Using GCC to produce readable assembly?

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.otest.o:     file format elf32-i386Disassembly 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.

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.

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)