> On 11 Mar 2025, at 12:04, Jonathan Wakely via Gcc-help <gcc-help@xxxxxxxxxxx> wrote:
>
> 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.
Trying the example above on this site, BinaryNinja gives, as suggested:
uint64_t foo(int32_t arg1) __pure
{
return arg1 * 5;
}
