From 321ee42771f4301ad1e9057aef726b3ca17bc688 Mon Sep 17 00:00:00 2001
From: Kevin Reid <kpreid@switchb.org>
Date: Mon, 27 Jan 2025 09:14:09 -0800
Subject: [PATCH] =?UTF-8?q?Don=E2=80=99t=20copy=20buffer=20data=20back=20t?=
 =?UTF-8?q?o=20JS=20if=20unnecessary,=20in=20WebGPU=20backend.=20(#7007)?=
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---
 CHANGELOG.md               |  4 ++++
 wgpu/src/backend/webgpu.rs | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f0a7b70b17..07315f2932 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -81,6 +81,10 @@ By @brodycj in [#6924](https://github.com/gfx-rs/wgpu/pull/6924).
 
 - Stop naga causing undefined behavior when a ray query misses. By @Vecvec in [#6752](https://github.com/gfx-rs/wgpu/pull/6752).
 
+#### WebGPU
+
+- Improve efficiency of dropping read-only buffer mappings. By @kpreid in [#7007](https://github.com/gfx-rs/wgpu/pull/7007).
+
 ### Documentation
 
 - Improved documentation around pipeline caches and `TextureBlitter`. By @DJMcNab in [#6978](https://github.com/gfx-rs/wgpu/pull/6978) and [#7003](https://github.com/gfx-rs/wgpu/pull/7003).
diff --git a/wgpu/src/backend/webgpu.rs b/wgpu/src/backend/webgpu.rs
index 154f01f02a..adc05210f8 100644
--- a/wgpu/src/backend/webgpu.rs
+++ b/wgpu/src/backend/webgpu.rs
@@ -1368,7 +1368,11 @@ pub struct WebQueueWriteBuffer {
 #[derive(Debug)]
 pub struct WebBufferMappedRange {
     actual_mapping: js_sys::Uint8Array,
+    /// Copy of the mapped data that lives in the Rust/Wasm heap instead of JS,
+    /// so Rust code can borrow it.
     temporary_mapping: Vec<u8>,
+    /// Whether `temporary_mapping` has possibly been written to and needs to be written back to JS.
+    temporary_mapping_modified: bool,
     /// Unique identifier for this BufferMappedRange.
     ident: crate::cmp::Identifier,
 }
@@ -2672,6 +2676,7 @@ impl dispatch::BufferInterface for WebBuffer {
         WebBufferMappedRange {
             actual_mapping,
             temporary_mapping,
+            temporary_mapping_modified: false,
             ident: crate::cmp::Identifier::create(),
         }
         .into()
@@ -3780,11 +3785,18 @@ impl dispatch::BufferMappedRangeInterface for WebBufferMappedRange {
 
     #[inline]
     fn slice_mut(&mut self) -> &mut [u8] {
+        self.temporary_mapping_modified = true;
         &mut self.temporary_mapping
     }
 }
 impl Drop for WebBufferMappedRange {
     fn drop(&mut self) {
+        if !self.temporary_mapping_modified {
+            // For efficiency, skip the copy if it is not needed.
+            // This is also how we skip copying back on *read-only* mappings.
+            return;
+        }
+
         // Copy from the temporary mapping back into the array buffer that was
         // originally provided by the browser
         let temporary_mapping_slice = self.temporary_mapping.as_slice();