From 260275d69484365edd8c430f7a421e33c75d7403 Mon Sep 17 00:00:00 2001
From: Janis <janis@nirgendwo.xyz>
Date: Sun, 5 Jan 2025 16:10:30 +0100
Subject: [PATCH] rendergraph: barriers against write-after-read

---
 crates/renderer/src/render_graph.rs | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/crates/renderer/src/render_graph.rs b/crates/renderer/src/render_graph.rs
index 583da85..d7d17f0 100644
--- a/crates/renderer/src/render_graph.rs
+++ b/crates/renderer/src/render_graph.rs
@@ -305,8 +305,6 @@ impl RenderGraph {
             }
         }
 
-        let mut dag = petgraph::stable_graph::StableDiGraph::new();
-
         #[derive(Debug, Clone, Copy)]
         enum PassNode {
             First,
@@ -314,13 +312,23 @@ impl RenderGraph {
             Last,
         }
 
+        let mut dag = petgraph::stable_graph::StableDiGraph::new();
+
         let root = dag.add_node(PassNode::First);
-        let mut last_write = self
+        let mut last_write: BTreeMap<GraphResourceId, (NodeIndex, Access)> = self
             .resources
             .keys()
             .filter_map(|id| self.accesses.get(id).map(|access| (*id, (root, *access))))
             .collect::<BTreeMap<_, _>>();
 
+        let mut last_read: BTreeMap<GraphResourceId, (NodeIndex, Access)> = BTreeMap::new();
+
+        // TODO: rewrite finding edges properly.
+        // finding out if this graph is cyclical is actually non-trivial
+        // some pass might require both a read of a resource 1, and a read of a resource 2, where 2 is the product of another pass writing to resource 1.
+        // this could be resolved by copying resource 1 before the write pass.
+        // tl;dr: write-after-read makes this all more complicated
+
         // insert edges between write->read edges of 2 passes
         for (i, pass) in self.pass_descs.iter().enumerate() {
             let node = dag.add_node(PassNode::Pass(i));
@@ -341,6 +349,7 @@ impl RenderGraph {
                     tracing::trace!("adding edge between {other:?} and {node:?} for {rid:?} with ({before:?} -> {after:?})");
                     dag.add_edge(other, node, (rid, (before, after)));
                 }
+                last_read.insert(rid, (node, after));
             }
 
             let mut write_accesses = BTreeMap::new();
@@ -356,6 +365,12 @@ impl RenderGraph {
 
             for (rid, after) in write_accesses {
                 last_write.insert(rid, (node, after));
+                if let Some(&(other, read)) = last_read.get(&rid)
+                    && other != node
+                {
+                    tracing::trace!("adding edge between {other:?} and {node:?} for {rid:?} with ({read:?} -> {after:?}) (WaR)");
+                    dag.add_edge(other, node, (rid, (read, after)));
+                }
             }
         }