[LLVMdev] need help understanding getelementptr assembler instruction

[Reid Spencer]

Robert,

I'm going to comment because it is important that people get this
straight. 

If you're learning LLVM, please read the GEP FAQ at
http://llvm.org/GetElementPtr.html.  If there is a deficiency there
(some question not answered) please let me know and I'll add it.

On Mon, 2006-11-13 at 11:23 -0800, Robert Mykland wrote:
> Ram:
> 
> Let me explain and hopefully Reid or Chris will correct if I have it 
> wrong.  The first 0 is the index into a possible array of sbyte[13] 
> arrays that this pointer points to.  

Yes. The confusion arises with GVs because:

%str1 = internal constant [13 x sbyte] c"Hello World\0a\00"

is of type [13 x sbyte]* not [13 x sbyte]

That isn't plainly clear from the assembly but all GlobalValues are
constants of pointer type. 

Think of %str1 as some magical register floating around that points to
some place in memory. The fact that it has the same value as the address
of the first element in the array is just a mathematical coincidence.
You still have to index through the pointer and then into the array.

> I expect GetElementPtr works this 
> way is to keep down the proliferation of arrays of arrays in the type 
> table.  

I don't think so.  It works this way because of the LLVM type system.

> So you are pointing at the zeroth, and in this case only, array 
> of sbyte[13].

Yes.  Although, I prefer to think of it as indexing through the GVar
pointer.

>   The next 0 actually indexes into the array of sbyte, so 
> you are pointing at index 0, or pointing at the 'H' in "Hello World".

Right.

> 
> Hope this helps,
> 
> -- Robert.
> 
> Ram Bhamidipaty wrote:
> > I am trying to understand the hello word assember example. This is
> > my version:
> >
> > %str1 = internal constant [13 x sbyte] c"Hello World\0a\00"
> >
> > declare int %printf(sbyte*, ...)
> >
> > implementation   ; Functions:
> >
> > int %main() {
> >   %str2 = getelementptr [13 x sbyte]* %str1, long 0, long 0
> >   call int(sbyte*, ...) *%printf(sbyte* %str2)
> >   ret int 0
> > }
> >
> > Why is getelementptr being given two "long 0" indices?
> >
> > Thanks for any help.
> > -Ram
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