Gcc stack optimization

Question

Hi I have a question about the possible optimization of the gcc (or g ++) stack ..

Sample code in FreeBSD (does UNIX variance matter here?):

void main() { char bing[100]; .. string buffer = ....; .. } 

What I found in gdb for the coredump of this program is that the bing address is actually less than this buffer (namely, the bing [0] &] buffer.

I think that this is completely contrary to what was said in the textbook. Could there be some compiler optimization that reorganizes the stack structure in this way?

This seems to be the only possible explanation, but I'm not sure.

If you're interested, coredump is caused by overflow of the bing buffer into the buffer (but this also confirms & bing [0] & buffer).

Thanks!

Answer 1

Compilers are free to organize stack frames (provided they even use stacks) in any way.

They can do this for alignment reasons or for performance reasons or for no reason. It would be unreasonable to suggest any particular order.

If you did not invoke undefined behavior by overflowing the buffer, you probably would never know how it should be.

The compiler can not only regroup its variables, but also optimize them from existence, if he can establish that they are not used. With code:

 #include <stdio.h> int main (void) { char bing[71]; int x = 7; bing[0] = 11; return 0; } 

Compare normal assembler output:

 main: pushl %ebp movl %esp, %ebp andl $-16, %esp subl $80, %esp movl %gs:20, %eax movl %eax, 76(%esp) xorl %eax, %eax movl $7, (%esp) movb $11, 5(%esp) movl $0, %eax movl 76(%esp), %edx xorl %gs:20, %edx je .L3 call __stack_chk_fail .L3: leave ret 

with insanely optimized:

 main: pushl %ebp xorl %eax, %eax movl %esp, %ebp popl %ebp ret 

Pay attention to something missing in the latter? Yes, there is no stack manipulation to create space for bing or x . They do not exist. In fact, the whole sequence of code comes down to the following:

• set the return code to 0.
• return.

Answer 2

The compiler can freely place local variables on the stack (or store them in a register or do something else with them), however, it considers it necessary: ​​the C and C ++ language standards do not say anything about these implementation details, nor does POSIX or UNIX . I doubt that your textbook told you otherwise, and if that were the case, I would look for a new textbook.