[LLVMdev] newbie question for type comparison

[Lu Zhao]

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:

(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?

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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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> >   
> 
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