How do you get assembler output from C/C++ source

Question

How does one do this?

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

Answer 1

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 agressive 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!

Answer 2

Use the -S option to gcc (or g++).

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.

By default this will output a file helloworld.s. The output file can be still be set by using the -o option.

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

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.

Answer 3

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.

Answer 4

From: http://www.delorie.com/djgpp/v2faq/faq8_20.html

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

in alternative to PhirePhly's answer Or just use -S as everyone said.

Answer 5

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.

Answer 6

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