diff --git a/arena/arena.go b/arena/arena.go
index a842390..cbdcd10 100644
--- a/arena/arena.go
+++ b/arena/arena.go
@@ -1,6 +1,7 @@
 package arena
 
 import (
+	"fmt"
 	"unsafe"
 
 	"git.brut.systems/judah/xx/mem"
@@ -36,6 +37,30 @@ func MakeSlice[T any](arena Arena, len, cap int) []T {
 	return unsafe.Slice((*T)(ptr), cap)[:len]
 }
 
+// ExtendSlice returns a slice pointing to the same underlying memory
+// but with an increased capacity.
+//
+// Note: If extending the slice results in non-contiguous memory, ExtendSlice will panic.
+func ExtendSlice[T any](arena Arena, slice []T, newcap int) []T {
+	if cap(slice) >= newcap {
+		return slice
+	}
+
+	count := newcap - cap(slice)
+	ptr, err := arena(ACTION_ALLOC, mem.Sizeof[T]()*uintptr(count), mem.Alignof[T](), nil)
+	if err != nil {
+		panic(err)
+	}
+
+	base := uintptr(unsafe.Pointer(unsafe.SliceData(slice)))
+	expected := base + uintptr(cap(slice)*int(mem.Sizeof[T]()))
+	if uintptr(ptr) != expected {
+		panic(fmt.Sprintf("extendslice: extension of %d was non-contiguous (%d vs %d)", count, uintptr(ptr), expected))
+	}
+
+	return unsafe.Slice((*T)(unsafe.SliceData(slice)), newcap)[:len(slice)]
+}
+
 // Reset restores an Arena to its initial state.
 //
 // Note: Accessing memory returned by an Arena after calling Reset is unsafe and may result in a fault.
diff --git a/arena/arena_test.go b/arena/arena_test.go
index 1dfaaed..9b8a7aa 100644
--- a/arena/arena_test.go
+++ b/arena/arena_test.go
@@ -28,3 +28,20 @@ func TestMakeSlice(t *testing.T) {
 
 	testx.Expect(t, p != &s[0], "p = %p, expected %p", p, &s[0])
 }
+
+func TestExtendSlice(t *testing.T) {
+	a := arena.Linear(1024 * mem.Kilobyte)
+	defer arena.Reset(a)
+
+	s := arena.MakeSlice[int](a, 0, 5)
+	testx.Expect(t, cap(s) == 5)
+
+	s = arena.ExtendSlice(a, s, cap(s)+2)
+	testx.Expect(t, cap(s) == 7)
+
+	testx.ShouldPanic(t, func() {
+		// cause a non-contiguous slice extension (which will panic)
+		_ = arena.New[int](a)
+		s = arena.ExtendSlice(a, s, cap(s)*2)
+	})
+}
diff --git a/mem/mem.go b/mem/mem.go
index cbb3041..7a2aea3 100644
--- a/mem/mem.go
+++ b/mem/mem.go
@@ -72,10 +72,8 @@ func Copy(dst, src unsafe.Pointer, size uintptr) unsafe.Pointer {
 //
 // Returns dst.
 func Clear(dst unsafe.Pointer, value byte, count uintptr) unsafe.Pointer {
-	b := (*byte)(dst)
-	for range count {
-		*b = value
-		b = (*byte)(unsafe.Add(unsafe.Pointer(b), 1))
+	for i := range count {
+		*(*byte)(unsafe.Add(unsafe.Pointer(dst), i)) = value
 	}
 	return dst
 }
diff --git a/mem/mem_test.go b/mem/mem_test.go
index 1d22501..2c3f2cd 100644
--- a/mem/mem_test.go
+++ b/mem/mem_test.go
@@ -27,6 +27,50 @@ func TestBitCast(t *testing.T) {
 	}
 }
 
+func TestMemoryPrimitives(t *testing.T) {
+	t.Run("copy", func(t *testing.T) {
+		{ // non-overlapping
+			a := []int{1, 2, 3, 4, 5, 6}
+			b := make([]int, 3)
+			mem.Copy(unsafe.Pointer(&b[0]), unsafe.Pointer(&a[0]), uintptr(len(b))*unsafe.Sizeof(int(0)))
+
+			for i := range len(b) {
+				testx.Expect(t, a[i] == b[i], "%d != %d [%d]", a[i], b[i], i)
+			}
+		}
+
+		{ // overlapping
+			a := []int{1, 2, 3, 4, 5}
+			b := &a[0]
+			mem.Copy(unsafe.Pointer(b), unsafe.Pointer(&a[0]), uintptr(len(a))*unsafe.Sizeof(int(0)))
+
+			for i := range len(a) {
+				testx.Expect(t, a[i] == i+1, "[%d] %d != %d", i, a[i], i+1)
+			}
+		}
+	})
+
+	t.Run("clear", func(t *testing.T) {
+		{ // zero
+			v := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+			mem.Clear(unsafe.Pointer(&v[0]), 0, uintptr(len(v))*unsafe.Sizeof(int(0)))
+
+			for i := range len(v) {
+				testx.Expect(t, v[i] == 0, "[%d] %d != 0", i, v[i])
+			}
+		}
+
+		{ // fill
+			v := []byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+			mem.Clear(unsafe.Pointer(&v[0]), 0xFF, uintptr(len(v))*unsafe.Sizeof(int(0)))
+
+			for i := range len(v) {
+				testx.Expect(t, v[i] == 0xFF, "[%d] %d != 0xFF", i, v[i])
+			}
+		}
+	})
+}
+
 func TestAllocationPrimitives(t *testing.T) {
 	t.Run("reserve, unreserve", func(t *testing.T) {
 		data, err := mem.Reserve(1 * mem.Gigabyte)