llvm dev - Oct 2009 - [LLVMdev] Dereference PointerType?

Hello,

I'm wondering if it's possible to dereference a PointerType. I have an
AllocaInst and although I can find the number of elements allocated, (using
Instruction::getOperand(0)), I can't find a way to get the size of each
element. What I'd like to do is:

AllocaInst *alloca;

PointerType *ptr_type = dynamic_cast<PointerType*>(alloca);
assert(ptr_type);
Type *allocated_type = ptr_type->dereference(); // this is the operation
that doesn't seem to exist.
size_t size;
if (isa<PointerType>(allocated_type)) {
  size = sizeof(void*) * 8;
} else {
  size = allocated_type->getPrimitiveSizeInBits();
}
// size now equals the size (in bits) of the type allocated

If it doesn't exist, how do you manage without it? Is there another way to
write the above code?

Thanks,

Daniel
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2009/10/20 Daniel Waterworth <da.waterworth at googlemail.com>
> Hello,
>
> I'm wondering if it's possible to dereference a PointerType. I have
an
> AllocaInst and although I can find the number of elements allocated, (using
> Instruction::getOperand(0)), I can't find a way to get the size of each
> element. What I'd like to do is:
>
> AllocaInst *alloca;
>
> PointerType *ptr_type = dynamic_cast<PointerType*>(alloca);
> assert(ptr_type);
> Type *allocated_type = ptr_type->dereference(); // this is the operation
> that doesn't seem to exist.
> size_t size;
> if (isa<PointerType>(allocated_type)) {
>   size = sizeof(void*) * 8;
> } else {
>   size = allocated_type->getPrimitiveSizeInBits();
> }
> // size now equals the size (in bits) of the type allocated
>
> If it doesn't exist, how do you manage without it? Is there another way
to
> write the above code?
>
> Thanks,
>
> Daniel
>
Spot the deliberate mistake [
PointerType *ptr_type = dynamic_cast<PointerType*>(alloca);

should be,

const PointerType *ptr_type = dynamic_cast<const
PointerType*>(alloca->getType());

and,

Type *allocated_type = ptr_type->dereference();

should probably be,

const Type *allocated_type = ptr_type->dereference();

Daniel
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Daniel Waterworth <da.waterworth at googlemail.com> writes:

[snip]

Use the getElementType method of PointerType.
>> size_t size;
>> if (isa<PointerType>(allocated_type)) {
>>   size = sizeof(void*) * 8;
>> } else {
>>   size = allocated_type->getPrimitiveSizeInBits();
>> }
>> // size now equals the size (in bits) of the type allocated
This looks suspicious to me. Maybe you intented

if (isa<PointerType>(allocated_type)) {
  size = sizeof(void*);
} else {
  size = allocated_type->getPrimitiveSizeInBits() * 8;
}

but take a look at getPrimitiveSizeInBits' documentation to make sure
that it is really what you want:

http://llvm.org/doxygen/classllvm_1_1Type.html

-- 
Óscar

Hi Daniel,
>     Type *allocated_type = ptr_type->dereference(); // this is the
>     operation that doesn't seem to exist.
Type *allocated_type = ptr_type->getElementType();

Ciao,

Duncan.