perf(arena3): inline size_class_index_for in allocation hot path

oscars/Cargo.toml

@@ -28,3 +28,7 @@ default = ["mark_sweep"]
 std = []
 mark_sweep = []
 gc_allocator = ["dep:allocator-api2", "mark_sweep"]
+
+[[bench]]
+name = "arena3_alloc"
+harness = false

oscars/benches/arena3_alloc.rs

@@ -0,0 +1,63 @@
+use criterion::{Criterion, criterion_group, criterion_main};
+use oscars::alloc::arena3::ArenaAllocator;
+use std::hint::black_box as bb;
+
+fn bench_arena3_alloc(c: &mut Criterion) {
+    let mut group = c.benchmark_group("Arena3 Allocation");
+
+    group.bench_function("Scenario A: Pure Bump Allocation", |b| {
+        b.iter_batched(
+            || {
+                let alloc = ArenaAllocator::default().with_arena_size(1024 * 64);
+                alloc
+            },
+            |mut alloc| {
+                for i in 0..1000 {
+                    let ptr = alloc.try_alloc(bb(i)).unwrap();
+                    bb(ptr);
+                }
+            },
+            criterion::BatchSize::SmallInput,
+        );
+    });
+
+    group.bench_function("Scenario B: Bulk Allocation Loop", |b| {
+        let mut alloc = ArenaAllocator::default().with_arena_size(1024 * 1024 * 10);
+        b.iter(|| {
+            for i in 0..1000 {
+                let ptr = alloc.try_alloc(bb(i)).unwrap();
+                bb(ptr);
+            }
+        });
+    });
+
+    group.bench_function("Scenario C: Free List Reuse", |b| {
+        b.iter_batched(
+            || {
+                let mut alloc = ArenaAllocator::default().with_arena_size(1024 * 64);
+                let mut ptrs = Vec::new();
+                for i in 0..1000 {
+                    ptrs.push(alloc.try_alloc(i).unwrap());
+                }
+                for ptr in ptrs {
+                    unsafe {
+                        alloc.free_slot_typed(ptr.as_ptr());
+                    }
+                }
+                alloc
+            },
+            |mut alloc| {
+                for i in 0..1000 {
+                    let ptr = alloc.try_alloc(bb(i)).unwrap();
+                    bb(ptr);
+                }
+            },
+            criterion::BatchSize::SmallInput,
+        );
+    });
+
+    group.finish();
+}
+
+criterion_group!(benches, bench_arena3_alloc);
+criterion_main!(benches);

oscars/src/alloc/arena3/mod.rs

@@ -31,6 +31,7 @@ impl From<LayoutError> for ArenaAllocError {
 
 const SIZE_CLASSES: &[usize] = &[16, 24, 32, 48, 64, 96, 128, 192, 256, 512, 1024, 2048];
 
+#[inline(always)]
 fn size_class_index_for(size: usize) -> usize {
     let idx = SIZE_CLASSES.iter().copied().position(|sc| sc >= size);
     debug_assert!(