Using GCC to produce readable assembly

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

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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

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Using the -S switch to GCC on x86 based systems produces a dump of AT amp 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  lt main gt    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  lt puts plt gt   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  lt main gt    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  lt puts plt gt   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

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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

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If you compile with debug symbols  you can use objdump to produce a more readable disassembly   gt 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  quot wide quot  mode  it doesn t line-wrap the machine-code bytes -Mintel  use GAS binutils MASM-like  intel syntax noprefix syntax instead of AT amp T -S  interleave source lines with disassembly   You could put something like alias disas  quot objdump -drwCS -Mintel quot  in your    bashrc  Example   gt  gcc -g -c test c  gt  objdump -d -M intel -S test o  test o      file format elf32-i386   Disassembly of section  text   00000000  lt main gt    include  lt stdio h gt   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  quot test quot       9    c7 04 24 00 00 00 00    mov    DWORD PTR  esp  0x0   10    e8 fc ff ff ff          call   11  lt main 0x11 gt       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

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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

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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

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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

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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

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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  lt stdio h gt   void func       printf   hello world n          using the following command line   -x c -std c99 -O3   and Colourise would generate the following

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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    gt as --help   as command line to see more help for the assembler tool inside gcc

[c] Using GCC to produce readable assembly?

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