[LLVMdev] Retrieving Underlying Type from AllocaInst

[Curtis Faith]

Reid,

Thanks, I ended up doing the cast this way as it works for both AllocaInst and GlobalVariable. Since a store expects a pointer it should work for anything that I put into my symbol table.

- Curtis

On May 28, 2010, at 10:22 AM, Reid Kleckner wrote:

> You should be able to use the second alternative that Nick proposed:
> cast<PointerType*>(pointer_value->getType())->getElementType()
> 
> Reid
> 
> On Fri, May 28, 2010 at 9:37 AM, Curtis Faith <curtis at curtisfaith.com> wrote:
>> Thanks Nick,
>> Unfortunately, that is indeed what I asked for but not what I really am
>> looking for.
>> My naive approach is to store symbol table entries as Value* objects so I
>> can allocate global variables and alloca variables and place them into the
>> symbol table and the rest of the code didn't need to know which kind they
>> were, in general. Loads and Stores of these types (as well as other
>> instructions) seem to work fine as long as the types being pointed to match
>> the type of the value being stored.
>> So what I was really looking for, I suppose, was
>> a Value class virtual member function something like getLoadStoreType or
>> perhaps getReferencedType which returned the type as it would be used in a
>> load or store. It appears that both load and store do handle the pointer
>> dereferencing automatically for both types,
>> i.e. AllocaInst and GlobalVariable both dereference the pointers
>> automatically.
>> I can cast the value I get from the symbol table to a AllocaInst, of course,
>> in the case where it is one, but this requires a test for one and is just as
>> messy as first getting the type and then testing it for being
>> a PointerType and less general since a GlobalVariable also store its type as
>> a PointerType which point to the actual type. They, unfortunately, do not
>> share a common superclass other than Value itself.
>> - Curtis
>> 
>> On May 28, 2010, at 1:06 AM, Nick Lewycky wrote:
>> 
>> Curtis Faith wrote:
>> 
>> Is there a recommended way to retrieve the original type from an
>> 
>> AllocaInst object?
>> 
>> For example, I am creating alloca instructions using the IRBuilder
>> 
>> interface like:
>> 
>> alloca = builder.CreateAlloca( Type::getDoubleTy( context ), 0,
>> 
>> variableName.c_str() );
>> 
>> and I place the alloca into a symbol table.
>> 
>> Later when I am generating instructions for an assignment operation, I
>> 
>> want to check the type of the variable allocated with the CreateAlloca
>> 
>> instruction so I can see if I need to do type conversion before the
>> 
>> store instruction, However, I cannot simply use:
>> 
>> alloca->getType()->isDoubleTy() etc.
>> 
>> because this doesn't return true in the above case. The ID of the Ty
>> 
>> member of the VTy member of the alloca is PointerTyID instead of
>> 
>> DoubleTyID as the construction for AllocaInst::AllocaInst seems to
>> 
>> create a PointerType pointing to the original type.
>> 
>> I was hoping for a member function of the alloca that would return the
>> 
>> original type that was passed to CreateAlloca. I can test for a
>> 
>> PointerTyID and then perform a cast to PointerType and then call the
>> 
>> getElementType() function on the PointerType but that seems a bit messy.
>> 
>> There's alloca->getAllocatedType() which merely calls
>> getType()->getElementType() for you.
>> 
>> http://llvm.org/doxygen/classllvm_1_1AllocaInst.html .
>> 
>> Nick
>> 
>> 
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