How the Lua stack works and how to use it

When it comes to interacting with Lua, writing Lua extensions, or using the Lua C API, it’s important to understand how the Lua stack works and how to use it.

What is stored in the heap

In the Lua heap, various Lua objects are stored. Objects in Lua include but are not limited to the following types:

Table: A table in Lua is a dynamic data structure used to store associative arrays. It can be used as a general container that can store different types of values and use keys of any type to access and manipulate the data.
Function: A function in Lua is an executable block of code that can be called and executed. Functions can receive parameters and return results, and can be used to implement specific functionality or encapsulate reusable code.
String: A string in Lua is a sequence of characters used to represent text data. Strings can contain letters, numbers, symbols, etc., and support various operations, such as splicing, search, replacement, etc.
User data (userdata): User data in Lua is a special type of object that can be used to encapsulate and operate C language data structures. Userdata allows passing data from C to Lua for processing and manipulation in Lua.

In addition to the above basic types of objects, Lua’s heap can also store other types of data, such as threads, light userdata, etc. The storage and management of these objects are taken care of by Lua’s garbage collector.

It should be noted that Lua’s heap stores the data of the object itself, not the object’s reference or pointer. When objects are no longer referenced or used, the garbage collector will automatically reclaim the memory space occupied by these objects so that subsequent objects can be used. Therefore, when using Lua, developers do not need to manually allocate or release the memory of objects, and the garbage collector will automatically handle the object’s life cycle.

Key points in using Lua heap

Memory management: Lua’s heap is memory managed by the Lua virtual machine. It uses a garbage collection mechanism to automatically reclaim memory that is no longer used, reducing the complexity of manual memory management. In general, there is no need to manually manage Lua heap memory.
Object creation and destruction: With appropriate Lua syntax and function calls, different types of objects such as tables, strings, and functions can be created on the heap. These objects are automatically destroyed by the garbage collection mechanism when no longer needed. For special types of objects (such as C data), it may be necessary to manually release related resources.
References and reference counting: Heap objects in Lua are accessed and manipulated through references. When an object is referenced, its reference count is incremented. When the reference count reaches zero, the object is reclaimed by the garbage collection mechanism. Therefore, when working with objects, care needs to be taken to correctly increment and decrement the reference count to avoid memory leaks and access to invalid objects.
Avoid unnecessary object creation: Since the creation of heap objects requires allocating memory and performing additional operations, frequent creation and destruction of objects may affect performance. Therefore, it is recommended to create objects only when necessary and try to reuse existing objects to avoid unnecessary overhead.
Use memory efficiently: Try to avoid storing a large number of large objects on the heap to reduce memory usage. For large data sets or data that requires temporary storage, you can use appropriate data structures or caching strategies to optimize memory usage.

In short, Lua’s heap is an area used to store dynamically allocated data, managed through a built-in garbage collection mechanism. When using the Lua heap, you should pay attention to properly creating and destroying objects, managing reference counts, avoiding unnecessary object creation, and using memory efficiently to ensure the performance of the program and the effectiveness of memory management.

Lua operates the heap through memory management functions

lua_newstate

- Description: Create a Lua state (Lua virtual machine instance).
- Function prototype: lua_State* lua_newstate(lua_Alloc f, void* ud);
- Parameters:
  - f: Memory allocation function, used to allocate memory in Lua state.
  - ud: User data passed to the memory allocation function.
- Return value: Returns a pointer to the newly created Lua state.
- Best practice: Use this function to create a Lua virtual machine instance, which allocates a block of memory to store Lua state and related data structures.

lua_close

- Description: Close and destroy Lua state and release related memory.
- Function prototype: void lua_close(lua_State* L);
- Parameters:
  - L: Pointer to Lua state (Lua virtual machine instance).
- Best practice: After using the Lua virtual machine, call this function to release related memory resources.

Lua’s memory management is handled by the Lua virtual machine instance (lua_State), which uses a mechanism called garbage collection to automatically manage memory. The garbage collector automatically tracks and reclaims memory that is no longer in use to avoid memory leaks and memory overflows.

A Lua virtual machine instance maintains a heap memory area for storing Lua objects (such as tables, functions, strings, etc.). When a Lua object is created, the virtual machine allocates enough memory on the heap to hold the object’s data. When an object is no longer referenced, the garbage collector detects that the object is unreachable and reclaims the memory it occupies for subsequent object allocation.

In addition to automatic memory management, Lua also provides some functions for manual memory management, such as:

lua_newuserdata: Used to create a new user data object and allocate enough memory to store its data.
lua_pushlightuserdata: Push a pointer onto the stack as lightweight user data without triggering garbage collection.
lua_Alloc: The type of memory allocation function, you can customize the memory allocation strategy.

By using these functions, developers can perform more flexible and precise memory operations in scenarios that require manual memory management.

To summarize, Lua manages heap memory through virtual machine instances and garbage collection mechanisms. When developers use Lua, they can operate and control heap memory by creating and destroying virtual machine instances, using automatic memory management and manual memory management functions, etc.

Lua heap performance optimization

Avoid frequent object creation and destruction: Frequent object creation and destruction will increase the pressure of garbage collection and lead to performance degradation. Try to reuse existing objects and avoid unnecessary object creation, especially in loops or frequently called code segments. Object pooling or caching technology can be used to reuse objects to reduce the overhead of memory allocation and recycling.
Reduce unnecessary object copies: Object copies take up additional memory and CPU time. When necessary, you can use references or pointers to avoid unnecessary object copies. For example, you can pass by reference or pass a pointer to an object instead of the entire object.
Pay attention to reference counting: Lua’s garbage collection mechanism uses reference counting to manage the life cycle of objects. Ensure that the object’s reference count is increased and decreased at the correct time to avoid reference counting errors that may cause the object to be recycled incorrectly or prematurely.
Avoid circular references: Circular references refer to objects forming a mutual reference relationship that prevents them from being garbage collected. Avoid creating circular references, or if circular references are required, use weak references or break them manually.
Properly set garbage collection parameters: Lua provides some parameters for adjusting the behavior of the garbage collection mechanism, such as garbage collection thresholds, collection intervals, etc. Depending on the specific scenario, these parameters can be adjusted to achieve better performance and memory management.
Pay attention to memory leaks: A memory leak refers to the inability to access and release objects that are no longer used, resulting in a continuous increase in memory usage. Ensure that references to objects are released promptly when they are no longer needed to avoid memory leaks. Especially for objects that involve external resources (such as files, network connections, etc.), ensure that they are properly released when not in use.
Use appropriate data structures: Choosing appropriate data structures can optimize memory usage and access efficiency. Based on actual needs, select appropriate table structures, string types, and data collections to improve performance and save memory.
Understand and utilize Lua’s optimization techniques: Lua provides some optimization techniques, such as local variable caching, avoiding too many intermediate variables, using tail recursion optimization, etc. Understanding and correctly applying these techniques can improve code execution efficiency and heap performance.
Processing large data in batches: For the case of processing large data, consider processing the data in batches instead of loading all the data into the heap at once. This reduces memory usage and improves processing efficiency.
Avoid unnecessary data copying: In Lua, certain operations may cause data to be copied, such as string concatenation, deep copy of tables, etc. Try to avoid unnecessary data copy operations, especially operations on large data structures. If you really need to perform operations, you can consider using in-place modification to reduce the cost of data copying.
Use lightweight data structures: In Lua, some data structures are relatively lightweight and occupy less memory. For example, use lightweight tables (light userdata) instead of full Lua tables, or use numerically indexed arrays (array) instead of associative arrays (table). Depending on specific needs, choosing the appropriate data structure can reduce memory overhead.
Avoid frequent garbage collection: Garbage collection is a resource-intensive operation, and triggering garbage collection frequently will affect performance. Avoid generating too many temporary objects and garbage data, and reasonably control the frequency and scale of garbage collection to reduce performance losses.
Reasonable use of extension libraries: Lua provides a wealth of extension libraries, which can improve performance and memory management effects by using these libraries. For example, use the bit library for bit operations, use the ffi library for more efficient C language interaction, etc. Choosing the right extension library and using it correctly can optimize heap performance.
Perform regular performance analysis and optimization: Regular performance analysis and optimization are key to maintaining heap performance. Use performance analysis tools (such as Lua Profiler) to identify performance bottlenecks and code segments with high memory usage, and make targeted optimizations and improvements.

To sum up, optimizing the performance of the Lua heap requires comprehensive consideration of factors such as memory management, garbage collection, data structure selection, and code writing. By avoiding frequent object creation and destruction, reducing unnecessary object copies, reasonably setting garbage collection parameters, and using appropriate data structures, heap performance and memory utilization efficiency can be improved. At the same time, regular performance analysis and optimization are important steps to continuously maintain heap performance.

What is stored in the stack

In Lua, the stack is a data structure used to store and operate data. It is the core component of the Lua virtual machine. The Lua stack is a last-in-first-out (LIFO) data structure used to store and track Lua function parameters, local variables, intermediate results, and return values from function calls.

Stored on the stack are copies of various Lua values. Lua values can be of different types such as integers, floating point numbers, strings, Boolean values, tables, functions, etc. When executing code in Lua, the stack is used to store these values and operate on them.

The bottom of the stack is the location with index 1, also known as the bottom of the stack. The top of the stack is at index n, where n is the number of elements currently in the stack, also known as the top of the stack. The values in the stack can be accessed and manipulated through the index between the top and bottom of the stack.

In Lua, the use of the stack is very flexible and can be used in many aspects such as function calling, parameter passing, return value processing, and local variable storage. Stack operation functions (such as lua_pushxxx, lua_pop, lua_getxxx, lua_setxxx, etc.) are used to push, pop, and access data on the stack.

It should be noted that the data on the stack is temporarily stored and is not persisted. When the function call ends or certain operations are completed, the data on the stack will be automatically popped for subsequent operations. Therefore, when using the stack, you need to pay attention to the life cycle of the data and ensure that data is pushed and popped onto the stack correctly when needed to avoid data loss or errors.

In Lua, a stack is an array-based data structure used to store and manipulate Lua values. It has the following characteristics:

Indexing mechanism:

The Lua stack uses integer indexing to access stack elements, with indexes starting at 1 and increasing. The element index at the top of the stack is 1, increasing in sequence.
A negative index represents an offset from the top of the stack, -1 represents the element at the top of the stack, -2 represents the element below the top of the stack, and so on.

2. Push operation:

- By using different types of lua_pushxxx functions, various types of values can be pushed onto the stack. For example, lua_pushnumber is used to push a number onto the top of the stack, and lua_pushstring is used to push a string onto the top of the stack.
- The push operation will change the position of the top of the stack, and the index of the top of the stack will automatically increase.

3. Pop stack operation:

- By using the lua_pop function, you can pop a value from the top of the stack and decrement the top index of the stack.
- The stack pop operation will remove the value on the top of the stack, and the top index of the stack will be automatically decremented.

4. Top of stack control:

- The lua_gettop function is used to get the top index of the stack, which is the number of elements in the stack.
- The lua_settop function is used to set the top index of the stack, which can increase or decrease the number of elements in the stack.

Stacks are used very frequently in the Lua C API. It is used for operations such as passing parameters, saving local variables, and returning function results. For example, when you call a C function from a Lua script, the function parameters are pushed onto the stack, and the C function can retrieve these parameters through the stack and process them accordingly. Similarly, the return value of a C function can also be returned to the Lua script through the stack.

Understanding the techniques and best practices for using the Lua stack is important for writing efficient and reliable Lua extensions. Here are some suggestions:

Debugging and error handling: Use the lua_gettop function to check the status of the stack to ensure that the number of elements in the stack is correct to avoid stack overflow or empty stack errors.
Parameter and return value handling: Pay attention to stack order, push parameters onto the stack in the correct order, and correctly obtain and handle return values in C functions.
Memory management: The memory allocated in the stack needs to be released in time to avoid memory leaks. You can use lua_newuserdata to create user data objects to manage custom data structures. Avoid excessive stack operations: Frequent push and pop operations will lead to performance degradation. Minimize unnecessary stack operations and optimize code logic.
The correct order of using the stack: When performing complex operations, you need to ensure that the state of the stack is correct and follow the first-in, last-out principle. For example, if you need to get the value in the table, you first need to push the table itself onto the stack, then push the key onto the stack, and finally call lua_gettable to get the value.
Pay attention to the range of the index: the stack index must be within the valid range and must not exceed the boundaries of the stack. Out-of-range index accesses can cause unpredictable results or crashes.
Saving and restoring the stack: When you need to perform nested Lua calls or use multiple Lua state machines, you can use the lua_pushvalue and lua_replace functions to save and restore the state of the stack. Make sure to use the same stack in different contexts.
Error handling and stack recovery: When writing Lua C extensions, catch and handle errors in a timely manner to avoid unhandled exceptions. When an error occurs, attention needs to be paid to restoring the stack to the correct state to avoid residual data in the stack from affecting subsequent operations.

Points to note when using:

Stack index range: Lua stack uses 1-based indexing, that is, the index of the first element is 1. Be careful not to exceed the valid index range of the stack, otherwise it will cause undefined behavior or crash.
Saving and restoring the stack: Before executing a Lua C call, you can use lua_pushvalue to copy the stack value that needs to be saved, and then use lua_replace to copy the saved value after the call. The stack value is restored back to keep the stack in a consistent state.
Clearing the stack: Before executing a Lua C call, you can use lua_settop to clear the stack so that it only retains a specified number of elements. This helps avoid stack overflows and confusing states.
Balance of the stack: When performing stack operations, the balance of the stack must be maintained. That is, each time a value is pushed in, the stack must be restored to its original state through a corresponding number of stack pop operations to avoid excessive growth of the stack.
Correct use order of the stack: When performing complex operations, ensure that the state of the stack is correct. For example, if you need to get a value from a table, first push the table itself onto the stack, then push the keys, and finally use lua_gettable to get the value. Pay attention to the order of operations and follow the principle of first in, last out.
Error handling and stack recovery: When writing Lua C extensions, errors must be caught and handled in a timely manner to avoid unhandled exceptions. When an error occurs, attention should be paid to restoring the stack to the correct state to prevent residual data in the stack from affecting subsequent operations.
Avoid frequent stack operations: Frequent push and pop operations will lead to performance degradation. Minimize unnecessary stack operations and optimize code logic. You can use local variables or temporary variables to reduce the number of stack operations.

The stack performs operations when:

Function call: When a function is called, the function’s parameters and local variables will be pushed onto the stack. After the function is executed, the stack will be cleared.
Expression evaluation: When evaluating an expression, it involves pushing operands and operators onto the stack. For example, variables, constants, operators, etc. are pushed onto the stack for calculation.
Control flow statements: In control flow statements such as conditional judgments and loop statements, the stack may be used to store temporary data such as the results of conditional judgments and loop variables.
Function return value: When the function completes execution and returns a value, the return value will be pushed onto the stack for use by the calling function.
Coroutine: A coroutine is a lightweight thread that can pause and resume execution. When coroutines are switched, the state of the stack will be saved and restored.

It should be noted that the execution of the stack is handled by the interpreter or virtual machine, and developers do not directly operate the stack when writing code. Stack operations are performed implicitly by the interpreter or virtual machine during code execution. Understanding the execution operations of the stack can help you understand the underlying mechanism of code execution and debug errors.

The reasons why local variables or temporary variables can reduce the number of stack operations are as follows:
Reduce stack read and write operations: When using local variables or temporary variables, you can directly read and write operations in registers or memory without accessing data through the stack. This can avoid frequent read and write operations on the stack and improve code execution efficiency.

Reduce stack push and pop operations: Each time a value is pushed onto the stack or popped from the stack, a corresponding stack operation is required. When using local variables or temporary variables, you can store the value in the local variable or temporary variable before the stack operation, and then use the variable directly to avoid unnecessary stack operations.
Reduce the depth of the stack: When frequent stack operations are performed, the depth of the stack will increase and occupy more memory space. When using local variables or temporary variables, unnecessary stack operations can be avoided, thereby reducing the depth of the stack and saving memory space.

When using local variables or temporary variables, you can reduce the read and write operations on the stack and the stack push/pop operations. Here is a simple example code that demonstrates the use of local variables and temporary variables:

-- use local variables
local a = 10
local b = 20
local c = a + b
print(c) -- output: 30

-- Use temporary variables
function calculateSum(x, y)
  local temp = x + y
  return temp
end

local result = calculateSum(5, 3)
print(result) -- Output: 8

In the above example, we used local variables and temporary variables to store intermediate results or temporary values, avoiding frequent read and write operations on the stack and stack push/pop operations. This can improve the execution efficiency of the code.

Note that in actual development, unless necessary, it is usually recommended to use local variables instead of global variables to limit the scope of variables and improve the readability and maintainability of the code. At the same time, reasonable use of temporary variables can simplify complex calculations or logic and improve the understandability of the code.

The above code examples are for demonstration purposes only. In actual use, please use local variables and temporary variables flexibly according to specific situations and needs.