[C/C++] Explore the secrets and advantages of memory alignment

Table of contents

I. Introduction

Second, what is memory alignment?

3. The principle of memory alignment

4. Advantages of memory alignment

5. How to achieve memory alignment? (Just watch this section)

1. Example of using #pragma pack to achieve memory alignment

7. Practical applications of memory alignment

8. Conclusion

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I. Introduction

• The physical structure of computer memory is in bytes, and each byte has a unique address. When a program accesses data in memory, the computer can read or write the data faster if the data happens to be at a naturally aligned address. If data is not stored in proper memory alignment, the computer may need to perform multiple read or write operations, slowing access and increasing the load on the processor.
• In the field of computer science, memory is the core of storing and processing data. However, data storage involves not only the storage unit itself, but also how the data is placed in memory. This involves the concept of Memory Alignment, a mysterious yet critical technology that is crucial to optimizing program performance and improving memory access efficiency.

Second, what is memory alignment?

• Memory Alignment refers to a rule or convention for placing data at a specific address when storing data in computer memory. A computer's memory is addressed in bytes, and memory alignment requires data to be stored in multiples of its size. In other words, memory alignment requires that the storage address of data in memory must be an integral multiple of the size of the data type. The main purpose of memory alignment is to optimize memory access speed and memory access efficiency.
• For example, a 4-byte (32-bit) integer should be placed in memory at addresses 4, 8, 12, etc., rather than 3, 7, 11, etc. This specification ensures that data storage always starts at the correct byte location, allowing the processor to access the data efficiently.
• The reason for memory alignment is the way modern computers access memory. Most computer architectures require that data must be read from memory into a cache line before being loaded into processor registers. A cache line is a fixed-size block, usually 64 bytes or 128 bytes. If the data is not stored in a memory-aligned manner, then a piece of data may span multiple cache lines, which will result in multiple memory accesses and reduce access efficiency.

3. The principle of memory alignment

Explanation of the principles of memory alignment

1. Processor and memory structure: The memory of modern computers is usually divided into consecutive byte addresses, and each byte has a unique number. When the processor is executing, it needs to read data from memory and then load it into registers for operation. However, processors do not always transfer data in units of single bytes, but in larger units such as words (2 bytes) or double words (4 bytes).
2. Cache line: Processors typically work with a cache, a small, high-speed memory located inside the processor that is used to temporarily store data read from memory. The basic storage unit of the cache is the cache line, which is a fixed-size memory block, usually 64 bytes or 128 bytes. When the processor reads data from memory, it loads the entire cache line into the cache.
3. Memory access and alignment requirements: Processors usually require that before data is loaded from memory into a register or cache, the starting address of the data must be an integral multiple of the data type size, that is, memory alignment requirements are met. If the data is not aligned as required, the processor may need to make additional memory accesses, causing performance degradation. This is because the processor cannot load unaligned data directly from memory and must perform additional operations, such as combining multiple memory accesses, to obtain sufficient data.
4. Data spans cache lines: If data is not stored according to alignment requirements, it may span multiple cache lines. When the processor needs to access data that spans cache lines, multiple cache lines may need to be loaded, and existing cache lines may even need to be flushed. This will cause additional memory access overhead and reduce performance.

Summarize

• The principle of memory alignment is to meet the memory access requirements of the processor and ensure that data is stored as an integral multiple of its size, so that the processor can access the data in the most efficient way. This alignment reduces unnecessary memory accesses, improving data transfer speeds and program performance. The compiler usually automatically sets memory alignment according to the rules of the target architecture, but in specific cases, developers may need to manually control memory alignment to meet specific needs.

4. Advantages of memory alignment

1. Improved access speed: Memory alignment allows the processor to access data in a more efficient manner. When data is stored according to memory alignment requirements, the processor can obtain more data with a single memory access, thereby reducing the need for multiple memory accesses. This helps increase program execution speed.
2. Reduce the number of memory accesses: Memory alignment can reduce unnecessary memory accesses. If data is not stored according to memory alignment, the processor may require multiple memory accesses to obtain a data item, which results in increased latency and reduced performance. With memory alignment, the processor can read the required data from memory more efficiently.
3. Improve cache efficiency: Modern computers are usually equipped with multi-level cache, and cache lines are the smallest cache units. Memory alignment ensures that data is stored according to the cache line size, thereby better utilizing the cache and reducing unnecessary cache refreshes and data loads.
4. Hardware compatibility: Some hardware architectures and communication protocols have strict requirements for memory alignment. If data is not stored as required, communication errors or hardware failures may result. Memory alignment can ensure that data meets hardware requirements and improve system stability and reliability.
5. Correctness guarantee: In composite data types (such as structures and unions), memory alignment can ensure the correct order of members and prevent data layout confusion, thereby ensuring the correctness of the program.
6. Optimize program performance: Memory alignment can be used as part of program performance optimization. By following memory alignment rules, developers can make the most of the processor's optimization features, allowing programs to achieve better performance on different platforms.

5. How to achieve memory alignment? (Just watch this section)

• In C or C++ programming, #pragma packa preprocessing directive used to control the memory alignment of structures, unions, and class members. Memory alignment means that when allocating variables in memory, the variable is placed at a location whose address is a multiple of its size in order to improve access speed and memory access efficiency. By default, the compiler will do the appropriate memory alignment based on the target architecture and compilation options, but sometimes you may need to manually control the memory alignment, which is #pragma packwhere .
• #pragma pack(n)Sets the memory alignment of structures, unions, and class members to n bytes. Typically, n is 1, 2, 4, 8, or other suitable positive integer. For example, #pragma pack(1)setting the memory alignment to 1 byte means that the structure members will be aligned on 1-byte boundaries.
• Use #pragma packto optimize memory usage in some specific cases, or to ensure that it matches the requirements of specific hardware or communication protocols. However, it should be noted that setting a memory alignment value that is too small may cause the efficiency of memory access to decrease, and may even cause program errors in some cases. Therefore, #pragma packyou need to be careful when using it to make sure you understand its effects and make appropriate settings where needed.

1. #pragma packExample of using to implement memory alignment

#include <iostream>

// 默认的内存对齐方式
struct DefaultAlignment {
    char a;
    int b;
    short c;
};

// 使用 #pragma pack(1) 设置内存对齐为 1 字节
#pragma pack(1)
struct PackedAlignment {
    char a;
    int b;
    short c;
};
#pragma pack() // 恢复默认的内存对齐设置，不然有可能会对系统产生影响

int main(int argc,char* argv[])
{
    std::cout << "Size of DefaultAlignment: " << sizeof(struct DefaultAlignment) << std::endl;
    std::cout << "Size of PackedAlignment: " << sizeof(struct PackedAlignment) << std::endl;
    return 0;
}

Output value

Size of DefaultAlignment: 12
Size of PackedAlignment: 7

7. Practical applications of memory alignment

Memory alignment has a wide range of practical applications in computer programming, involving performance optimization, hardware compatibility, data communication and other fields. The following are some practical application scenarios of memory alignment:

1. Embedded Systems Development: In embedded systems, resources are often constrained and performance and efficiency are critical. Memory alignment can help maximize the use of limited memory, improve data access speed, and adapt to different hardware platforms.
2. Network communications: In data communications, different computers may have different endianness (big endian or little endian). Memory alignment ensures that data is arranged as expected bytes during transmission, thereby avoiding communication errors and parsing issues.
3. Structures and unions: Structures and unions are composite data types, and memory alignment ensures that structure members are arranged in the expected order and bytes. This is important for operations such as bit manipulation, data serialization and deserialization in data structures.
4. Hardware driver development: In drivers that interact with hardware, memory alignment ensures that the driver correctly reads and writes hardware registers, as well as data packets that communicate with the hardware.
5. Graphics processing: Data structures in graphics processing (e.g., vertex data, texture data) often need to be aligned for efficient access and rendering on graphics hardware.
6. High-performance computing: In high-performance computing and scientific computing, the memory layout and alignment of data can affect the parallelism and vectorization efficiency of the calculation.
7. Data serialization and deserialization: During data exchange and storage, the data needs to be serialized into a byte stream and then deserialized into original data. Memory alignment ensures consistent serialization and deserialization.
8. Cross-platform development: In multi-platform development, different hardware architectures and operating systems may have different requirements for memory alignment. Developers may need to manually set memory alignment to ensure code consistency across different platforms.

8. Conclusion

• Although memory alignment may be overlooked in daily programming, it has an important impact on the performance and stability of the program. By understanding the principles and benefits of memory alignment, developers can better optimize code, improve application performance, and ensure compatibility with specific hardware and communication protocols. Therefore, whether you are a beginner or an experienced developer, it is worth in-depth study and application of memory alignment technology to contribute to code optimization and performance improvement.