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Examining the assembly listing generated by the C++ compiler - I

Rama Krishna Vavilala

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26 May 2002

Examining the assembly listing generated by the C++ compiler

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Introduction

VC++ compiler can create a text file that shows the assembly code generated for a C/C++ file. I have often used this file to see the kind of code the compiler generates. This file provides a great insight into some of the concepts like exception handling, vtables, vbtables etc. A very elementary knowledge of assembly language is sufficient to understand the output of the listing file. (Refer to Matt Pietrek's article on basic assembly language for a brief introduction to assembly language.) The purpose of this article (which is one of the two articles in the series) is to see how the listing file aids us in understanding the inner workings of the C++ compiler.

Setting the Listing File

You can set the C/C++ compiler options to generate the listing file in Project Settings dialog of VC6 as shown below.

In VC++.NET you can set the same option in the Project Properties dialog.

The different types of listing generated by the compiler are :-

Assembly code only (.asm)
Assembly code and machine code. (.cod)
Assembly code with source code. (.asm)
Assembly code with machine and the source code. (.cod)

Viewing the Listing File (s)

Lets examine the listing generated for the following application.

#include <stdio.h>


int main(int argc, char* argv[])
{
    printf("Hello World!");
    return 0;
}

1. Assembly Only Listing (/FA)

The assembly listing is place in a file with .asm extension in the intermediate directory. For example if the file name is main.cpp then there would be a main.asm file in the intermediate directory. Here is a code snippet of the main function from the listing file:-

PUBLIC  _main
PUBLIC  ??_C@_0N@GCDOMLDM@Hello?5World?$CB?$AA@ ; `string'

EXTRN   _printf:NEAR
; COMDAT ??_C@_0N@GCDOMLDM@Hello?5World?$CB?$AA@

; File g:\wksrc\compout\main.cpp

CONST   SEGMENT
??_C@_0N@GCDOMLDM@Hello?5World?$CB?$AA@ DB 'Hello World!', 00H ; `string'

; Function compile flags: /Ogty

CONST   ENDS
;   COMDAT _main

_TEXT   SEGMENT
_argc$ = 8
_argv$ = 12
_main   PROC NEAR ; COMDAT

; Line 5

    push    OFFSET FLAT:??_C@_0N@GCDOMLDM@Hello?5World?$CB?$AA@
    call    _printf
    add esp, 4
; Line 6

    xor eax, eax
; Line 7

    ret 0
_main   ENDP
END

Let's try to examine the listing.

Lines beginning with ; are comments
PUBLIC _main means that the _main function is shared with other files (as opposed to static functions). For static functions there is no prefix.
CONST SEGMENT indicates beginning of a CONST data segment. VC++ compiler places constant data like strings in this section. So we see that the string "Hello World" is placed in the CONST segment. Altering any data in the const segment would cause an access violation exception to be thrown. More on this later.
_TEXT SEGMENT marks the beginning of another segment. The compiler places all the code in this segment.
_argc$ = 8 and _argv$ = 12 indicate the stack positions of the arguments argc and argv. In this case it means if you add 8 to the stack pointer (CPU register ESP) you get the address of the argc parameter. The offset of 4 would be for the return address.
_main PROC NEAR signals the start of the function _main. Notice that for C functions (functions declared with extern "C") the name is prefixed with _ for C++ function the name is decorated.
Next we see that the compiler pushes the address of the string "Hello World" to the stack and calls the function printf. After the function call is over the stack pointer is incremented by 4 (as printf has C calling convention )
EAX is the register which hold the return value of a function. We see that EAX is XORed with itself. (This is a quick way to set a register to zero.) This is because our original code contains returns 0 from function main.
Finally the ret 0 is the instruction for returning from the function. The numeric argument 0 following the ret instruction indicates the amount by which the stack pointer should be incremented.

So this was assembly only listing. Lets see how the other three listings look like.

2. Assembly With Source Code (/FAs)

This listing gives far more clearer picture then the first one. It shows the source text as well as the assembly code.

_TEXT   SEGMENT
_argc$ = 8
_argv$ = 12
_main   PROC NEAR ; COMDAT


; 5    :    printf("Hello World");


    push    OFFSET FLAT:??_C@_0M@KPLPPDAC@Hello?5World?$AA@
    call    _printf
    add esp, 4

; 6    :    return 0;


    xor eax, eax

; 7    : }

3. Assembly With Machine Code (/FAc)

The listing shows the instruction codes as well as the instruction mnemonics. This listing is normally generated in a .cod file. So for this example the listing could be seen in main.cod file.

;   COMDAT 

_main   _TEXT SEGMENT
_argc$  = 8 
_argv$ = 12 
_main PROC
NEAR  ; COMDAT        

; Line 5 

00000 68 00 00 00 00 push OFFSET     
            FLAT:??_C@_0M@KPLPPDAC@Hello?5World?$AA@
                 
00005 e8 00 00 00 00 call _printf    
0000a   83 c4 04 add esp,  4   

; Line 6 

0000d 33  c0    xor eax, eax     

; Line 7 

0000f c3  ret   0        
_main   ENDP

4. Assembly, Machine and Source Code (/FAsc)

This listing is also generated in a .cod file. As expected this shows source code as well as the machine code with the assembly.

;   COMDAT _main

_TEXT   SEGMENT
_argc$ = 8
_argv$ = 12
_main   PROC NEAR  ; COMDAT


; 5    :    printf("Hello World");


  00000 68 00 00 00 00   push    
      OFFSET FLAT:??_C@_0M@KPLPPDAC@Hello?5World?$AA@
  00005 e8 00 00 00 00   call    _printf
  0000a 83 c4 04     add     esp, 4

; 6    :    return 0;


  0000d 33 c0        xor     eax, eax

; 7    : }


  0000f c3       ret     0
_main   ENDP

So we see all the four types of listing generated by the compiler. In general it is not necessary to look at the machine code. In most cases Assembly with Source (/FAs) is the most useful listing.

Having seen the different types of listings and how to generate the listings let's see what useful information we could gather from a listing.

Const Segment

We saw that the compiler placed the constant string "Hello World" in CONST segment. Let's study the implications of this through the following sample application.

#include <stdio.h> <stdio.h>


char* szHelloWorld = "Hello World";

int main(int argc, char* argv[])
{
    printf(szHelloWorld);

    szHelloWorld[1] = 'o';
    szHelloWorld[2] = 'l';
    szHelloWorld[3] = 'a';
    szHelloWorld[4] = '\'';

    printf(szHelloWorld);

    return 0;
}

This sample app first prints "Hello World", tries to convert the string "Hello" to "Hola'" and finally prints altered string. Lets build and run this application. To our amazement we see that the application crashes with access violation exception and the line szHelloWorld[2] = 'l';.

Let's alter the line

char* szHelloWorld = "Hello World";

char szHelloWorld[] = "Hello World";

The application runs successfully this time. Examining the listing shows us why.

In the first case the data "Hello World" is placed in the CONST segment which is a read only segment

CONST	SEGMENT
?szHelloWorld@@3PADA DB 'Hello World', 00H ; szHelloWorld

CONST	ENDS

In the second case the data is placed in _DATA segment which is a read write segment

_DATA	SEGMENT
?szHelloWorld@@3PADA DB 'Hello World', 00H ; szHelloWorld

_DATA	ENDS

Function Inlining

One of the most useful thing that can be seen from the assembly listing is whether a function is inlined or not. The inline directive or _declspec(inline) does not force the compiler to make a function inline. There are variety of factors (mostly unknown) which determine whether a function is inlined or not. Nor does it mean that not adding inline directive before a function would not make it inline. Assembly listing is very valuable in finding out whether a function is inlined or not. Lets take the following example :-

void ConvertStr(char* argv)
{
    szHelloWorld[1] = 'o';
    szHelloWorld[2] = 'l';
    szHelloWorld[3] = 'a';
    szHelloWorld[4] = '\'';
}

int main(int argc, char* argv[])
{
    printf(szHelloWorld);
    ConvertStr(szHelloWorld);
    printf(szHelloWorld);
    return 0;
}

Lets examine the listing for the main function.

_main   PROC NEAR       ; COMDAT


; 15   :    printf(szHelloWorld);


push OFFSET FLAT:?szHelloWorld@@3PADA    ; szHelloWorld

call  _printf

; 16   :    

; 17   :    ConvertStr(szHelloWorld);

; 18   : 

; 19   :    printf(szHelloWorld);


push OFFSET FLAT:?szHelloWorld@@3PADA    ; szHelloWorld

mov BYTE PTR ?szHelloWorld@@3PADA+1, 111    ; 0000006fH

mov BYTE PTR ?szHelloWorld@@3PADA+2, 108    ; 0000006cH

mov BYTE PTR ?szHelloWorld@@3PADA+3, 97 ; 00000061H

mov BYTE PTR ?szHelloWorld@@3PADA+4, 39 ; 00000027H

call    _printf
add esp, 8

; 20   : 

; 21   :    return 0;


    xor eax, eax

; 22   : }


    ret 0

Note that we don't see any call instruction for ConvertStr instead we see lots of move BYTE PTR instructions which modify the characters in the string (stuff done by the ConvertStr function). This indicates that ConvertStr was in fact expanded inline.

Let's disable inline function expansion of ConvertStr by using _declspec(noinline).

_main   PROC NEAR  ; COMDAT


; 15   :    printf(szHelloWorld);


    push    OFFSET FLAT:?szHelloWorld@@3PADA    
        ; szHelloWorld


    call    _printf

; 16   :    ConvertStr(szHelloWorld);


    push    OFFSET FLAT:?szHelloWorld@@3PADA    
        ; szHelloWorld


    call    ?ConvertStr@@YAXPAD@Z   ; ConvertStr


; 17   :    printf(szHelloWorld);


    push    OFFSET FLAT:?szHelloWorld@@3PADA            
        ; szHelloWorld


    call    _printf
    add esp, 12  ; 0000000cH


; 18   :    return 0;


    xor eax, eax

; 19   : }


    ret 0
_main   ENDP

As expected we see a call instruction to ConvertStr function. Whether or not a function gets inlined is up to the discretion of the compiler. In some cases when an inline function calls another inline function only one function is inlined. Use of #pragma inline_depth() and #pragma inline_recursion() sometimes seems to help. Again the listing file is very useful in showing whether a function was inlined or not.

Destructors

In order to examine the behavior of the destructors, let's take the following example.

class SmartString
{
private:
    char* m_sz;

public:
    SmartString(char* sz)
    {
        m_sz = new char[strlen(sz) + 1];
        strcpy(m_sz, sz);
    }
	
    char* ToStr()
    {
        return m_sz;
    }

    _declspec(noinline) ~SmartString()
    {
        delete[] m_sz;
    }
};

int main(int argc, char* argv[])
{
    SmartString sz1("Hello World");
    printf(sz1.ToStr());

    return 0;
}

The code generated looks like the following.

; 36   : {


    push    ecx

; 37   :    SmartString sz1("Hello World");


push    OFFSET FLAT:??_C@_0M@KPLPPDAC@Hello?5World?$AA@
lea ecx, DWORD PTR _sz1$[esp+8]
call    ??0SmartString@@QAE@PAD@Z       
    ; SmartString::SmartString


; 38   :    printf(sz1.ToStr());


    mov eax, DWORD PTR _sz1$[esp+4]
    push    eax
    call    _printf
    add esp, 4

; 39   : 

; 40   :    return 0;


    lea ecx, DWORD PTR _sz1$[esp+4]

    call    ??1SmartString@@QAE@XZ 
        ; SmartString::~SmartString


    xor eax, eax

; 41   : }


    pop ecx
    ret 0
_main   ENDP

This is quite straight forward and we see clearly the call to the destructor before the function exits. What is interesting is to see how destructors work for array of objects. So we modify the app a little bit.

int main(int argc, char* argv[])
{
    SmartString arr[2];

    arr[0] = ("Hello World");
    arr[1] = ("Hola' World");

    printf(arr[0].ToStr());
    printf(arr[1].ToStr());
	
    return 0;
}

The last few lines of the main function now looks like following.

push    OFFSET FLAT:??1SmartString@@QAE@XZ  
; SmartString::~SmartString


push    2
push    4
lea eax, DWORD PTR _arr$[ebp]
push    eax
call    ??_I@YGXPAXIHP6EX0@Z@Z
xor eax, eax
leave
ret 0

This code is a function call to a function ??_I@YGXPAXIHP6EX0@Z@Z which in English stands for "vector destructor iterator" (can be seen in the listing). This function is automatically generated by the compiler. A translation of the above assembly code in C++ would be :-

vector_destructor_iterator(arr, 2, 4, 
        &SmartString::SmartString);

What exactly is this "vector destructor iterator"? We know that when an array of objects goes out of scope, destructor for each of the objects in array is called. This is what vector destructor iterator does. Lets examine the code of vector destructor iterator and try to reverse engineer it.

PUBLIC  ??_I@YGXPAXIHP6EX0@Z@Z              
; `vector destructor iterator'


; Function compile flags: /Ogsy

;   COMDAT ??_I@YGXPAXIHP6EX0@Z@Z

_TEXT   SEGMENT
___t$ = 8
___s$ = 12
___n$ = 16
___f$ = 20
??_I@YGXPAXIHP6EX0@Z@Z PROC NEAR            
; `vector destructor iterator', COMDAT


    push    ebp
    mov ebp, esp
    mov eax, DWORD PTR ___n$[ebp]
    mov ecx, DWORD PTR ___s$[ebp]
    imul    ecx, eax
    push    edi
    mov edi, DWORD PTR ___t$[ebp]
    add edi, ecx
    dec eax
    js  SHORT $L912
    push    esi
    lea esi, DWORD PTR [eax+1]
$L911:
    sub edi, DWORD PTR ___s$[ebp]
    mov ecx, edi
    call    DWORD PTR ___f$[ebp]
    dec esi
    jne SHORT $L911
    pop esi
$L912:
    pop edi
    pop ebp
    ret 16          ; 00000010H

??_I@YGXPAXIHP6EX0@Z@Z ENDP             
; `vector destructor iterator'

From our previous discussions we know that __t$=8 etc. denote the parameters of the function. We already know the way function was called from the assembly code of the _main function. Based on this we can figure out the signature of the function as :-

typedef void (*DestructorPtr)(void* object);
void vector_destructor_iterator(void* _t, 
        int _n, int _s, DestructorPtr _f)

The function implementation can be reverse engineered to something like

void vector_destructor_iterator(void* _t, int _n, int _s, DestructorPtr _f)
{
    unsigned char* ptr = _t + _s*_n;

    while(_n--)
    {
        ptr -= size;
        _f(ptr);
    }					
}

Basically, it calls destructor for every object in the array. Now we can figure out what the individual parameters mean.

vector_destructor_iterator(arr, 2, 4, 
    &SmartString::SmartString);

The first parameter is obviously the pointer to the array.
The second parameter us the number of elements in the array
The third parameter is size of the individual elements. In our case </li> sizeof(SmartString) (= 4).
The fourth parameter is the address of the destructor function.

In this case the compiler knew exactly the number of elements in the array. So it passes 2 as the size of array to the vector destructor iterator. The question is what happens in case of dynamic arrays allocated using new. In that case the compiler cannot figure out the exact size of the array as the array is allocated at runtime. To find that we modify the application once again.

int main(int argc, char* argv[])
{
    SmartString arr = new SmartString[2];
    arr[0] = "Hello World";
    arr[1] = "Hola' World";

    printf(arr[0].ToStr());
    printf(arr[1].ToStr());
		
    delete [] arr;

    return 0;
}

The assembly listing now looks like:-

; 30   :    

; 31   :    delete [] arr;


    push    3
    mov ecx, esi
    call    ??_ESmartString@@QAEPAXI@Z
    pop edi

; 32   : 

; 33   :    return 0;


    xor eax, eax
    pop esi

We see that there is a new function which has come into picture - ??_ESmartString@@QAEPAXI@Z. The function in English would be called "vector deleting destructor". The assembly code for the vector deleting destructor :-

PUBLIC  ??_ESmartString@@QAEPAXI@Z          
; SmartString::`vector deleting destructor'


; Function compile flags: /Ogsy

;   COMDAT ??_ESmartString@@QAEPAXI@Z

_TEXT   SEGMENT
___flags$ = 8
??_ESmartString@@QAEPAXI@Z PROC NEAR            
; SmartString::`vector deleting destructor', COMDAT


; _this$ = ecx

    push    ebx
    mov bl, BYTE PTR ___flags$[esp]
    test    bl, 2
    push    esi
    mov esi, ecx
    je  SHORT $L896
    push    edi
    push    OFFSET FLAT:??1SmartString@@QAE@XZ  
    ; SmartString::~SmartString


    lea edi, DWORD PTR [esi-4]
    push    DWORD PTR [edi]
    push    4
    push    esi
    call    ??_I@YGXPAXIHP6EX0@Z@Z
    test    bl, 1
    je  SHORT $L897
    push    edi
    call    ??3@YAXPAX@Z ; operator delete

    pop ecx
$L897:
    mov eax, edi
    pop edi
    jmp SHORT $L895
$L896:
    mov ecx, esi
    call    ??1SmartString@@QAE@XZ          
    ; SmartString::~SmartString


    test    bl, 1
    je  SHORT $L899
    push    esi
    call    ??3@YAXPAX@Z ; operator delete

    pop ecx
$L899:
    mov eax, esi
$L895:
    pop esi
    pop ebx
    ret 4
??_ESmartString@@QAEPAXI@Z ENDP             
; SmartString::`vector deleting destructor'


_TEXT   ENDS

This function has a __thiscall calling convention meaning the first parameter this is put in ECX. This is the pseudo calling convention used to invoke member functions. The C++ pseudo code for this would be :-

void SmartString::vector_deleting_destructor(int flags)
{
    if (flags & 2)
    {
        int numElems = *((unsigned char*)this - 4);
        vector_destructor_iterator(this, numElems, 
            4, &SmartString::SmartString);
    }
    else
    {
        this->~SmartString();
    }

    if (flags & 1)
        delete ((unsigned char*)this - 4);
}

So we see that the number of elements is stored just before the first element of the array. This means that the new[] operator should allocate extra 4 bytes. Looking at the assembly generated for the new[] call confirms this.

; 23   : SmartString* arr = new SmartString[2];


    push    12      ; 0000000cH

    call    ??2@YAPAXI@Z    ; operator new

The new operator takes the size to be allocated as a parameter. We see that the number 12 is pushed on the stack. The size of SmartString is only 4 bytes, and the total size of two elements is 8 bytes. So the new operator does allocate extra 4 bytes to mark the number of elements in the array. In order to further confirm this overload the operator new[] in SmartString. It can be seen that the amount of memory requested is always 4 bytes more than the actual memory required to store the array.

Constructors

Lets see the assembly code for the section where the new operator is called.

; 23   : SmartString* arr = new SmartString[2];


    push    12  ; 0000000cH

    call    ??2@YAPAXI@Z    ; operator new

    test    eax, eax
    pop ecx
    je  SHORT $L980
    push    2
    pop ecx
    push    OFFSET FLAT:??0SmartString@@QAE@XZ  
    ; SmartString::SmartString


    push    ecx
    lea esi, DWORD PTR [eax+4]
    push    4
    push    esi
    mov DWORD PTR [eax], ecx
    call    ??_H@YGXPAXIHP6EPAX0@Z@Z
    jmp SHORT $L981
$L980:
    xor esi, esi
$L981:

The ??_H@YGXPAXIHP6EPAX0@Z@Z function is "vector constructor iterator" something similar to vector destructor iterator.

Translated in pseudo C++ code this would look like

unsigned char* allocated = 
    new unsigned char[12]; //Allocate 12 bytes 4 for the size


if (allocated != NULL)
{
    //Put the size in the first four bytes

    //The actual array starts at allocated + 4

    *(int*)allocated = 4; 
	
    vector_constructor_iterator(allocated + 4, 
        4, 2, &SmartString::SmartString);
}

The vector constructor iterator works in the same way as vector destructor iterator. It calls the constructor for all elements in the array.

Exceptions

In all my examples I have disabled exception handling before compiling the application. Enabling exception handling causes lot of extra code to be emitted by the compiler. This has been described in great detail in by Vishal Kocchar.

Calling Conventions

Nemja Trifunjovic describes calling conventions in great detail in his article.

Conclusion

I have tried to examine some aspects of inner workings of the C++ compiler using assembly listing. Examining the assembly listing gives a clear picture of what compiler does under the hood with the C++ code. This will help us in writing better and efficient C++ code. There are lots of other things that can be found out from the assembly listing. In the next article I will discuss how the compiler implements vftables, vbtables and RTTI.

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