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

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
> http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
>

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

That's correct,
	X = getElementPtr <Ty>* ptr, long 0
is a NOP.  It does exactly what you said: X = &b[0].

	X = getElementPtr <Ty>* ptr
is a NOP too.

--Vikram

-------------------------------------------------------------------------
 VIKRAM S. ADVE
 Assistant Professor                            E-MAIL: vadve at cs.uiuc.edu
 Department of Computer Science                 PHONE:  (217) 244-2016
 Univ. of Illinois at Urbana-Champaign          FAX:    (217) 244-6869
 1304 W. Springfield Ave.              WWW: http://www.cs.uiuc.edu/~vadve
 Urbana IL 61801-2987.
-------------------------------------------------------------------------

> -----Original Message-----
> From: Brian R. Gaeke [mailto:gaeke at uiuc.edu]
> Sent: Friday, September 27, 2002 10:17 PM
> To: Vikram S. Adve
> Cc: Brian R. Gaeke; llvmdev at cs.uiuc.edu
> Subject: Another question about GetElementPtr Instruction
>
>
>
> 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
> > > http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
> > >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> --
> gaeke at uiuc.edu
>