llvm dev - Jun 2010 - [LLVMdev] Question on Load and GetElementPtr instructions

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

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 <xiaolong.snake at
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?
>
>
> Thanks,
> Xiaolong

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 <xiaolong.snake at 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?

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.