From 92556fdfd0514f7e889761e1bb6eea17efc13feb Mon Sep 17 00:00:00 2001
From: bjorn3 <bjorn3@users.noreply.github.com>
Date: Tue, 12 Jan 2021 11:26:14 +0100
Subject: [PATCH] Use StableHashMap instead of HashMap + BTreeSet for
 diagnostics
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This is a significant reduction in the release mode compile times of
bevy_diagnostics

```
Benchmark #1: touch crates/bevy_diagnostic/src/lib.rs && cargo build --release -p bevy_diagnostic -j1
  Time (mean ± σ):      3.645 s ±  0.009 s    [User: 3.551 s, System: 0.094 s]
  Range (min … max):    3.632 s …  3.658 s    20 runs
```

```
Benchmark #1: touch crates/bevy_diagnostic/src/lib.rs && cargo build --release -p bevy_diagnostic -j1
  Time (mean ± σ):      2.938 s ±  0.012 s    [User: 2.850 s, System: 0.090 s]
  Range (min … max):    2.919 s …  2.969 s    20 runs
```
---
 crates/bevy_diagnostic/src/diagnostic.rs         | 16 +++++-----------
 .../src/log_diagnostics_plugin.rs                |  4 ++--
 2 files changed, 7 insertions(+), 13 deletions(-)

diff --git a/crates/bevy_diagnostic/src/diagnostic.rs b/crates/bevy_diagnostic/src/diagnostic.rs
index 549ecdb20f7558..4a7cd780fbfa7a 100644
--- a/crates/bevy_diagnostic/src/diagnostic.rs
+++ b/crates/bevy_diagnostic/src/diagnostic.rs
@@ -1,5 +1,5 @@
-use bevy_utils::{Duration, HashMap, Instant, Uuid};
-use std::collections::{BTreeSet, VecDeque};
+use bevy_utils::{Duration, StableHashMap, Instant, Uuid};
+use std::collections::VecDeque;
 
 /// Unique identifier for a [Diagnostic]
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq, PartialOrd, Ord)]
@@ -101,13 +101,13 @@ impl Diagnostic {
 /// A collection of [Diagnostic]s
 #[derive(Debug, Default)]
 pub struct Diagnostics {
-    diagnostics: HashMap<DiagnosticId, Diagnostic>,
-    ordered_diagnostics: BTreeSet<DiagnosticId>,
+    // This uses a [`StableHashMap`] to ensure that the iteration order is deterministic between
+    // runs when all diagnostics are inserted in the same order.
+    diagnostics: StableHashMap<DiagnosticId, Diagnostic>,
 }
 
 impl Diagnostics {
     pub fn add(&mut self, diagnostic: Diagnostic) {
-        self.ordered_diagnostics.insert(diagnostic.id);
         self.diagnostics.insert(diagnostic.id, diagnostic);
     }
 
@@ -134,10 +134,4 @@ impl Diagnostics {
     pub fn iter(&self) -> impl Iterator<Item = &Diagnostic> {
         self.diagnostics.values()
     }
-
-    pub fn ordered_iter(&self) -> impl Iterator<Item = &Diagnostic> {
-        self.ordered_diagnostics
-            .iter()
-            .filter_map(move |k| self.diagnostics.get(k))
-    }
 }
diff --git a/crates/bevy_diagnostic/src/log_diagnostics_plugin.rs b/crates/bevy_diagnostic/src/log_diagnostics_plugin.rs
index 276480786f6118..6e5d2636d76703 100644
--- a/crates/bevy_diagnostic/src/log_diagnostics_plugin.rs
+++ b/crates/bevy_diagnostic/src/log_diagnostics_plugin.rs
@@ -78,7 +78,7 @@ impl LogDiagnosticsPlugin {
                     Self::log_diagnostic(diagnostic);
                 }
             } else {
-                for diagnostic in diagnostics.ordered_iter() {
+                for diagnostic in diagnostics.iter() {
                     Self::log_diagnostic(diagnostic);
                 }
             }
@@ -96,7 +96,7 @@ impl LogDiagnosticsPlugin {
                     debug!("{:#?}\n", diagnostic);
                 }
             } else {
-                for diagnostic in diagnostics.ordered_iter() {
+                for diagnostic in diagnostics.iter() {
                     debug!("{:#?}\n", diagnostic);
                 }
             }