[LLVMdev] casting Constant * to value *?

[Alexander Poddey]

Hi Jon,

yes, I read through the kaleidoscope at the beginning of my project, which 
is about manipulating IR code.

Kaleidoscope example is nice, but naturally can not cover every possible 
situation.

What I really like about the  CPPBackend is, that I can code anything in c / 
c++ and see how this could be achieved in the API (e.g. accessing nested 
structs). I then use these code snippets as entry points to llvm source code 
browsing and seeing how I could use it for my needs.

Is there a better way / better C++Api code generator I should use instead?

Thanks
Alex


Jonathan Roelofs wrote:

> Alex,
> 
> Seeing your questions go by on the list, ISTM that you are focusing a
> lot on the CPPBackend, which tends to generate ugly code generators, and
> isn't particularly the best example to be following unless you're really
> stuck and the doxygen docs aren't helping... has anyone pointed you to
> the Kaleidoscope tutorial? http://llvm.org/docs/tutorial/LangImpl3.html
> 
> 
> Cheers,
> 
> Jon
> 
> 
> On 3/18/15 11:35 AM, Alexander Poddey wrote:
>> John, you are right.
>> I 'browsed' the doxygen's inheritance diagram.
>> Shouldn't I then be able to cast Constant * to Value*?
>>
>> Plugging the retrieved Constant* (from ConstantExpr::getGetElementPtr)
>> into Instruction->setOperand compiles, but gives me an assertion failure
>> at runtime.
>> I have no access to the code at the moment. I will gather more
>> information possibly tomorrow.
>>
>> Thanks
>> Alex
>>
>>
>>
>>
>> John Criswell wrote:
>>
>>> On 3/17/15 8:40 PM, Alexander Poddey wrote:
>>>> Hi all,
>>>>
>>>> extracting datafields of globals, the API code ends up in a Constant *
>>>>
>>>>
>>>> Constant* const_ptr_103 =
>>>> ConstantExpr::getGetElementPtr(gvar_struct_foo, const_ptr_103_indices);
>>>>
>>>> it can be used to initialize e.g. a new instruction like:
>>>>
>>>> StoreInst* void_119 = new StoreInst(const_float_102, const_ptr_103,
>>>> false, label_entry_113);
>>>>
>>>>
>>>> But how about replacing the operand of an already existing instruction
>>>> using a Constant *?
>>>>
>>>> Instruction->setOperand(1,__needs_a_value*_here__);
>>>
>>> If you look at the inheritance tree, you'll see that an llvm::Constant
>>> is a subclass of llvm::Value.  Therefore, you can use an llvm::Constant
>>> anywhere in which an llvm::Value is needed.
>>>
>>> To look at the inheritance tree, look at the LLVM doxygen documentation.
>>>
>>> Regards,
>>>
>>> John Criswell
>>>>
>>>>
>>>> Thx
>>>> Alex
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>>
>>
>>
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