Assembly Code in C++

I would like to learn reading the assembly code generated by the compiler. Where and how coul开发者_Go百科d I assess the assembly code generated from C++ ?

Thanks

Your compiler likely has an option to generate assembly code output, optionally interleaved with the corresponding source code. In Microsoft Visual C++ v10 this is /Fa.

Or, just look at the two side-by-side in your debugger.

However you look at this, be sure to compare the versions built with and without optimization. It's amazing to see how much can be discarded by today's compilers without affecting the operation of the program.

If you are using gcc, use the -S argument and the compiler's output won't go through the assembler.

From your object file:

$ g++ -g -c -Wall yourfile.cpp -o yourfile.o

Then:

$ gdb yourfile.o

Once in GDB you could use disassemble command to view the generated assembly.

So, if your C++ source is:

int f() { return 1; }

You can do in GDB:

(gdb) disassemble f

And the output will be:

Dump of assembler code for function f:
0x00000000 <f+0>:       push   %ebp
0x00000001 <f+1>:       mov    %esp,%ebp
0x00000003 <f+3>:       mov    $0x1,%eax
0x00000008 <f+8>:       pop    %ebp
0x00000009 <f+9>:       ret

For GCC and objdump. Using GCC to produce readable assembly?

For Visual Studio, if you are using the IDE, you can modify the C/C++ 'Output Files' property in your project's properties, and change 'Assembler Output' to 'Assembly with Source Code'

This is also the '/Fas' flag for Visual C++ compiler.

//a.cpp
#include <iostream>

int main()
{
   std::cout << "hello";
} 

On gcc you can use the -S option, i.e gcc -S a.cpp generates a.s

(a.s):

        .file   "a.cpp"
.lcomm __ZStL8__ioinit,1,1
        .def    ___main;        .scl    2;      .type   32;     .endef
        .section .rdata,"dr"
LC0:
        .ascii "hello\0"
        .text
.globl _main
        .def    _main;  .scl    2;      .type   32;     .endef
_main:
        pushl   %ebp
        movl    %esp, %ebp
        andl    $-16, %esp
        subl    $16, %esp
        call    ___main
        movl    $LC0, 4(%esp)
        movl    $__ZSt4cout, (%esp)
        call    __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
        movl    $0, %eax
        leave
        ret
        .def    ___tcf_0;       .scl    3;      .type   32;     .endef
___tcf_0:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        movl    $__ZStL8__ioinit, (%esp)
        call    __ZNSt8ios_base4InitD1Ev
        leave
        ret
        .def    __Z41__static_initialization_and_destruction_0ii;       .scl
3;      .type   32;     .endef
__Z41__static_initialization_and_destruction_0ii:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        cmpl    $1, 8(%ebp)
        jne     L3
        cmpl    $65535, 12(%ebp)
        jne     L3
        movl    $__ZStL8__ioinit, (%esp)
        call    __ZNSt8ios_base4InitC1Ev
        movl    $___tcf_0, (%esp)
        call    _atexit
L3:
        leave
        ret
        .def    __GLOBAL__I_main;       .scl    3;      .type   32;     .endef
__GLOBAL__I_main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        movl    $65535, 4(%esp)
        movl    $1, (%esp)
        call    __Z41__static_initialization_and_destruction_0ii
        leave
        ret
        .section        .ctors,"w"
        .align 4
        .long   __GLOBAL__I_main
        .def    __ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc;
.scl    2;      .type   32;     .endef
        .def    __ZNSt8ios_base4InitD1Ev;       .scl    2;      .type   32;
.endef
        .def    __ZNSt8ios_base4InitC1Ev;       .scl    2;      .type   32;
.endef
        .def    _atexit;        .scl    2;      .type   32;     .endef