How do you get assembler output from C/C++ source in GCC?

Question

How does one do this?

If I want to analyze how something is getting compiled, how would I get the emitted assembly code?

Southwest

For tips on making the asm output human readable, see also: How to remove “noise” from GCC/clang assembly output? – Deservedly

Answer 1

Use the -S option to gcc (or g++), optionally with -fverbose-asm which works well at the default -O0 to attach C names to asm operands as comments. It works less well at any optimization level, which you normally want to use to get asm worth looking at.

gcc -S helloworld.c

This will run the preprocessor (cpp) over helloworld.c, perform the initial compilation and then stop before the assembler is run. For useful compiler options to use in that case, see How to remove "noise" from GCC/clang assembly output? (or just look at your code on Matt Godbolt's online Compiler Explorer which filters out directives and stuff, and has highlighting to match up source lines with asm using debug information.)

By default, this will output the file helloworld.s. The output file can be still be set by using the -o option, including -o - to write to standard output for pipe into less.

gcc -S -o my_asm_output.s helloworld.c

Of course, this only works if you have the original source. An alternative if you only have the resultant object file is to use objdump, by setting the --disassemble option (or -d for the abbreviated form).

objdump -S --disassemble helloworld > helloworld.dump

-S interleaves source lines with normal disassembly output, so this option works best if debugging option is enabled for the object file (-g at compilation time) and the file hasn't been stripped.

Running file helloworld will give you some indication as to the level of detail that you will get by using objdump.

Other useful objdump options include -rwC (to show symbol relocations, disable line-wrapping of long machine code, and demangle C++ names). And if you don't like AT&T syntax for x86, -Mintel. See the man page.

So for example, objdump -drwC -Mintel -S foo.o | less. -r is very important with a .o that only has 00 00 00 00 placeholders for symbol references, as opposed to a linked executable.

Previdi

an addition use : objdump -M intel -S --disassemble helloworld > helloworld.dump to get the object dump in intel syntax compatible with nasm on linux. – Dori

If you have a single function to optimize/check, then you can give a try to online Interactive C++ Compilers i.e. godbolt – Church

@touchStone: GAS .intel_syntax is not compatible with NASM. It's more like MASM (e.g. mov eax, symbol is a load, unlike in NASM where it's a mov r32, imm32 of the address), but not totally compatible with MASM either. I do highly recommend it as a nice format to read, especially if you like to write in NASM syntax though. objdump -drwC -Mintel | less or gcc foo.c -O1 -fverbose-asm -masm=intel -S -o- | less are useful. (See also How to remove “noise” from GCC/clang assembly output?). -masm=intel works with clang, too. – Deservedly

Better use gcc -O -fverbose-asm -S – Taxidermy

Why objdump -S has no noise whereas gcc -S has noise output ? – Beastly

Answer 2

This will generate assembly code with the C code + line numbers interwoven, to more easily see which lines generate what code (-S -fverbose-asm -g -O2):

# Create assembler code:
g++ -S -fverbose-asm -g -O2 test.cc -o test.s

# Create asm interlaced with source lines:
as -alhnd test.s > test.lst

It was found in Algorithms for programmers, page 3 (which is the overall 15th page of the PDF).

Eslinger

Sadly, as on OS X doesn't know these flags. If it did, though, you could probably one-line this using -Wa to pass options to as. – Stylish

g++ -g -O0 -c -fverbose-asm -Wa,-adhln test.cpp > test.lst would be the short hand version of this. – Cecum

You can also use either gcc -c -g -Wa,-ahl=test.s test.c or gcc -c -g -Wa,-a,-ad test.c > test.txt – Percentage

A blog post explaining this in more detail, including the one-command version like legends and Lu'u posted. But why -O0? That's full of loads/stores that make it hard to track a value, and doesn't tell you anything about how efficient the optimized code will be. – Deservedly

Answer 3

The following command line is from Christian Garbin's blog:

g++ -g -O -Wa,-aslh horton_ex2_05.cpp >list.txt

I ran G++ from a DOS window on Windows XP, against a routine that contains an implicit cast

cd C:\gpp_code
g++ -g -O -Wa,-aslh horton_ex2_05.cpp > list.txt

Output:

horton_ex2_05.cpp: In function `int main()':
horton_ex2_05.cpp:92: warning: assignment to `int' from `double'

The output is assembled generated code, interspersed with the original C++ code (the C++ code is shown as comments in the generated assembly language stream)

  16:horton_ex2_05.cpp **** using std::setw;
  17:horton_ex2_05.cpp ****
  18:horton_ex2_05.cpp **** void disp_Time_Line (void);
  19:horton_ex2_05.cpp ****
  20:horton_ex2_05.cpp **** int main(void)
  21:horton_ex2_05.cpp **** {
 164                    %ebp
 165                            subl $128,%esp
?GAS LISTING C:\DOCUME~1\CRAIGM~1\LOCALS~1\Temp\ccx52rCc.s
166 0128 55                    call ___main
167 0129 89E5          .stabn 68,0,21,LM2-_main
168 012b 81EC8000      LM2:
168      0000
169 0131 E8000000      LBB2:
169      00
170                    .stabn 68,0,25,LM3-_main
171                    LM3:
172                            movl $0,-16(%ebp)

Driskill

@Paladin - Not necessarily. The OP was about getting the assembler output equivalent of the C/C++ source code, this gets the Listing, which I agree is more useful for understanding what the compiler and optimizer is doing. But it would cause the assembler itself to barf, as it is not expecting the line numbers, and compiled bytes off tot he left of the assembly instructions. – Mantic

Use at least -O2, or whatever optimization options you actually use when building your project, if you want to see how gcc optimizes your code. (Or if you use LTO, like you should, then you have to disassemble the linker output to see what you really get.) – Deservedly

@PeterCordes there is an easier way, see this question – Mariande

Answer 4

Use the -S switch:

g++ -S main.cpp

Or also with gcc:

gcc -S main.c

Also see this.

Southwest

Answer 5

-save-temps

This was mentioned in METADATA's answer, but let me further exemplify it.

The big advantage of this option over -S is that it is very easy to add it to any build script, without interfering much in the build itself:

gcc -save-temps -c -o main.o main.c

main.c

#define INC 1

int myfunc(int i) {
    return i + INC;
}

and now, besides the normal output main.o, the current working directory also contains the following files:

• main.i is a bonus and contains the preprocessed file:

  # 1 "main.c"
  # 1 "<built-in>"
  # 1 "<command-line>"
  # 31 "<command-line>"
  # 1 "/usr/include/stdc-predef.h" 1 3 4
  # 32 "<command-line>" 2
  # 1 "main.c"
  
  
  int myfunc(int i) {
      return i + 1;
  }
  

• main.s contains the desired generated assembly:

      .file    "main.c"
      .text
      .globl    myfunc
      .type    myfunc, @function
  myfunc:
  .LFB0:
      .cfi_startproc
      pushq    %rbp
      .cfi_def_cfa_offset 16
      .cfi_offset 6, -16
      movq    %rsp, %rbp
      .cfi_def_cfa_register 6
      movl    %edi, -4(%rbp)
      movl    -4(%rbp), %eax
      addl    $1, %eax
      popq    %rbp
      .cfi_def_cfa 7, 8
      ret
      .cfi_endproc
  .LFE0:
      .size    myfunc, .-myfunc
      .ident    "GCC: (Ubuntu 8.3.0-6ubuntu1) 8.3.0"
      .section    .note.GNU-stack,"",@progbits
  

Docs: https://gcc.gnu.org/onlinedocs/gcc/Developer-Options.html#index-save-temps

-save-temps=obj

If you want to do it for a large number of files, consider using instead:

-save-temps=obj

which saves the intermediate files to the same directory as the -o object output instead of the current working directory, thus avoiding potential basename conflicts.

For example:

gcc -save-temps -c -o out/subdir/main.o subdir/main.c

leads to the creation of files:

out/subdir/main.i
out/subdir/main.o
out/subdir/main.s

Clearly an Apple plot to take over the world.

-save-temps -v

Another cool thing about this option is if you add -v:

gcc -save-temps -c -o main.o -v main.c

it actually shows the explicit files being used instead of ugly temporaries under /tmp, so it is easy to know exactly what is going on, which includes the preprocessing / compilation / assembly steps:

/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu main.c -mtune=generic -march=x86-64 -fpch-preprocess -fstack-protector-strong -Wformat -Wformat-security -o main.i
/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -fpreprocessed main.i -quiet -dumpbase main.c -mtune=generic -march=x86-64 -auxbase-strip main.o -version -fstack-protector-strong -Wformat -Wformat-security -o main.s
as -v --64 -o main.o main.s

It was tested in Ubuntu 19.04 (Disco Dingo) amd64, GCC 8.3.0.

CMake predefined targets

CMake automatically provides a targets for the preprocessed file:

make help

shows us that we can do:

make main.s

and that target runs:

Compiling C source to assembly CMakeFiles/main.dir/main.c.s
/usr/bin/cc    -S /home/ciro/hello/main.c -o CMakeFiles/main.dir/main.c.s

so the file can be seen at CMakeFiles/main.dir/main.c.s.

It was tested on CMake 3.16.1.

Septuagenarian

Answer 6

If what you want to see depends on the linking of the output, then objdump on the output object file/executable may also be useful in addition to the aforementioned gcc -S. Here's a very useful script by Loren Merritt that converts the default objdump syntax into the more readable NASM syntax:

#!/usr/bin/perl -w
$ptr='(BYTE|WORD|DWORD|QWORD|XMMWORD) PTR ';
$reg='(?:[er]?(?:[abcd]x|[sd]i|[sb]p)|[abcd][hl]|r1?[0-589][dwb]?|mm[0-7]|xmm1?[0-9])';
open FH, '-|', '/usr/bin/objdump', '-w', '-M', 'intel', @ARGV or die;
$prev = "";
while(<FH>){
    if(/$ptr/o) {
        s/$ptr(\[[^\[\]]+\],$reg)/$2/o or
        s/($reg,)$ptr(\[[^\[\]]+\])/$1$3/o or
        s/$ptr/lc $1/oe;
    }
    if($prev =~ /\t(repz )?ret / and
       $_ =~ /\tnop |\txchg *ax,ax$/) {
       # drop this line
    } else {
       print $prev;
       $prev = $_;
    }
}
print $prev;
close FH;

I suspect this can also be used on the output of gcc -S.

Shahjahanpur

Still, this script is a dirty hack which doesn't fully convert the syntax. E.g. mov eax,ds:0x804b794 is not very NASMish. Also, sometimes it just strips useful information: movzx eax,[edx+0x1] leaves the reader to guess whether the memory operand was byte or word. – Marleenmarlen

To disassemble in NASM syntax in the first place, use Agner Fog's objconv. You can get it to disassemble to stdout with output file = /dev/stdout, so you can pipe into less for viewing. There's also ndisasm, but it only disassembles flat binaries, and doesn't know about object files (ELF / PE). – Deservedly

Answer 7

Well, as everyone said, use the -S option.

If you use the -save-temps option, you can also get the preprocessed file (.i), assembly file (.s) and object file (*.o) (get each of them by using -E, -S, and -c, respectively).

Aragonite

Answer 8

As everyone has pointed out, use the -S option to GCC. I would also like to add that the results may vary (wildly!) depending on whether or not you add optimization options (-O0 for none, -O2 for aggressive optimization).

On RISC architectures in particular, the compiler will often transform the code almost beyond recognition in doing optimization. It's impressive and fascinating to look at the results!

Sturdy

Answer 9

Use the -S option:

gcc -S program.c

Proclus

Answer 10

As mentioned before, look at the -S flag.

It's also worth looking at the '-fdump-tree' family of flags, in particular -fdump-tree-all, which lets you see some of GCC's intermediate forms. These can often be more readable than assembler (at least to me), and let you see how optimisation passes perform.

Waitabit

Answer 11

If you're looking for LLVM assembly:

llvm-gcc -emit-llvm -S hello.c

Prow

The question was about GCC, not LLVM. – Apron

Answer 12

I don't see this possibility among answers, probably because the question is from 2008, but in 2018 you can use Matt Goldbolt's online website https://godbolt.org

You can also locally git clone and run his project https://github.com/mattgodbolt/compiler-explorer

Digged

The accepted answer has been updated with that information. – Apron

Answer 13

Here is a solution for C using GCC:

gcc -S program.c && gcc program.c -o output

1. Here the first part stores the assembly output of the program in the same file name as the program, but with a changed .s extension, you can open it as any normal text file.

2. The second part here compiles your program for actual usage and generates an executable for your Program with a specified file name.

The program.c used above is the name of your program and output is the name of the executable you want to generate.

Cummine

You can also use gcc -O2 -save-temps foo.c to compile+assemble+link, but save the intermediate .s and .o files, instead of separately running a build that only compiles to asm. (But also a .i preprocessed C file). So it's fewer steps, but produces files you don't want. – Deservedly

Answer 14

From the FAQ How to get GCC to generate assembly code:

gcc -c -g -Wa,-a,-ad [other GCC options] foo.c > foo.lst

as an alternative to PhirePhly's answer.

Or just use -S as everyone said.

Sterigma

Answer 15

Here are the steps to see/print the assembly code of any C program on your Windows:

In a console/terminal command prompt:

1. Write a C program in a C code editor like Code::Blocks and save it with filename extension .c

2. Compile and run it.

3. Once run successfully, go to the folder where you have installed your GCC compiler and enter the following command to get a ' .s ' file of the ' .c' file

   cd C:\gcc
   gcc -S complete path of the C file  ENTER
   

   An example command (as in my case)

   gcc  -S D:\Aa_C_Certified\alternate_letters.c
   

   This outputs a '.s' file of the original '.c' file.

4. After this, type the following command

   cpp filename.s ENTER
   

   Example command (as in my case)

   cpp alternate_letters.s  <enter>
   

This will print/output the entire assembly language code of your C program.

Olivann

Answer 16

Recently I wanted to know the assembly of each functions in a. This is how I did it:

gcc main.c    // 'main.c' source file
gdb a.exe     // 'gdb a.out' in Linux

In GDB:

disass main   // Note here 'main' is a function
              // Similarly, it can be done for other functions.

Okun

Answer 17

Use "-S" as an option. It displays the assembly output in the terminal.

Truancy

To display in the terminal, use gcc foo.c -masm=intel -fverbose-asm -O3 -S -o- |less. -S on its own creates foo.s. – Deservedly

This is already covered by previous answers. – Apron