Use mimalloc for benchmarks (#7029)

`mimalloc` is a _much_ faster allocator, especially important when doing
a lot of small allocations (which egui does).

We use `mimalloc` in Rerun, and I recommend everyone to use it.

## The difference it makes

![image](https://github.com/user-attachments/assets/b22e0025-bc5e-4b3c-94e0-74ce46e86f85)

crates/egui/src/lib.rs

@@ -400,6 +400,9 @@
 //! profile-with-puffin = ["profiling/profile-with-puffin"]
 //! ```
 //!
+//! ## Custom allocator
+//! egui apps can run significantly (~20%) faster by using a custom allocator, like [mimalloc](https://crates.io/crates/mimalloc) or [talc](https://crates.io/crates/talc).
+//!
 
 #![allow(clippy::float_cmp)]
 #![allow(clippy::manual_range_contains)]

crates/egui_demo_app/Cargo.toml

@@ -86,6 +86,7 @@ env_logger = { version = "0.10", default-features = false, features = [
   "auto-color",
   "humantime",
 ] }
+mimalloc.workspace = true
 rfd = { version = "0.15.3", optional = true }
 
 # web:

crates/egui_demo_app/src/main.rs

@@ -4,6 +4,9 @@
 #![allow(rustdoc::missing_crate_level_docs)] // it's an example
 #![allow(clippy::never_loop)] // False positive
 
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc; // Much faster allocator, can give 20% speedups: https://github.com/emilk/egui/pull/7029
+
 // When compiling natively:
 fn main() -> eframe::Result {
     for arg in std::env::args().skip(1) {

crates/egui_demo_lib/Cargo.toml

@@ -56,8 +56,9 @@ serde = { workspace = true, optional = true }
 
 [dev-dependencies]
 criterion.workspace = true
-egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
 egui = { workspace = true, features = ["default_fonts"] }
+egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
+mimalloc.workspace = true                                            # for benchmarks
 rand = "0.9"
 
 [[bench]]

crates/egui_demo_lib/benches/benchmark.rs

@@ -8,6 +8,9 @@ use egui::{Button, Id, RichText, TextureId, Ui, UiBuilder, Vec2};
 use egui_demo_lib::LOREM_IPSUM_LONG;
 use rand::Rng as _;
 
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc; // Much faster allocator
+
 /// Each iteration should be called in their own `Ui` with an intentional id clash,
 /// to prevent the Context from building a massive map of `WidgetRects` (which would slow the test,
 /// causing unreliable results).

crates/epaint/Cargo.toml

@@ -101,6 +101,7 @@ backtrace = { workspace = true, optional = true }
 
 [dev-dependencies]
 criterion.workspace = true
+mimalloc.workspace = true
 similar-asserts.workspace = true
 
 

crates/epaint/benches/benchmark.rs

@@ -5,6 +5,9 @@ use epaint::{
     TessellationOptions, Tessellator, TextureAtlas, Vec2,
 };
 
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc; // Much faster allocator
+
 fn single_dashed_lines(c: &mut Criterion) {
     c.bench_function("single_dashed_lines", move |b| {
         b.iter(|| {