From 2a6dfd2545b78b91ddb690f30068742212862f81 Mon Sep 17 00:00:00 2001
From: Damian Tarnawski <gthetarnav@gmail.com>
Date: Sat, 23 Aug 2025 16:55:12 +0200
Subject: [PATCH] Avoid overlap issues when correcting memory after resize in
 _reserve_soa

---
 base/runtime/core_builtin_soa.odin        | 19 ++++++--
 tests/core/runtime/test_core_runtime.odin | 58 +++++++++++++++++------
 2 files changed, 58 insertions(+), 19 deletions(-)

diff --git a/base/runtime/core_builtin_soa.odin b/base/runtime/core_builtin_soa.odin
index 7548f6735..46c767751 100644
--- a/base/runtime/core_builtin_soa.odin
+++ b/base/runtime/core_builtin_soa.odin
@@ -266,6 +266,12 @@ _reserve_soa :: proc(array: ^$T/#soa[dynamic]$E, capacity: int, zero_memory: boo
 		// from: |x x y y z z _ _ _|
 		// to:   |x x _ y y _ z z _|
 
+		// move old data to the end of the new allocation to avoid overlap
+		old_start := uintptr(new_data) + uintptr(new_size - old_size)
+		mem_copy(rawptr(old_start), old_data, old_size)
+
+		// now:  |_ _ _ x x y y z z|
+
 		for i in 0..<field_count {
 			type := si.types[i].variant.(Type_Info_Multi_Pointer).elem
 
@@ -273,18 +279,21 @@ _reserve_soa :: proc(array: ^$T/#soa[dynamic]$E, capacity: int, zero_memory: boo
 			new_offset = align_forward_int(new_offset, max_align)
 
 			new_data_elem := rawptr(uintptr(new_data) + uintptr(new_offset))
-			old_data_elem := rawptr(uintptr(new_data) + uintptr(old_offset))
+			old_data_elem := rawptr(old_start + uintptr(old_offset))
 
-			mem_copy(new_data_elem, old_data_elem, type.size * old_cap)
+			old_size_elem := type.size * old_cap
+			new_size_elem := type.size * capacity
+
+			mem_copy(new_data_elem, old_data_elem, old_size_elem)
 
 			(^rawptr)(uintptr(array) + i*size_of(rawptr))^ = new_data_elem
 
 			if zero_memory {
-				mem_zero(old_data_elem, int(uintptr(new_data_elem) - uintptr(old_data_elem)))
+				mem_zero(rawptr(uintptr(new_data_elem) + uintptr(old_size_elem)), new_size_elem - old_size_elem)
 			}
 
-			old_offset += type.size * old_cap
-			new_offset += type.size * capacity
+			old_offset += old_size_elem
+			new_offset += new_size_elem
 		}
 
 		return nil
diff --git a/tests/core/runtime/test_core_runtime.odin b/tests/core/runtime/test_core_runtime.odin
index d74edd3f8..1bc100348 100644
--- a/tests/core/runtime/test_core_runtime.odin
+++ b/tests/core/runtime/test_core_runtime.odin
@@ -181,28 +181,58 @@ test_map_get :: proc(t: ^testing.T) {
 
 @(test)
 test_soa_array_allocator_resize :: proc(t: ^testing.T) {
+
 	arena: runtime.Arena
 	context.allocator = runtime.arena_allocator(&arena)
 	defer runtime.arena_destroy(&arena)
 
-	array, err := make(#soa[dynamic][2]int, 2, 3)
-	array[0] = [2]int{1, 2}
-	array[1] = [2]int{3, 4}
+	{
+		// |1 3 _ 2 4 _|
+		// |1 3 _ _ 2 4 _ _|
 
-	testing.expect_value(t, err, nil)
-	testing.expect_value(t, len(array), 2)
-	testing.expect_value(t, cap(array), 3)
+		array, err := make(#soa[dynamic][2]int, 2, 3)
+		array[0] = [2]int{1, 2}
+		array[1] = [2]int{3, 4}
 
-	err = resize(&array, 4)
+		testing.expect_value(t, err, nil)
+		testing.expect_value(t, len(array), 2)
+		testing.expect_value(t, cap(array), 3)
 
-	testing.expect_value(t, err, nil)
-	testing.expect_value(t, len(array), 4)
-	testing.expect_value(t, cap(array), 4)
+		err = resize(&array, 4)
 
-	testing.expect_value(t, array[0], [2]int{1, 2})
-	testing.expect_value(t, array[1], [2]int{3, 4})
-	testing.expect_value(t, array[2], [2]int{0, 0})
-	testing.expect_value(t, array[3], [2]int{0, 0})
+		testing.expect_value(t, err, nil)
+		testing.expect_value(t, len(array), 4)
+		testing.expect_value(t, cap(array), 4)
+
+		testing.expect_value(t, array[0], [2]int{1, 2})
+		testing.expect_value(t, array[1], [2]int{3, 4})
+		testing.expect_value(t, array[2], [2]int{0, 0})
+		testing.expect_value(t, array[3], [2]int{0, 0})
+	}
+
+	{
+		// |1 4 2 5 3 6|
+		// |1 4 _ _ 2 5 _ _ 3 6 _ _|
+
+		array, err := make(#soa[dynamic][3]int, 2, 2)
+		array[0] = [3]int{1, 2, 3}
+		array[1] = [3]int{4, 5, 6}
+
+		testing.expect_value(t, err, nil)
+		testing.expect_value(t, len(array), 2)
+		testing.expect_value(t, cap(array), 2)
+
+		err = resize(&array, 4)
+
+		testing.expect_value(t, err, nil)
+		testing.expect_value(t, len(array), 4)
+		testing.expect_value(t, cap(array), 4)
+
+		testing.expect_value(t, array[0], [3]int{1, 2, 3})
+		testing.expect_value(t, array[1], [3]int{4, 5, 6})
+		testing.expect_value(t, array[2], [3]int{0, 0, 0})
+		testing.expect_value(t, array[3], [3]int{0, 0, 0})
+	}
 }
 
 @(test)