llvm dev - Dec 2011 - [LLVMdev] GetElementPtr

On Thu, Dec 8, 2011 at 2:37 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> Thanks.
>
> So basically then this operation is not target-independent like the docs
> suggest?
>
>
> On Thu, Dec 8, 2011 at 1:23 PM, Eli Friedman <eli.friedman at
gmail.com>wrote:
>
>> On Thu, Dec 8, 2011 at 11:49 AM, Ryan Taylor <ryta1203 at
gmail.com> wrote:
>> > After some thought, to put this more simply (more direct), it
would be
>> fine
>> > if the getelementptr is lowered into IR assuming an x86
architecture.
>> The
>> > real problem is that I don't want the x86 code generation, I
still want
>> to
>> > deal in LLVM IR just with the GEP lowered, and it lowered for the
x86
>> > architecture is fine.
>> >
>> > Is there any way to do this?
>>
>> It should be straightforward to write, but I don't think
there's any
>> existing code to do this.  You can grab the necessary information
>> about the target out of TargetData.
>>
>> -Eli
>>
>
>
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Ok, thanks, this makes sense. But there is no way to get the SelectionDAG
to do it for me via something like visitGetElementrPtr?

On Thu, Dec 8, 2011 at 2:54 PM, Eli Friedman <eli.friedman at gmail.com>
wrote:
> > On Thu, Dec 8, 2011 at 2:37 PM, Ryan Taylor <ryta1203 at
gmail.com> wrote:
> >>
> >> Thanks.
> >>
> >> So basically then this operation is not target-independent like
the docs
> >> suggest?
>
> It is target-independent in the sense that a frontend can generate
> code using GEP's without worrying about how a given target actually
> lays out structures.  It isn't target-independent in the sense that
> the actual computed offset in bytes isn't the same for every target.
> See also
> http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
> .
>
> -Eli
>
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