[LLVMdev] Question on Load and GetElementPtr instructions

[Xiaolong Tang]

Hey,

Considering the following instruction:

%20 = load %struct.Node** getelementptr inbounds (%struct.Node*
@head, i32 0, i32 0), align 16 ; <%struct.Node*> [#uses=1]

What is the type of the first operand of the instruction (i.e.,
getOperand(0))?

I thought the operand is a "GetElementPtr" instruction, however, the
predicate "isa<Instruction>()" over the operand returns false.
So, I am confused. Can someone clarify the above instruction for me?

By the way, why does not LLVM factor out the operand and instead
generate such two instructions:

%19 = getelementptr inbounds (%struct.Node* @head, i32 0, i32 0)
%20 = load %struct.Node** %19, align 16 ; <%struct.Node*> [#uses=1]

What it the cause? What are the differences between the
"GetElementPtr inbounds" instruction and just the "GetElementPtr"
instruction?

Thanks,
Xiaolong

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[NAKAMURA Takumi]

Good evening, Tang.

> What is the type of the first operand of the instruction (i.e.,
> getOperand(0))?

It might be ConstantExpr.
You may use dyn_cast<GEPOperator>(getPointerOperand()).
Also GetElementPtrInst* can be casted to GEPOperator.

...Takumi

2010/6/18 Xiaolong Tang <xiaolon...@gmail.com>:

[Xiaolong Tang]

Hello Nakamura,

> > What is the type of the first operand of the instruction (i.e.,
> > getOperand(0))?
>
> It might be ConstantExpr.
> You may use dyn_cast<GEPOperator>(getPointerOperand()).
> Also GetElementPtrInst* can be casted to GEPOperator.

You are right. Now I use isa<GEPOperator> as the guard.
Thanks.

Best,
Xiaolong

>
>
> 2010/6/18 Xiaolong Tang <xiaolon...@gmail.com>:
> >
> > Hey,
> >
> > Considering the following instruction:
> >
> >  %20 = load %struct.Node** getelementptr inbounds (%struct.Node*
> >          @head, i32 0, i32 0), align 16 ; <%struct.Node*> [#uses=1]
> >
> > What is the type of the first operand of the instruction (i.e.,
> > getOperand(0))?
> >
> > I thought the operand is a "GetElementPtr" instruction, however, the
> > predicate "isa<Instruction>()" over the operand returns false.
> > So, I am confused. Can someone clarify the above instruction for me?
> >
> > By the way, why does not LLVM factor out the operand and instead
> > generate such two instructions:
> >
> >  %19 = getelementptr inbounds (%struct.Node* @head, i32 0, i32 0)
> >  %20 = load %struct.Node** %19, align 16 ; <%struct.Node*> [#uses=1]
> >
> > What it the cause?  What are the differences between the
> > "GetElementPtr inbounds" instruction and just the "GetElementPtr"
> > instruction?

[Duncan Sands]

Hi Xiaolong,

> Considering the following instruction:
>
> %20 = load %struct.Node** getelementptr inbounds (%struct.Node*
> @head, i32 0, i32 0), align 16 ;<%struct.Node*> [#uses=1]
>
> What is the type of the first operand of the instruction (i.e.,
> getOperand(0))?
>
> I thought the operand is a "GetElementPtr" instruction, however, the
> predicate "isa<Instruction>()" over the operand returns false.
> So, I am confused. Can someone clarify the above instruction for me?

it is not an Instruction, it is a ConstantExpr. Several expressions like
GEP exist both as instructions and as constants, see the ConstantExpr class.

Ciao,

Duncan.