On Tue, 11 Mar 2025 at 09:01, David Brown via Gcc-help
<gcc-help@xxxxxxxxxxx> wrote:
>
> On 11/03/2025 07:26, manoj alladawar via Gcc-help wrote:
> > Dear GCC Community,
> >
> > I hope this message finds you well.
> >
> > I am currently working on a Debian-based system running Ubuntu 22.04 and
> > using GCC version 11.4.0. I have compiled a C source file using GCC with
> > the -O2 optimization flag, resulting in an optimized binary.
> >
> > I am now interested in generating high-level C code from this optimized
> > binary for analysis and understanding. Could you kindly advise me on which
> > decompiler would be most suitable for this purpose? Specifically, I would
> > appreciate recommendations for decompilers that perform effectively on
> > GCC-based optimized code.
> >
> > Your guidance and insights would be greatly appreciated.
> >
> > Thank you for your time and support. Kind regards,
> > Manoj Alladawar
> >
>
> I am not sure what you are trying to get at here.
>
> Suppose we have the function :
>
> int foo(int x) {
> int y = 0;
> for (int i = 0; i < 5; i++) {
> y += x;
> }
> return y;
> }
>
> When optimising, gcc will just multiply "x" by 5.
>
> Are you hoping to be able to run gcc on this, then decompile that output
> and get a result :
>
> int foo(int x) {
> return x * 5;
> }
>
> or even :
>
> int foo(int x) {
> return (x << 2) + x;
> }
>
> ?
>
> If so, I think no such tool is possible in the general case.
>
> There /are/ decompilers that turn assembly into compilable C. But the
> structure and details is often lost - an original "for" loop might end
> up as a "goto" loop, for example. Once you have fed in normal C code
> with optimisation, things like inter-procedural optimisations, constant
> propagation, inlining, and other re-arrangements will mean that the
> regurgitated C code is incomprehensible. It should be possible to
> compile it again and get the same semantics - that's the point of a
> decompiler. And it might be possible for security analysis tools to
> gather information about vulnerabilities. But it's not going to help
> you understand how the optimiser is working.
>
> I think the best tool for looking at optimisation of small sections of
> code is the <https://godbolt.org> online compiler explorer. You can put
> in C code (or many other languages) and look at the generated assembly -
> using an x86 compiler or any other target processor if you prefer.
>
> <https://godbolt.org/z/dG8dd57ej>
I recently learnt of dogbolt.org for disassembling.
