Using GCC to produce readable assembly?

Question

I was wondering how to use GCC on my C source file to dump a mnemonic version of the machine code so I could see what my code was being compiled into. You can do this with Java but I haven't been able to find a way with GCC.

I am trying to re-write a C method in assembly and seeing how GCC does it would be a big help.

Answer 1

I would like to add to these answers that if you give gcc the flag -fverbose-asm, the assembler it emits will be a lot clearer to read.

Answer 2

If you compile with debug symbols, you can use objdump to produce a more readable disassembly.

>objdump --help
[...]
-S, --source             Intermix source code with disassembly
-l, --line-numbers       Include line numbers and filenames in output

objdump -drwC -Mintel is nice:

• -r shows symbol names on relocations (so you'd see puts in the call instruction below)
• -R shows dynamic-linking relocations / symbol names (useful on shared libraries)
• -C demangles C++ symbol names
• -w is "wide" mode: it doesn't line-wrap the machine-code bytes
• -Mintel: use GAS/binutils MASM-like .intel_syntax noprefix syntax instead of AT&T
• -S: interleave source lines with disassembly.

You could put something like alias disas="objdump -drwCS -Mintel" in your ~/.bashrc

---

Example:

> gcc -g -c test.c
> objdump -d -M intel -S test.o

test.o:     file format elf32-i386


Disassembly of section .text:

00000000 <main>:
#include <stdio.h>

int main(void)
{
   0:   55                      push   ebp
   1:   89 e5                   mov    ebp,esp
   3:   83 e4 f0                and    esp,0xfffffff0
   6:   83 ec 10                sub    esp,0x10
    puts("test");
   9:   c7 04 24 00 00 00 00    mov    DWORD PTR [esp],0x0
  10:   e8 fc ff ff ff          call   11 <main+0x11>

    return 0;
  15:   b8 00 00 00 00          mov    eax,0x0
}
  1a:   c9                      leave  
  1b:   c3                      ret

Answer 3

You can use gdb for this like objdump.

This excerpt is taken from http://sources.redhat.com/gdb/current/onlinedocs/gdb_9.html#SEC64

---

Here is an example showing mixed source+assembly for Intel x86:

  (gdb) disas /m main
Dump of assembler code for function main:
5       {
0x08048330 :    push   %ebp
0x08048331 :    mov    %esp,%ebp
0x08048333 :    sub    $0x8,%esp
0x08048336 :    and    $0xfffffff0,%esp
0x08048339 :    sub    $0x10,%esp

6         printf ("Hello.\n");
0x0804833c :   movl   $0x8048440,(%esp)
0x08048343 :   call   0x8048284 

7         return 0;
8       }
0x08048348 :   mov    $0x0,%eax
0x0804834d :   leave
0x0804834e :   ret

End of assembler dump.

Answer 4

Using the -S switch to GCC on x86 based systems produces a dump of AT&T syntax, by default, which can be specified with the -masm=att switch, like so:

gcc -S -masm=att code.c

Whereas if you'd like to produce a dump in Intel syntax, you could use the -masm=intel switch, like so:

gcc -S -masm=intel code.c

(Both produce dumps of code.c into their various syntax, into the file code.s respectively)

In order to produce similar effects with objdump, you'd want to use the --disassembler-options= intel/att switch, an example (with code dumps to illustrate the differences in syntax):

 $ objdump -d --disassembler-options=att code.c

 080483c4 <main>:
 80483c4:   8d 4c 24 04             lea    0x4(%esp),%ecx
 80483c8:   83 e4 f0                and    $0xfffffff0,%esp
 80483cb:   ff 71 fc                pushl  -0x4(%ecx)
 80483ce:   55                      push   %ebp
 80483cf:   89 e5                   mov    %esp,%ebp
 80483d1:   51                      push   %ecx
 80483d2:   83 ec 04                sub    $0x4,%esp
 80483d5:   c7 04 24 b0 84 04 08    movl   $0x80484b0,(%esp)
 80483dc:   e8 13 ff ff ff          call   80482f4 <puts@plt>
 80483e1:   b8 00 00 00 00          mov    $0x0,%eax
 80483e6:   83 c4 04                add    $0x4,%esp 
 80483e9:   59                      pop    %ecx
 80483ea:   5d                      pop    %ebp
 80483eb:   8d 61 fc                lea    -0x4(%ecx),%esp
 80483ee:   c3                      ret
 80483ef:   90                      nop

and

$ objdump -d --disassembler-options=intel code.c

 080483c4 <main>:
 80483c4:   8d 4c 24 04             lea    ecx,[esp+0x4]
 80483c8:   83 e4 f0                and    esp,0xfffffff0
 80483cb:   ff 71 fc                push   DWORD PTR [ecx-0x4]
 80483ce:   55                      push   ebp
 80483cf:   89 e5                   mov    ebp,esp
 80483d1:   51                      push   ecx
 80483d2:   83 ec 04                sub    esp,0x4
 80483d5:   c7 04 24 b0 84 04 08    mov    DWORD PTR [esp],0x80484b0
 80483dc:   e8 13 ff ff ff          call   80482f4 <puts@plt>
 80483e1:   b8 00 00 00 00          mov    eax,0x0
 80483e6:   83 c4 04                add    esp,0x4
 80483e9:   59                      pop    ecx
 80483ea:   5d                      pop    ebp
 80483eb:   8d 61 fc                lea    esp,[ecx-0x4]
 80483ee:   c3                      ret    
 80483ef:   90                      nop

Answer 5

Use the -S (note: capital S) switch to GCC, and it will emit the assembly code to a file with a .s extension. For example, the following command:

gcc -O2 -S foo.c

will leave the generated assembly code on the file foo.s.

Ripped straight from http://www.delorie.com/djgpp/v2faq/faq8_20.html (but removing erroneous -c)

Answer 6

Did you try gcc -S -fverbose-asm -O source.c then look into the generated source.s assembler file ?

The generated assembler code goes into source.s (you could override that with -o assembler-filename ); the -fverbose-asm option asks the compiler to emit some assembler comments "explaining" the generated assembler code. The -O option asks the compiler to optimize a bit (it could optimize more with -O2 or -O3).

If you want to understand what gcc is doing try passing -fdump-tree-all but be cautious: you'll get hundreds of dump files.

BTW, GCC is extensible thru plugins or with MELT (a high level domain specific language to extend GCC).