> On 11 Mar 2025, at 15:50, David Brown <david.brown@xxxxxxxxxxxx> wrote:
>
> On 11/03/2025 15:31, Hans Åberg wrote:
>>> 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;
>> }
>
> That is nice to see. I will play with dogbolt.org when I get the chance - it's an interesting idea. I am particularly interested in how it deals with code for embedded devices (like Cortex-M Thumb2 code), since that is my most used target.
>
> I think that once you have more advanced optimisations like inter-procedural optimisations, it's going to be very difficult for a decompiler to deduce a good structure. It is not at all uncommon for a single C file to have only two or three externally linked functions and dozens of static functions - the optimiser will then inline most of these. Mix in partial inlining, cold code outlining, cloning, and - for maximum fun - LTO, and I think it will be very difficult to get useful results from decompilation.
>
> However, it could still be a useful tool for small sections of code that are structured appropriately.
For showing the effects of optimization, perhaps there should be a tool or compiler option that compiles source code directly into C++ code.
