PaperMC/paper-server/patches/features/0008-Optimize-Voxel-Shape-Merging.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sun, 3 May 2020 22:35:09 -0400
Subject: [PATCH] Optimize Voxel Shape Merging

This method shows up as super hot in profiler, and also a high "self" time.

Upon analyzing, it appears most usages of this method fall down to the final
else statement of the nasty ternary.

Upon even further analyzation, it appears then the majority of those have a
consistent list 1.... One with Infinity head and Tails.

First optimization is to detect these infinite states and immediately return that
VoxelShapeMergerList so we can avoid testing the rest for most cases.

Break the method into 2 to help the JVM promote inlining of this fast path.

Then it was also noticed that VoxelShapeMergerList constructor is also a hotspot
with a high self time...

Well, knowing that in most cases our list 1 is actualy the same value, it allows
us to know that with an infinite list1, the result on the merger is essentially
list2 as the final values.

This let us analyze the 2 potential states (Infinite with 2 sources or 4 sources)
and compute a deterministic result for the MergerList values.

Additionally, this lets us avoid even allocating new objects for this too, further
reducing memory usage.

diff --git a/net/minecraft/world/phys/shapes/IndirectMerger.java b/net/minecraft/world/phys/shapes/IndirectMerger.java
index 60b56a5086b8aad0fad693f686b89138b3a4c80d..5b079ec43c259368d0eed4e8385880712abda4f7 100644
--- a/net/minecraft/world/phys/shapes/IndirectMerger.java
+++ b/net/minecraft/world/phys/shapes/IndirectMerger.java
@@ -10,12 +10,32 @@ public class IndirectMerger implements IndexMerger {
     private final int[] firstIndices;
     private final int[] secondIndices;
     private final int resultLength;
+    // Paper start
+    private static final int[] INFINITE_B_1 = {1, 1};
+    private static final int[] INFINITE_B_0 = {0, 0};
+    private static final int[] INFINITE_C = {0, 1};
+    // Paper end
 
     public IndirectMerger(DoubleList lower, DoubleList upper, boolean excludeUpper, boolean excludeLower) {
         double d = Double.NaN;
         int size = lower.size();
         int size1 = upper.size();
         int i = size + size1;
+        // Paper start - optimize common path of infinity doublelist
+        double tail = lower.getDouble(size - 1);
+        double head = lower.getDouble(0);
+        if (head == Double.NEGATIVE_INFINITY && tail == Double.POSITIVE_INFINITY && !excludeUpper && !excludeLower && (size == 2 || size == 4)) {
+            this.result = upper.toDoubleArray();
+            this.resultLength = upper.size();
+            if (size == 2) {
+                this.firstIndices = INFINITE_B_0;
+            } else {
+                this.firstIndices = INFINITE_B_1;
+            }
+            this.secondIndices = INFINITE_C;
+            return;
+        }
+        // Paper end
         this.result = new double[i];
         this.firstIndices = new int[i];
         this.secondIndices = new int[i];
diff --git a/net/minecraft/world/phys/shapes/Shapes.java b/net/minecraft/world/phys/shapes/Shapes.java
index e1b4c4b53844b0755e0640a05e8782fd9a7700a2..e759221fb54aa510d2d8bbba47e1d794367aec6d 100644
--- a/net/minecraft/world/phys/shapes/Shapes.java
+++ b/net/minecraft/world/phys/shapes/Shapes.java
@@ -279,9 +279,22 @@ public final class Shapes {
     }
 
     @VisibleForTesting
-    protected static IndexMerger createIndexMerger(int size, DoubleList list1, DoubleList list2, boolean excludeUpper, boolean excludeLower) {
+    private static IndexMerger createIndexMerger(int size, DoubleList list1, DoubleList list2, boolean excludeUpper, boolean excludeLower) { // Paper - private
+        // Paper start - fast track the most common scenario
+        // doublelist is usually a DoubleArrayList with Infinite head/tails that falls to the final else clause
+        // This is actually the most common path, so jump to it straight away
+        if (list1.getDouble(0) == Double.NEGATIVE_INFINITY && list1.getDouble(list1.size() - 1) == Double.POSITIVE_INFINITY) {
+            return new IndirectMerger(list1, list2, excludeUpper, excludeLower);
+        }
+        // Split out rest to hopefully inline the above
+        return lessCommonMerge(size, list1, list2, excludeUpper, excludeLower);
+    }
+
+    private static IndexMerger lessCommonMerge(int size, DoubleList list1, DoubleList list2, boolean excludeUpper, boolean excludeLower) {
+        // Paper end - fast track the most common scenario
         int i = list1.size() - 1;
         int i1 = list2.size() - 1;
+        // Paper note - Rewrite below as optimized order if instead of nasty ternary
         if (list1 instanceof CubePointRange && list2 instanceof CubePointRange) {
             long l = lcm(i, i1);
             if (size * l <= 256L) {
@@ -289,14 +302,21 @@ public final class Shapes {
             }
         }
 
-        if (list1.getDouble(i) < list2.getDouble(0) - 1.0E-7) {
+        // Paper start - Identical happens more often than Disjoint
+        if (i == i1 && Objects.equals(list1, list2)) {
+            if (list1 instanceof IdenticalMerger) {
+                return (IndexMerger) list1;
+            } else if (list2 instanceof IdenticalMerger) {
+                return (IndexMerger) list2;
+            }
+            return new IdenticalMerger(list1);
+        } else if (list1.getDouble(i) < list2.getDouble(0) - 1.0E-7) {
+            // Paper end - Identical happens more often than Disjoint
             return new NonOverlappingMerger(list1, list2, false);
         } else if (list2.getDouble(i1) < list1.getDouble(0) - 1.0E-7) {
             return new NonOverlappingMerger(list2, list1, true);
         } else {
-            return (IndexMerger)(i == i1 && Objects.equals(list1, list2)
-                ? new IdenticalMerger(list1)
-                : new IndirectMerger(list1, list2, excludeUpper, excludeLower));
+            return new IndirectMerger(list1, list2, excludeUpper, excludeLower); // Paper - Identical happens more often than Disjoint
         }
     }