为什么GCC会分配不使用的堆栈空间？

Question

In many functions the function prologue starts with sub $0x1c,%esp , which is later removed by add $0x1c,%esp . The function can be as simple as doing virtually nothing. This space is never used however, and has become an annoyance to work with as I need full control over the stack of my functions. Any idea what causes this, and how can I remove it? I'm compiling with -O2 -Wall -c -m32 -fno-stack-protector -fno-defer-pop -fno-builtin -march=i386 .

Answer 1

It would help to see more code but it's to do with stack alignment. You can mess around with this in GCC using -mpreferred-stack-boundary. The default on my machine is 4 so things are aligned to 16 bytes. I'm pretty confident this has nothing to do with stack protection. To see what happens with stack protection compile the program with -fno-stack-check and -fstack-check and diff the results. You'll see something like

>   orl $0, -12300(%esp)