[LLVMdev] newbie question for type comparison

[John Criswell]

Lu Zhao wrote:
> Hi John,
>
> Thank you a lot. That clarifies some my confusions. What I want to do
> is to use both methods, get ALL struct allocation and a SPECIFIC struct
> allocation, at different situations. Here, I've got a couple of more
> questions.
>
> 1. For getting ALL struct allocation, when I use
>
>         if( (AI = dyn_cast<AllocaInst>(&*i)))
>           if(AI->getOperand(0)->getType()->getTypeID() ==
> Type::StructTyID) {
>
> to get all alloca instructions allocating a structure, it does not
> work. In my gdb session, I've got the following dumps:
>   
I'm not sure why the above doesn't work.  My best guess is that the 
allocated type is not an operand (since it is not an LLVM Value) but is 
an attribute (for lack of a better word) of the alloca instruction.

My suggestion is to use the getAllocatedType() method of the AllocaInst 
class.  I'm pretty sure it does what you want:

if (AI->getAllocatedType()->getTypeID() == Type::StructTyID)


> (gdb) call AI->dump()
>         %s = alloca %struct.S           ; <%struct.S*> [#uses=4]
> (gdb) call AI->getOperand(0)->dump()
>  i32 1
> (gdb) call AI->getOperand(0)->getType()->dump()
> i32
>
> AI->getOperand(0)->getType() returns i32 instead of what I expect,
> {i32, i32}.
>
> 2. For TargetData size, for those struct allocations that I don't know
> its size in advance, I want to get its size and use the size in a memcpy
> function, which makes a copy of the struct. Is the ABI type size the
> correct one?
>   
Yes, getABITypeSize() is what you want since it returns the amount of 
memory allocated by an alloca instruction for the given type.

If you haven't used it already, you may want to use the LLVM Doxygen 
documentation (http://llvm.org/doxygen/hierarchy.html).  It has lots of 
information on the LLVM API, and it gets rebuilt nightly to reflect 
recent changes in the LLVM programming API.

-- John T.

> Thanks.
> Lu
>
> On Mon, 21 Apr 2008 09:08:54 -0500
> John Criswell <criswell at cs.uiuc.edu> wrote:
>
>   
>> Lu Zhao wrote:
>>     
>>> Hi,
>>>
>>> I want to extract all instruction of the form "alloca %struct.S",
>>> where $struct.S is defined as a struct
>>>
>>>         %struct.S = type { i32, i32 }
>>>
>>> I'm using the following loop:
>>>
>>> for(inst_iterator i = inst_begin(F), e = inst_end(F); i!= e; i++)
>>> {
>>>         AllocaInst* ai;
>>>         if( (ai = dyn_cast<AllocaInst>(&*i))){
>>>           if(ai->getOperand(0)->getType()->getTypeID() ==
>>>                 Type::StructTyID)
>>>                         allocs.push_back(ai);
>>>          }
>>> }
>>>
>>>       
>> I might be misunderstanding something, but the above code finds all
>> alloca instructions that allocate structure types.  This is not quite
>> what you want if you're looking specifically for type {i32, i32}.
>>
>> To search specifically for that type, do the following:
>>
>> 1) Create a "new" structure type representing {i32. i32} using the
>> static StructType::get() method.  I think the code would look
>> something like this:
>>
>> Type * MyStructType = StructType::get (Type::Int32Ty, Type::Int32Ty,
>> 0);
>>
>> 2) Compare the pointer of MyStructType to the pointer for the
>> alloca's allocation type:
>>
>> if (ai->getOperand(0)->getType() == MyStructType)
>>
>> The above works because LLVM will only create one instance of each
>> type (i.e. if there's already a {i32, i32}, StructType::get() returns
>> a pointer to that instead of creating a new object).  This is what
>> allows the pointer comparison to work.
>>
>>     
>>> However, I had the impression that the type comparison can be done
>>> by a pointer comparison. Can anyone please suggest me a better way
>>> to do it?
>>>
>>> Also, I want to get the size of %struct.S, should I use the
>>> following code?
>>>
>>> const TargetData &TD = getAnalysis<TargetData>();
>>> unsigned StructSize =
>>>          TD.getABITypeSizeInBits(ai->getOperand(0)->getType());
>>>
>>>       
>> There are different interpretations of "size" that TargetData
>> supports, but without knowing what you're trying to do, I'd have to
>> guess that this is probably what you want.
>>
>> -- John T.
>>
>>     
>>> Thanks a lot.
>>> Lu
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>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>       
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