Move forward update links out of Node struct

Cargo.toml

@@ -75,7 +75,6 @@ hmac = "0.12"
 sha2 = "0.10"
 bitvec = "1"
 flate2 = "1"
-smallvec = "1.9.0"
 enum_dispatch = "0.3"
 petgraph = "0.7"
 thiserror = "2"

crates/redpiler/Cargo.toml

@@ -15,5 +15,4 @@ tracing = { workspace = true }
 petgraph = { workspace = true }
 itertools = { workspace = true }
 rustc-hash = { workspace = true }
-smallvec = { workspace = true }
 enum_dispatch = { workspace = true }

crates/redpiler/src/backend/direct/compile.rs

@@ -7,7 +7,6 @@ use mchprs_world::TickEntry;
 use petgraph::visit::EdgeRef;
 use petgraph::Direction;
 use rustc_hash::FxHashMap;
-use smallvec::SmallVec;
 use std::sync::Arc;
 use tracing::trace;
 
@@ -28,6 +27,7 @@ fn compile_node(
     nodes_len: usize,
     nodes_map: &FxHashMap<NodeIdx, usize>,
     noteblock_info: &mut Vec<(BlockPos, Instrument, u32)>,
+    forward_links: &mut Vec<ForwardLink>,
     stats: &mut FinalGraphStats,
 ) -> Node {
     let node = &graph[node_idx];
@@ -73,8 +73,9 @@ fn compile_node(
     side_inputs.ss_counts[0] += (MAX_INPUTS - side_input_count) as u8;
 
     use crate::compile_graph::NodeType as CNodeType;
-    let updates = if node.ty != CNodeType::Constant {
-        graph
+    let fwd_link_begin = forward_links.len();
+    if node.ty != CNodeType::Constant {
+        let new_links = graph
             .edges_directed(node_idx, Direction::Outgoing)
             .sorted_by_key(|edge| nodes_map[&edge.target()])
             .into_group_map_by(|edge| std::mem::discriminant(&graph[edge.target()].ty))
@@ -89,12 +90,11 @@ fn compile_node(
 
                 let weight = edge.weight();
                 ForwardLink::new(target_id, weight.ty == LinkType::Side, weight.ss)
-            })
-            .collect()
-    } else {
-        SmallVec::new()
+            });
+        forward_links.extend(new_links);
     };
-    stats.update_link_count += updates.len();
+    let fwd_link_end = forward_links.len();
+    stats.update_link_count += fwd_link_end - fwd_link_begin;
 
     let ty = match &node.ty {
         CNodeType::Repeater {
@@ -132,7 +132,8 @@ fn compile_node(
         ty,
         default_inputs,
         side_inputs,
-        updates,
+        fwd_link_begin,
+        fwd_link_end,
         powered: node.state.powered,
         output_power: node.state.output_strength,
         locked: node.state.repeater_locked,
@@ -167,6 +168,7 @@ pub fn compile(
                 nodes_len,
                 &nodes_map,
                 &mut backend.noteblock_info,
+                &mut backend.forward_links,
                 &mut stats,
             )
         })

crates/redpiler/src/backend/direct/mod.rs

@@ -6,6 +6,7 @@ mod tick;
 mod update;
 
 use super::JITBackend;
+use crate::backend::direct::node::ForwardLink;
 use crate::compile_graph::CompileGraph;
 use crate::task_monitor::TaskMonitor;
 use crate::{block_powered_mut, CompilerOptions};
@@ -109,6 +110,7 @@ enum Event {
 #[derive(Default)]
 pub struct DirectBackend {
     nodes: Nodes,
+    forward_links: Vec<ForwardLink>,
     blocks: Vec<Option<(BlockPos, Block)>>,
     pos_map: FxHashMap<BlockPos, NodeId>,
     scheduler: TickScheduler,
@@ -128,12 +130,11 @@ impl DirectBackend {
         node.changed = true;
         node.powered = powered;
         node.output_power = new_power;
-        for i in 0..node.updates.len() {
-            let node = &self.nodes[node_id];
-            let update_link = unsafe { *node.updates.get_unchecked(i) };
-            let side = update_link.side();
-            let distance = update_link.ss();
-            let update = update_link.node();
+
+        for forward_link in &self.forward_links[node.fwd_link_begin..node.fwd_link_end] {
+            let side = forward_link.side();
+            let distance = forward_link.ss();
+            let update = forward_link.node();
 
             let update_ref = &mut self.nodes[update];
             let inputs = if side {
@@ -196,6 +197,7 @@ impl JITBackend for DirectBackend {
             }
         }
 
+        self.forward_links.clear();
         self.pos_map.clear();
         self.noteblock_info.clear();
         self.events.clear();
@@ -381,7 +383,7 @@ impl fmt::Display for DirectBackend {
                 "No Pos".to_string()
             };
             writeln!(f, "    n{} [ label = \"{}\\n({})\" ];", id, label, pos)?;
-            for link in node.updates.iter() {
+            for link in &self.forward_links[node.fwd_link_begin..node.fwd_link_end] {
                 let out_index = link.node().index();
                 let distance = link.ss();
                 let color = if link.side() { ",color=\"blue\"" } else { "" };

crates/redpiler/src/backend/direct/node.rs

@@ -1,5 +1,4 @@
 use mchprs_blocks::blocks::ComparatorMode;
-use smallvec::SmallVec;
 use std::num::NonZeroU8;
 use std::ops::{Index, IndexMut};
 
@@ -149,12 +148,22 @@ impl NonMaxU8 {
     }
 }
 
+// The `Node` struct's size is currently 64 bytes which happens to be the same
+// size as an L1 cache line on most modern processors. By forcing a 64-byte
+// alignment, we make sure that the entire `Node` can fit on one cache line,
+// preventing scenarios where we have to fetch 2 cache lines to read a single `Node`.
+#[repr(align(64))]
 #[derive(Debug, Clone)]
 pub struct Node {
     pub ty: NodeType,
     pub default_inputs: NodeInput,
     pub side_inputs: NodeInput,
-    pub updates: SmallVec<[ForwardLink; 10]>,
+
+    /// The index to the first forward link of this node.
+    pub fwd_link_begin: usize,
+    /// The index to after the last forward link of this node.
+    pub fwd_link_end: usize,
+
     pub is_io: bool,
 
     /// Powered or lit