[LLVMdev] Another question about GetElementPtr Instruction

[Brian R. Gaeke]

Hi,

I've another question about getelementptr.  So, in the twisted mind of my little
pass, I have a statement like this:

%reg228-mod = getelementptr %struct.SimpleStruct* %N.1, long 0          ; <%struct.SimpleStruct*> [#uses=2]

I'm not sure of the semantics of %a = getelementptr %b, long 0.  I want to
think that it is some kind of weird identity (no-op), judging from the operand
type and the result type that is printed out by llvm.  In this sense I suppose
it is like saying a = &(b[0]) (== b). I couldn't quite make it out from reading
the language reference.

So I want to go ahead and transform uses of %reg228-mod by replacing them with
uses of %N.1, in this example.  Is this right or am I way off?

Thanks for any help,

-Brian



> 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
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> >
> 
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-- 
gaeke at uiuc.edu