Using GCC to produce readable assembly? - c

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

If you compile with debug symbols (add -g to your GCC command line, even if you're also using -O31),
you can use objdump -S to produce a more readable disassembly interleaved with C source.
>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. If not on x86, or if you like AT&T syntax, omit -Mintel.
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
Note that this isn't using -r so the call rel32=-4 isn't annotated with the puts symbol name. And looks like a broken call that jumps into the middle of the call instruction in main. Remember that the rel32 displacement in the call encoding is just a placeholder until the linker fills in a real offset (to a PLT stub in this case, unless you statically link libc).
Footnote 1: Interleaving source can be messy and not very helpful in optimized builds; for that, consider https://godbolt.org/ or other ways of visualizing which instructions go with which source lines. In optimized code there's not always a single source line that accounts for an instruction but the debug info will pick one source line for each asm instruction.

Answer 2

If you give GCC the flag -fverbose-asm, it will
Put extra commentary information in the generated assembly code to make it more readable.
[...] The added comments include:
information on the compiler version and command-line options,
the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
hints on which high-level expressions correspond to the various assembly instruction operands.

Answer 3

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 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

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:

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; which I abandoned in 2017)

Answer 7

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 8

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

Answer 9

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

Answer 10

For risc-v dissasembly, these flags are nice:
riscv64-unknown-elf-objdump -d -S -l --visualize-jumps --disassembler-color=color --inlines
-d: disassemble, most basic flag
-S: intermix source. Note: must use -g flag while compiling
-l: line numbers
--visualize-jumps: fancy arrows, not too useful but why not. Sometimes get's too messy and actually makes reading the source harder. Taken from Peter Cordes's comment: --visualize-jumps=coloris also an option, to use different colors for different arrows
--disassembler-color=color: give the disassembly some color
--inlines: print out inlines
Maybe usefull:
-M numeric: Use numeric reg names instead of abi names, useful if you are doing cpu dev and don't know the abi names by heart
-M no-aliases: don't use psudoinstructions like li and call
Example:
main.o:
#include <stdio.h>
#include <stdint.h>
static inline void example_inline(const char* str) {
for (int i = 0; str[i] != 0; i++)
putchar(str[i]);
}
int main() {
printf("Hello world");
example_inline("Hello! I am inlined");
return 0;
}
I recommend to use -O0 if you want intermix sources. Intermix sources becomes very messy if using -O2.
Command:
riscv64-unknown-elf-gcc main.c -c -O0 -g
riscv64-unknown-elf-objdump -d -S -l --disassembler-color=color --inlines main.o
Dissasembly:
main.o: file format elf64-littleriscv
Disassembly of section .text:
0000000000000000 <example_inline>:
example_inline():
/Users/cyao/test/main.c:4
#include <stdio.h>
#include <stdint.h>
static inline void example_inline(const char* str) {
0: 7179 addi sp,sp,-48
2: f406 sd ra,40(sp)
4: f022 sd s0,32(sp)
6: 1800 addi s0,sp,48
8: fca43c23 sd a0,-40(s0)
000000000000000c <.LBB2>:
/Users/cyao/test/main.c:5
for (int i = 0; str[i] != 0; i++)
c: fe042623 sw zero,-20(s0)
10: a01d j 36 <.L2>
0000000000000012 <.L3>:
/Users/cyao/test/main.c:6 (discriminator 3)
putchar(str[i]);
12: fec42783 lw a5,-20(s0)
16: fd843703 ld a4,-40(s0)
1a: 97ba add a5,a5,a4
1c: 0007c783 lbu a5,0(a5)
20: 2781 sext.w a5,a5
22: 853e mv a0,a5
24: 00000097 auipc ra,0x0
28: 000080e7 jalr ra # 24 <.L3+0x12>
/Users/cyao/test/main.c:5 (discriminator 3)
for (int i = 0; str[i] != 0; i++)
2c: fec42783 lw a5,-20(s0)
30: 2785 addiw a5,a5,1
32: fef42623 sw a5,-20(s0)
0000000000000036 <.L2>:
/Users/cyao/test/main.c:5 (discriminator 1)
36: fec42783 lw a5,-20(s0)
3a: fd843703 ld a4,-40(s0)
3e: 97ba add a5,a5,a4
40: 0007c783 lbu a5,0(a5)
44: f7f9 bnez a5,12 <.L3>
0000000000000046 <.LBE2>:
/Users/cyao/test/main.c:7
}
46: 0001 nop
48: 0001 nop
4a: 70a2 ld ra,40(sp)
4c: 7402 ld s0,32(sp)
4e: 6145 addi sp,sp,48
50: 8082 ret
0000000000000052 <main>:
main():
/Users/cyao/test/main.c:9
int main() {
52: 1141 addi sp,sp,-16
54: e406 sd ra,8(sp)
56: e022 sd s0,0(sp)
58: 0800 addi s0,sp,16
/Users/cyao/test/main.c:10
printf("Hello world");
5a: 000007b7 lui a5,0x0
5e: 00078513 mv a0,a5
62: 00000097 auipc ra,0x0
66: 000080e7 jalr ra # 62 <main+0x10>
/Users/cyao/test/main.c:11
example_inline("Hello! I am inlined");
6a: 000007b7 lui a5,0x0
6e: 00078513 mv a0,a5
72: 00000097 auipc ra,0x0
76: 000080e7 jalr ra # 72 <main+0x20>
/Users/cyao/test/main.c:13
return 0;
7a: 4781 li a5,0
/Users/cyao/test/main.c:14
}
7c: 853e mv a0,a5
7e: 60a2 ld ra,8(sp)
80: 6402 ld s0,0(sp)
82: 0141 addi sp,sp,16
84: 8082 ret
PS. There are colors in the dissembled code

Answer 11

use -Wa,-adhln as option on gcc or g++ to produce a listing output to stdout.
-Wa,... is for command line options for the assembler part (execute in gcc/g++ after C/++ compilation). It invokes as internally (as.exe in Windows).
See
>as --help
as command line to see more help for the assembler tool inside gcc