llvm dev - May 2011 - [LLVMdev] Could LLVM or Clang go backward to modify c source code?

OK. Thank you.

What I want to do is: Fix source code in-place, then feed it to compiler
normally.
I assume I can fix source code in-place using clang::SourceManager, but I
cannot find the appropriate API.
Now I know that way is infeasible.

Thanks again.

2011/5/5 Joshua Warner <joshuawarner32 at gmail.com>
> Wen-Han,
>
> It sounds like there are two problems here: first is detecting when free is
> not properly called, and figuring out where to insert the call - which
seems
> like an intractable problem by iteself.  Second is a reversible translation
> from C to LLVM IR.  LLVM does (did?) have support for generating C code
> again on the back end, but the result doesn't look that much like the
> original code.  In order to translate changes from the IR back to the
source
> code, detailed information would have to be kept on what IR structures
> resulted from what source statements and how they interact.  There has been
> work on such reversible transformations done (I can't recall where
atm), but
> that was only in very simple cases that could be described by regular
> expressions.
>
> So the short answer is "no."
>
> -Joshua
>
> On Wed, May 4, 2011 at 9:16 PM, Wenhan Gu <nowar100 at gmail.com>
wrote:
>
>> Hi all,
>>
>> I have a source code:
>> int* p = malloc(...);
>>
>> After translating to llvm::Module, We can know the source code did not
>> call free.
>> I know we can hack this into Module.
>>
>> But my question is,
>> could we hack it back to source code?
>> After fixing, the source will become:
>> int *p = malloc(...);
>> free(p);
>>
>> Is this feasible?
>>
>> Thanks all very much.
>>
>> --
>> Best regards,
>> Wen-Han
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>

-- 
Best regards,
Wen-Han
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On 5 May 2011, at 06:19, Wenhan Gu wrote:
> OK. Thank you.
> 
> What I want to do is: Fix source code in-place, then feed it to compiler
normally.
Yes, this is possible.
> I assume I can fix source code in-place using clang::SourceManager, but I
cannot find the appropriate API.
You will want to use the rewriter API in clang, modify the code in a buffer, and
then tell clang that the file is out of date and to use the in-memory version.
> Now I know that way is infeasible.
It isn't infeasible, and APIs exist for doing it, however the general
problem of inserting missing free statements is hard.  You will need to perform
escape analysis in the clang AST, which requires cross-module code flow
checking.  If p is passed as an argument to a function, does the function store
it somewhere?  If you have an allocator function, then it will malloc() some
memory, initialise it, and return it. Where do you insert the free() call?

David
> Thanks again.
> 
> 2011/5/5 Joshua Warner <joshuawarner32 at gmail.com>
> Wen-Han, 
> 
> It sounds like there are two problems here: first is detecting when free is
not properly called, and figuring out where to insert the call - which seems
like an intractable problem by iteself.  Second is a reversible translation from
C to LLVM IR.  LLVM does (did?) have support for generating C code again on the
back end, but the result doesn't look that much like the original code.  In
order to translate changes from the IR back to the source code, detailed
information would have to be kept on what IR structures resulted from what
source statements and how they interact.  There has been work on such reversible
transformations done (I can't recall where atm), but that was only in very
simple cases that could be described by regular expressions.
> 
> So the short answer is "no."
> 
> -Joshua
> 
> On Wed, May 4, 2011 at 9:16 PM, Wenhan Gu <nowar100 at gmail.com>
wrote:
> Hi all,
> 
> I have a source code:
> int* p = malloc(...);
> 
> After translating to llvm::Module, We can know the source code did not call
free.
> I know we can hack this into Module.
> 
> But my question is,
> could we hack it back to source code?
> After fixing, the source will become:
> int *p = malloc(...);
> free(p);
> 
> Is this feasible?
> 
> Thanks all very much.
> 
> -- 
> Best regards,
> Wen-Han
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> 
> 
> 
> 
> -- 
> Best regards,
> Wen-Han
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

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