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Why is so much space allocated on the stack?

Posted by: admin November 29, 2017 Leave a comment

Questions:

This question comes from answering Stack Overflow question Why do books say, “the compiler allocates space for variables in memory”?, where I tried to demonstrate to the OP what happens when you allocate a variable on the stack and how the compiler generates code that knows the size of memory to allocate. Apparently the compiler allocates much more space than what is needed.

However, when compiling the following

#include <iostream>
using namespace std;

int main()
{
    int foo;
    return 0;
}

You get the following assembler output with Visual C++ 2012 compiled in debug mode with no optimisations on:

int main()
{
00A31CC0  push        ebp
00A31CC1  mov         ebp,esp
00A31CC3  sub         esp,0CCh  // Allocates 204 bytes here.
00A31CC9  push        ebx
00A31CCA  push        esi
00A31CCB  push        edi
00A31CCC  lea         edi,[ebp-0CCh]
00A31CD2  mov         ecx,33h
00A31CD7  mov         eax,0CCCCCCCCh
00A31CDC  rep stos    dword ptr es:[edi]
   int foo;
   return 0;
00A31CDE  xor         eax,eax
}

Adding one more int to my program makes the commented line above to the following:

00B81CC3  sub         esp,0D8h // Allocate 216 bytes

The question raised by @JamesKanze in my answer linked atop, is why the compiler, and apparently it’s not only Visual C++ (I haven’t done the experiment with another compiler), allocated 204 and 216 bytes respectively, where in the first case it only needs four and in the second it needs only eight?

This program creates a 32-bit executable.

From a technical perspective, why may it need to allocate 204 bytes instead of just 4?

EDIT:

Calling two functions and creating a double and two int in main, you get

 01374493  sub         esp,0E8h  // 232 bytes

For the same program as the edit above, it does this in release mode (no optimizations):

 sub    esp, 8                // Two ints
 movsd  QWORD PTR [esp], xmm0 // I suspect this is where my `double` goes
Answers:

This extra space is generated by the /Zi compile option. Which enables Edit + Continue. The extra space is available for local variables that you might add when you edit code while debugging.

You are also seeing the effect of /RTC, it initializes all local variables to 0xcccccccc so that it is easier to diagnose problems due to forgetting to initialize variables. Of course none of this code is generated in the default Release configuration settings.