[LLVMdev] newbie question for type comparison

John Criswell criswell at cs.uiuc.edu
Mon Apr 21 07:08:54 PDT 2008

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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