关于栈对齐(stack alignment)

最新推荐文章于 2024-12-17 19:18:10 发布

Sco_field

最新推荐文章于 2024-12-17 19:18:10 发布

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分类专栏： C/C++ 文章标签： alignment 编译器 performance 优化 system gcc

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栈对齐在保护模式内存管理中至关重要，栈指针应当根据段描述符的D标志保持16位或32位对齐。处理器不检查栈指针对齐，程序、任务和系统过程需确保正确对齐，否则可能导致性能下降或程序失败。编译器如VC++6.0和GCC在DEBUG与RELEASE版本间，由于优化策略不同，栈变量的占用空间也会有所变化。

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手册上介绍:

The stack pointer for a stack segment should be aligned on 16-bit (word) or 32-bit (double-word)
boundaries, depending on the width of the stack segment. The D flag in the segment descriptor
for the current code segment sets the stack-segment width (refer to Chapter 3, Protected-Mode
Memory Management of the Intel Architecture Software Developer’s Manual, Volume 3). The
PUSH and POP instructions use the D flag to determine how much to decrement or increment
the stack pointer on a push or pop operation, respectively. When the stack width is 16 bits, the
stack pointer is incremented or decremented in 16-bit increments; when the width is 32 bits, the
stack pointer is incremented or decremented in 32-bit increments.
The processor does not check stack pointer alignment. It is the responsibility of the programs,
tasks, and system procedures running on the processor to maintain proper alignment of stack
pointers. Misaligning a stack pointer can cause serious performance degradation and in some
instances program failures.

在实际测试时，需要视编译器而定，即便是相同的编译器，不同的优化策略得出的结果也有可能不同。

比如：

char ch1;

char ch2;

在VC++6.0的debug版本，分别占用4个字节，但若是RELEASE版本为了优化大小，则分别只占用1个字节。

再比如GCC，DEBUG版本直接就是分别占用1个字节。