llvm dev - Oct 2015 - How to generate IR code to dynamically compute object size ?

Hi,

We are using the following kind code to compile the size of a dynamically
allocated object, flowing the recommendation described here :

http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt

Our code was :

Value* ptr_size =
GetElementPtrInst::Create(ConstantPointerNull::get(dsp_type_ptr),
genInt64(fModule, 1), "ptr_size", entry_func_llvm_create_dsp);
CastInst* size_inst = new PtrToIntInst(ptr_size, fBuilder->getInt64Ty(),
"size", entry_func_llvm_create_dsp);
CallInst* call_inst1 = CallInst::Create(func_malloc, size_inst, "",
entry_func_llvm_create_dsp);

With LLVM 3.7 the prototype of GetElementPtrInst::Create has changed, so we
tried :

#if defined(LLVM_37)
    ConstantPointerNull* null_ptr =  ConstantPointerNull::get(dsp_type_ptr);
    Value* ptr_size = GetElementPtrInst::Create(null_ptr->getType(),
null_ptr, genInt64(fModule, 1), "ptr_size",
entry_func_llvm_create_dsp);
 #else
     Value* ptr_size =
GetElementPtrInst::Create(ConstantPointerNull::get(dsp_type_ptr),
genInt64(fModule, 1), "ptr_size", entry_func_llvm_create_dsp);
#endif
     CastInst* size_inst = new PtrToIntInst(ptr_size, fBuilder->getInt64Ty(),
"size", entry_func_llvm_create_dsp);
     CallInst* call_inst1 = CallInst::Create(func_malloc, size_inst,
"", entry_func_llvm_create_dsp);


But we get the following error when running the code : 

Assertion failed: (PointeeType ==
cast<PointerType>(Ptr->getType()->getScalarType())->getElementType()),
function Create, file
/opt/local/libexec/llvm-3.7/include/llvm/IR/Instructions.h, line 880.

Is there a cleaner and more official way to generate code to compute the size of
a dynamically allocated object with LLVM 3.7?

Thanks.

Stéphane Letz

On 10/1/2015 5:01 AM, Stéphane Letz via llvm-dev wrote:
>
> We are using the following kind code to compile the size of a dynamically
allocated object, flowing the recommendation described here :
>
> http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
>
Since you must know the type for the above to work, can't you use 
DataLayout::getTypeSizeInBits instead?

-Krzysztof

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