CODEWITHSUNDEEP
cgccassemblyx86-64compiler-optimization
David542
David542

Reputation: 110267

How to figure out which asm my server is using

What is the most straightforward way to determine which asm a server is using? For example, I have the following C program:

int main(void) {                                                                                                                                 
    int i = 4;
    return i;
}

And compiled to assembly via $ gcc -S main.c I get:

  1   .file "main.c"                                                                                                                                 
  2   .text
  3   .globl  main
  4   .type main, @function
  5 main:
  6 .LFB0:
  7   .cfi_startproc
  8   pushq %rbp
  9   .cfi_def_cfa_offset 16
 10   .cfi_offset 6, -16
 11   movq  %rsp, %rbp
 12   .cfi_def_cfa_register 6
 13   movl  $4, -4(%rbp)
 14   movl  -4(%rbp), %eax
 15   popq  %rbp
 16   .cfi_def_cfa 7, 8
 17   ret
 18   .cfi_endproc
 19 .LFE0:
 20   .size main, .-main
 21   .ident  "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4"
 22   .section  .note.GNU-stack,"",@progbits

I would like to use https://godbolt.org/ to select the closest language to compile online, but I'm not sure which one I should select (or even how to figure out how I can determine that). Basically all I know is "it looks like ATT syntax". How can I figure that out?

Upvotes: 1

Views: 120

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 364438

GCC identifies itself via metadata; in the asm this is the .ident "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4" directive.

You could have seen the same thing via gcc -v to just print your GCC version.

That tells you that your server is using crusty old GCC 4.8.4. (I'd really recommend using a newer compiler if you care about optimization; e.g. gcc 8 introduced simpler auto-vectorization that's better for arrays that are aligned, and lets the hardware handle misalignment by just using unaligned loads.)

So on https://godbolt.org/ you can get identical asm to what you get locally by setting the C compiler version to x86-64 GCC 4.8.4, and using -fstack-protector-strong. (Because Ubuntu configures their GCC to use that option). Normally you should also use -Wall -O2 at least, or -O3; looking at un-optimized asm is usually a waste of time. See How to remove "noise" from GCC/clang assembly output? - especially the link to Matt Godbolt's CPPcon talk.)

Pick C or C++ as the "language" on Godbolt; you'll have gcc and g++ 4.8.4 installed on your Ubuntu systmem.

Later Ubuntu versions enable -fPIE as part of the default options so you'd also have to enable that manually on Godbolt to match what you see locally. But I think 14.04 is too old for that.

(We can tell that this is x86-64 because the stack pointer is RSP not ESP. And yes, GCC on x86 defaults to AT&T syntax unless you use -masm=intel. The Godbolt compiler explorer defaults to Intel you can toggle that output option. It's just a different text syntax for the same machine instructions though; if you know how to read both the difference is irrelevant.)

It's possible that Godbolt will compile with slightly different header versions than you'd get locally; I think its setup has different compiler installs using the headers from one install instead of each having their own. I might be wrong about this, though. Usually it's fine.

Upvotes: 2

Related Questions