[LLVMdev] GetElementPtr

[John Criswell]

On 12/12/11 7:00 PM, Ryan Taylor wrote:
> So in this example:
>
> %idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
>
> Why is it picking i64 for the index but i32 for the offset?

I believe pointers and arrays are indexed using i64 (or some integer 
size matching the pointer size) and structure elements are indexed using 
i32.

-- John T.

>
>
> On Mon, Dec 12, 2011 at 4:58 PM, Ryan Taylor <ryta1203 at gmail.com 
> <mailto:ryta1203 at gmail.com>> wrote:
>
>
>
>     ---------- Forwarded message ----------
>     From: *Ryan Taylor* <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>>
>     Date: Mon, Dec 12, 2011 at 4:58 PM
>     Subject: Re: [LLVMdev] GetElementPtr
>     To: Eli Friedman <eli.friedman at gmail.com
>     <mailto:eli.friedman at gmail.com>>
>
>
>     Sorry,
>
>       So what I'm trying to ask is are the widths given (32, 64) for
>     the index and the offset the widths of the index and offset values
>     or the width of the type they are offsetting and indexing or both?
>
>       I apologize if you already answered this and I didn't get it.
>
>
>     On Mon, Dec 12, 2011 at 4:50 PM, Eli Friedman
>     <eli.friedman at gmail.com <mailto:eli.friedman at gmail.com>> wrote:
>
>         On Mon, Dec 12, 2011 at 4:46 PM, Ryan Taylor
>         <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>> wrote:
>         > So in the second example I gave, why is the pointer type 32
>         but the index
>         > type is 64?
>
>         Because that's what the frontend chose the generate?  I'm not sure
>         what you're trying to ask.
>
>         -Eli
>
>         > On Mon, Dec 12, 2011 at 4:43 PM, Eli Friedman
>         <eli.friedman at gmail.com <mailto:eli.friedman at gmail.com>>
>         > wrote:
>         >>
>         >> On Mon, Dec 12, 2011 at 4:38 PM, Ryan Taylor
>         <ryta1203 at gmail.com <mailto:ryta1203 at gmail.com>> wrote:
>         >> > By LLVM do you mean the backend? I'm not using the
>         backend, so is that
>         >> > i32
>         >> > on the 0 index the type of the index value or the type of
>         the value to
>         >> > which
>         >> > exists at that index?
>         >> >
>         >> > it seems the pointer itself has no width, it's arbitrary
>         and is handled
>         >> > in
>         >> > the lowering and is target dependent on the bus width.
>         >> >
>         >> > Basically, when I am computing offset I need to know the
>         sizes for add.
>         >> > The
>         >> > size of the pointer (dedicated by bus width) and the size
>         of the index.
>         >>
>         >> The size of the index is simply its type.  You would normally
>         >> sign-extend or truncate to the width of a pointer to do
>         arithmetic,
>         >> though.
>         >>
>         >> You can grab the pointer size from
>         TargetData::getPointerSizeInBits().
>         >>
>         >> -Eli
>         >
>         >
>
>
>
>
>
>
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