Add button benchmark (#6854)

This helped me benchmark the atomic layout (#5830) changes.

I also realized that the label benchmark wasn't testing the painting,
since the buttons at some point will be placed outside the screen_rect,
meaning it won't be painted.

This fixes it by benching the label in a child ui.

The `label &str` benchmark went from 483 ns to 535 ns with these
changes.

EDIT:

I fixed another benchmark problem, since the benchmark would show the
same widget millions of times for a single frame, the WidgetRects
hashmap would get huge, causing each iteration to slow down a bit more
and causing the benchmark to have unreliable results.

With this the `label &str` benchmark went from 535ns to 298ns. Also the
`label format!` benchmark now takes almost the same time (302 ns).
Before, it was a lot slower since it reused the same Context which
already had millions of widget ids.

crates/egui_demo_lib/README.md

@@ -14,3 +14,19 @@ The demo library is a separate crate for three reasons:
 * To remove the amount of code in `egui` proper.
 * To make it easy for 3rd party egui integrations to use it for tests.
   - See for instance https://github.com/not-fl3/egui-miniquad/blob/master/examples/demo.rs
+
+This crate also contains benchmarks for egui. 
+Run them with 
+```bash
+# Run all benchmarks
+cargo bench -p egui_demo_lib 
+
+# Run a single benchmark
+cargo bench -p egui_demo_lib "benchmark name"
+
+# Profile benchmarks with cargo-flamegraph (--root flag is necessary for MacOS)
+CARGO_PROFILE_BENCH_DEBUG=true cargo flamegraph --bench benchmark --root -p egui_demo_lib  -- --bench "benchmark name"
+
+# Profile with cargo-instruments
+CARGO_PROFILE_BENCH_DEBUG=true cargo instruments --profile bench --bench benchmark -p egui_demo_lib -t time -- --bench "benchmark name" 
+```

crates/egui_demo_lib/benches/benchmark.rs

@@ -1,11 +1,20 @@
 use std::fmt::Write as _;
 
-use criterion::{criterion_group, criterion_main, Criterion};
+use criterion::{criterion_group, criterion_main, BatchSize, Criterion};
 
 use egui::epaint::TextShape;
+use egui::load::SizedTexture;
+use egui::{Button, Id, TextureId, Ui, UiBuilder, Vec2};
 use egui_demo_lib::LOREM_IPSUM_LONG;
 use rand::Rng as _;
 
+/// 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).
+fn create_benchmark_ui(ctx: &egui::Context) -> Ui {
+    Ui::new(ctx.clone(), Id::new("clashing_id"), UiBuilder::new())
+}
+
 pub fn criterion_benchmark(c: &mut Criterion) {
     use egui::RawInput;
 
@@ -55,17 +64,62 @@ pub fn criterion_benchmark(c: &mut Criterion) {
     {
         let ctx = egui::Context::default();
         let _ = ctx.run(RawInput::default(), |ctx| {
-            egui::CentralPanel::default().show(ctx, |ui| {
-                c.bench_function("label &str", |b| {
-                    b.iter(|| {
+            c.bench_function("label &str", |b| {
+                b.iter_batched_ref(
+                    || create_benchmark_ui(ctx),
+                    |ui| {
                         ui.label("the quick brown fox jumps over the lazy dog");
-                    });
-                });
-                c.bench_function("label format!", |b| {
-                    b.iter(|| {
+                    },
+                    BatchSize::LargeInput,
+                );
+            });
+            c.bench_function("label format!", |b| {
+                b.iter_batched_ref(
+                    || create_benchmark_ui(ctx),
+                    |ui| {
                         ui.label("the quick brown fox jumps over the lazy dog".to_owned());
-                    });
-                });
+                    },
+                    BatchSize::LargeInput,
+                );
+            });
+        });
+    }
+
+    {
+        let ctx = egui::Context::default();
+        let _ = ctx.run(RawInput::default(), |ctx| {
+            let mut group = c.benchmark_group("button");
+
+            // To ensure we have a valid image, let's use the font texture. The size
+            // shouldn't be important for this benchmark.
+            let image = SizedTexture::new(TextureId::default(), Vec2::splat(16.0));
+
+            group.bench_function("1_button_text", |b| {
+                b.iter_batched_ref(
+                    || create_benchmark_ui(ctx),
+                    |ui| {
+                        ui.add(Button::new("Hello World"));
+                    },
+                    BatchSize::LargeInput,
+                );
+            });
+            group.bench_function("2_button_text_image", |b| {
+                b.iter_batched_ref(
+                    || create_benchmark_ui(ctx),
+                    |ui| {
+                        ui.add(Button::image_and_text(image, "Hello World"));
+                    },
+                    BatchSize::LargeInput,
+                );
+            });
+            group.bench_function("3_button_text_image_right_text", |b| {
+                b.iter_batched_ref(
+                    || create_benchmark_ui(ctx),
+                    |ui| {
+                        ui.add(Button::image_and_text(image, "Hello World").right_text("⏵"));
+                    },
+                    BatchSize::LargeInput,
+                );
             });
         });
     }