llvm dev - Sep 2002 - [LLVMdev] question about GetElementPtr Instruction

I have a question about GetElementPtr. 

Suppose I have an GetElementPtr Instruction GI:

%reg = getelementptr %ST* %s, uint 1, ubyte 2, ubyte 1, uint
5, uint 13

I want to check if this is the reference of a component of a
structure, how can I do that? Should I check which operand of
this instruction is 'ubyte' type? How can I do that in code?

should I use 
ubyte *a = dyn_cast<ubyte>GI.getoperand(i) or something else?

Thanks,
xiaodong

> I have a question about GetElementPtr.
>
> Suppose I have an GetElementPtr Instruction GI:
>
> %reg = getelementptr %ST* %s, uint 1, ubyte 2, ubyte 1, uint
> 5, uint 13
>
> I want to check if this is the reference of a component of a
> structure, how can I do that? Should I check which operand of
> this instruction is 'ubyte' type? How can I do that in code?

Yes, indices of type UByte are indices into structures; type Long are
indices into arrays.  Also, structure indices are always constants.

> should I use
> ubyte *a = dyn_cast<ubyte>GI.getoperand(i) or something else?
You are trying to cast the operand (a Value) to a particular primitive type,
but there is no such thing as class ubyte.  Instead:

	if (GI.getoperand(i)->getType() == Type::UByteTy) {
	  ConstantUInt* idx = cast<ConstantUInt>(GI.getoperand(i));
	  ...
	}

You could also do the following (since array indices are signed and
structure indices are unsigned and always constant, but I don't recommend
it):

	if (ConstantUInt* idx = dyn_cast<ConstantUInt>(GI.getoperand(i))) {
	  ...
	}

--Vikram

Dear LLVMdev,

As far as I can tell, what the "ubyte 2" value really means is the
2+1'th member of the structure...regardless of whether that structure's
3rd element actually starts at (pointer to structure) + 3*sizeof(ubyte)
or not.  So, is there any semantic connection between "ubyte" as
used in structure accesses and what getelementptr actually interprets
it as?  Or is it just called ubyte for fun?

-"Confused in Urbana", Brian 
> I have a question about GetElementPtr. 
> 
> Suppose I have an GetElementPtr Instruction GI:
> 
> %reg = getelementptr %ST* %s, uint 1, ubyte 2, ubyte 1, uint
> 5, uint 13
-- 
gaeke at uiuc.edu

You are correct, ubyte 2 simply means 3rd element of the structure.  This
quantity is always constant (it specifies the field number), so we could
have used any signed or unsigned integer type for it instead of ubyte.
UByte means we are limited to at most 256 structure fields but it also makes
the bytecode representation more compact.  (To remove this restriction, we
may migrate to UInt in the future and use some kind of Zipf encoding to
avoid increasing bytecode size in the common case.)

There is a routine TargetData::GetIndexedOffset to convert any ptr + index
list to the actual byte  offset for a particular target machine.  See
llvm/include/Target/TargetData.h.

--Vikram

> -----Original Message-----
> From: llvmdev-admin at cs.uiuc.edu [mailto:llvmdev-admin at cs.uiuc.edu]On
> Behalf Of Brian R. Gaeke
> Sent: Thursday, September 26, 2002 1:55 AM
> To: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] question about GetElementPtr Instruction
>
>
>
> Dear LLVMdev,
>
> As far as I can tell, what the "ubyte 2" value really means is
the
> 2+1'th member of the structure...regardless of whether that
structure's
> 3rd element actually starts at (pointer to structure) + 3*sizeof(ubyte)
> or not.  So, is there any semantic connection between "ubyte" as
> used in structure accesses and what getelementptr actually interprets
> it as?  Or is it just called ubyte for fun?
>
> -"Confused in Urbana", Brian
>
> > I have a question about GetElementPtr.
> >
> > Suppose I have an GetElementPtr Instruction GI:
> >
> > %reg = getelementptr %ST* %s, uint 1, ubyte 2, ubyte 1, uint
> > 5, uint 13
>
> --
> gaeke at uiuc.edu
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