Trying to understand gcc's complicated stack-alignment at the top of main that copies the return address

Question

hi I have disassembled some programs (linux) I wrote to understand better how it works, and I noticed that the main function always begins with:

lea    ecx,[         


        

Answer 1

This is done to keep the stack aligned to a 16-byte boundary. Some instructions require certain data types to be aligned on as much as a 16-byte boundary. In order to meet this requirement, GCC makes sure that the stack is initially 16-byte aligned, and allocates stack space in multiples of 16 bytes. This can be controlled using the option -mpreferred-stack-boundary=num. If you use -mpreferred-stack-boundary=2 (for a 2²=4-byte alignment), this alignment code will not be generated because the stack is always at least 4-byte aligned. However you could then have trouble if your program uses any data types that require stronger alignment.

According to the gcc manual:

On Pentium and PentiumPro, double and long double values should be aligned to an 8 byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16 byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

The lea loads the original stack pointer (from before the call to main) into ecx, since the stack pointer is about to modified. This is used for two purposes:

1. to access the arguments to the main function, since they are relative to the original stack pointer
2. to restore the stack pointer to its original value when returning from main