g++ - Using GCC to produce readable assembly?

generate with (9)

godbolt is a very useful tool, they list only has C++ compilers but you can use -x c flag in order to get it treat the code as C. It will then generate an assembly listing for your code side by side and you can use the Colourise option to generate colored bars to visually indicate which source code maps to the generated assembly. For example the following code:

#include <stdio.h>

void func()
  printf( "hello world\n" ) ;

using the following command line:

-x c -std=c99 -O3

and Colourise would generate the following:

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.

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)

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 -c foo.c

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


> 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
   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

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 <[email protected]>
 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


$ 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 <[email protected]>
 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

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).

I haven't given a shot to gcc, but in case of g++. The command below works for me. -g for debug build and -Wa,-adhln is passed to assembler for listing with source code

g++ -g -Wa,-adhln src.cpp

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.

The following tests have been done with Visual C++ compiler as it is used by the default Qt Creator install (I guess with no optimization flag). When using GCC, there is no big difference between Mystical's version and my "optimized" code. So the conclusion is that compiler optimizations take care off micro optimization better than humans (me at last). I leave the rest of my answer for reference.

It's not efficient to process images this way. It's better to use single dimension arrays. Processing all pixels is the done in one loop. Random access to points could be done using:

pointer + (x + y*width)*(sizeOfOnePixel)

In this particular case, it's better to compute and cache the sum of three pixels groups horizontally because they are used three times each.

I've done some tests and I think it's worth sharing. Each result is an average of five tests.

Original code by user1615209:

8193: 4392 ms
8192: 9570 ms

Mystical's version:

8193: 2393 ms
8192: 2190 ms

Two pass using a 1D array: first pass for horizontal sums, second for vertical sum and average. Two pass addressing with three pointers and only increments like this:

imgPointer1 = &avg1[0][0];
imgPointer2 = &avg1[0][SIZE];
imgPointer3 = &avg1[0][SIZE+SIZE];


8193: 938 ms
8192: 974 ms

Two pass using a 1D array and addressing like this:


8193: 932 ms
8192: 925 ms

One pass caching horizontal sums just one row ahead so they stay in cache:

// Horizontal sums for the first two lines
// Rest of the computation
    // Compute horizontal sum for next line
    // Final result

8193: 599 ms
8192: 652 ms


  • No benefits of using several pointers and just increments (I thought it would have been faster)
  • Caching horizontal sums is better than computing them several time.
  • Two pass is not three times faster, two times only.
  • It's possible to achieve 3.6 times faster using both a single pass and caching an intermediary result

I'm sure it's possible to do much better.

NOTE Please, note that I wrote this answer to target general performance issues rather than the cache problem explained in Mystical's excellent answer. At the beginning it was just pseudo code. I was asked to do tests in the comments... Here is a completely refactored version with tests.

c gcc assembly