interposers in the Z dimension (z is for die-stacking, and an interposer in a die stacking dimension is not currently manufacturable).

This commit makes the bit mask much more obvious that it should be one-hot encoded.

soheilshahrouz

Thanks Amir!

libs/librrgraph/src/base/rr_graph_storage.cpp

vpr/src/route/rr_graph_generation/rr_graph_interposer.cpp

libs/librrgraph/src/base/rr_graph_view.h

vpr/src/route/rr_graph_generation/interposer_cut.cpp

soheilshahrouz · 2025-11-17T21:20:49Z

vpr/src/route/rr_graph_generation/interposer_cut.cpp

+    }
+}
+
+void update_interposer_crossing_nodes_in_spatial_lookup_and_rr_graph_storage(const RRGraphView& rr_graph, const DeviceGrid& grid, RRGraphBuilder& rr_graph_builder, const std::vector<std::pair<RRNodeId, int>>& sg_node_indices) {


the line is too long
the function name is too long

vpr/src/route/rr_graph_generation/interposer_cut.h

vpr/src/route/rr_graph_generation/rr_graph.cpp

vaughnbetz

Looks good; some commenting upgrades and a few changes suggested.

vaughnbetz · 2025-11-17T17:46:05Z

libs/librrgraph/src/base/rr_graph_builder.h

+
+    /** @brief Removes a given list of RREdgeIds for the RR Graph.
+     * This method does not preserve the order of edges. If you're
+     * calling it after partition_edges has been called, you need


"you need to call it again" --> you will need to call partition_edges again

libs/librrgraph/src/base/rr_graph_builder.h

vaughnbetz · 2025-11-17T17:56:26Z

libs/librrgraph/src/base/rr_graph_storage.cpp

+    edge_remapped_.erase(edge_remapped_.begin() + edge_list_end + 1, edge_remapped_.end());
+
+    VTR_ASSERT(edge_dest_node_.size() == (starting_edge_count - rr_edges_to_remove.size()));
+}


Should set partitioned = false here

vaughnbetz · 2025-11-17T17:57:31Z

libs/librrgraph/src/base/rr_graph_storage.h

    }

+    /** @brief Returns a range of all edges in the RR Graph.
+     * This method does not depend on the edges begin correctly


begin -> being

vaughnbetz · 2025-11-17T17:58:33Z

libs/librrgraph/src/base/rr_graph_storage.h

    /** @brief Adds a batch of edges.*/
    void alloc_and_load_edges(const t_rr_edge_info_set* rr_edges_to_create);

+    /** @brief Removes a given list of RREdgeIds for the RR Graph.


'for the RR' -> 'from the RR'
'has been called, you need to call it again' -> 'has been called, you need to call partition_edges again'

Probably should say this operation is O(num_edges) in the RR graph, so it should not be called often and edges should be removed in bulk if at all possible.

vaughnbetz · 2025-11-17T22:00:09Z

vpr/src/route/rr_graph_generation/rr_graph_interposer.cpp

+
+        int layer = rr_graph.node_layer_low(src_node);
+
+        for (int cut_loc_y : grid.get_horizontal_interposer_cuts()[layer]) {


Explain this a bit. If the start (drive point) of the src node of the edge is on a different side of the interposer than the sink node of the edge drive point, the edge starts on a piece of wire that should be cut by the interposer. Mark it for removal.

vaughnbetz · 2025-11-17T23:02:49Z

vpr/src/route/rr_graph_generation/interposer_cut.cpp

+  * @brief Update a CHANY node's bounding box in RRGraph and SpatialLookup entries.
+  * This function assumes that the channel node actually crosses the cut location and
+  * might not function correctly otherwise.
+  *


comment the params -- there are a lot so good to document.

Could you put an assert in for the channel node crosses the cut location?

vaughnbetz · 2025-11-17T23:04:33Z

vpr/src/route/rr_graph_generation/interposer_cut.cpp

+  * might not function correctly otherwise.
+  *
+  * This is a low level function, you should use cut_channel_node that wraps this up in a nicer API.
+  */


Document params

vaughnbetz · 2025-11-17T23:05:02Z

vpr/src/route/rr_graph_generation/interposer_cut.cpp

+    if (node_direction == Direction::INC) {
+        // Anything to the right of cut_loc_x shouldn't exist
+        rr_graph_builder.set_node_coordinates(node, x_low, y_low, cut_loc_x, y_high);
+


Add assert that the node crosses the cut location (or just return without doing anything and update the comment).

vaughnbetz · 2025-11-17T23:06:31Z

vpr/src/route/rr_graph_generation/interposer_cut.cpp

+#include "rr_spatial_lookup.h"
+#include "vpr_error.h"
+#include "vtr_assert.h"
+


Comment what this file is for.
Add an ascii art showing where the interposer boundary is / what wires are cut / where channels are vs. the interposer for the same coordinate (e.g. y).

github-actions bot added VPR VPR FPGA Placement & Routing Tool lang-cpp C/C++ code labels Nov 11, 2025

AmirhosseinPoolad force-pushed the rr_graph_interposer_cut branch from 95dee19 to 9b6566b Compare November 12, 2025 19:01

AmirhosseinPoolad changed the title ~~[WIP] RR Graph - Cut interposer crossing wires~~ RR Graph - Cut interposer crossing wires Nov 12, 2025

github-actions bot added the docs Documentation label Nov 12, 2025

AlexandreSinger reviewed Nov 13, 2025

View reviewed changes

AmirhosseinPoolad force-pushed the rr_graph_interposer_cut branch from d4485f3 to f007bde Compare November 13, 2025 15:58

AmirhosseinPoolad requested a review from amin1377 November 14, 2025 15:19

amin1377 requested changes Nov 14, 2025

View reviewed changes

AmirhosseinPoolad added 9 commits November 16, 2025 14:54

Add interposer crossing wire cutting

998ba66

Fix edge cutting

2546789

Fix integration with scatter-gather patterns

21d5080

Add doxygen to interposer cut functions

72e8933

Fix some code duplication issues

bc9a40a

Remove experimental interposer status from documentation

b6f592a

Fix issue with interposers and 3D connection blocks

349d672

Address PR comments

e659e23

Change RR Graph warning enum to be clearer

e9876b3

This commit makes the bit mask much more obvious that it should be one-hot encoded.

AmirhosseinPoolad force-pushed the rr_graph_interposer_cut branch from f007bde to e9876b3 Compare November 16, 2025 19:54

AmirhosseinPoolad added 4 commits November 17, 2025 13:36

Address PR comments on RR Graph changes

2d6de4f

Add doxygen to interposer_cut.h

16f26ce

Fix formatting

ea396cd

Add .clangd to gitignore

4d95419

amin1377 approved these changes Nov 17, 2025

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soheilshahrouz reviewed Nov 17, 2025

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AmirhosseinPoolad added 5 commits November 17, 2025 16:59

Make remove_edges use ranges

e2cee0c

Pass RRNodeIds by value

4573aa9

Add newline to 2D scatter-gather comment

6b39edc

Rename interposer_cut to rr_graph_interposer

0fc6d07

Add static function declarations to rr_graph_interposer.cpp

dda2e43

AmirhosseinPoolad added 2 commits November 17, 2025 17:07

Add std::pair include

61ebf99

Fix formatting issues

561b5fb

github-actions bot added the libarchfpga Library for handling FPGA Architecture descriptions label Nov 17, 2025

Fix bug with checking if device has interposer cuts

b57b08f

AmirhosseinPoolad force-pushed the rr_graph_interposer_cut branch from 88f8e82 to b57b08f Compare November 17, 2025 22:48

amin1377 mentioned this pull request Nov 17, 2025

[WIP] Move node_tilable_track_nums_ to rr_graph_storage and add remove_node #3347

Open

vaughnbetz requested changes Nov 18, 2025

View reviewed changes


		int layer = rr_graph.node_layer_low(src_node);

		for (int cut_loc_y : grid.get_horizontal_interposer_cuts()[layer]) {

RR Graph - Cut interposer crossing wires #3342

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RR Graph - Cut interposer crossing wires #3342

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AmirhosseinPoolad commented Nov 11, 2025 •

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