llvm dev - Dec 2011 - [LLVMdev] GetElementPtr

Eli,

   Ok, thanks, this is a big help. So how can I use the TargetData (or get
the TargetData) without having a DAG?

On Thu, Dec 8, 2011 at 3:45 PM, Eli Friedman <eli.friedman at gmail.com>
wrote:
> On Thu, Dec 8, 2011 at 3:29 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
> > Ok, thanks, this makes sense. But there is no way to get the
> SelectionDAG to
> > do it for me via something like visitGetElementrPtr?
>
> SelectionDAGBuilder::visitGetElementPtr uses the same algorithm you
> want to use, but it is specialized for building a SelectionDAG, which
> is lower level than LLVM IR.
>
> -Eli
>
> >
> >
> > 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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On Thu, Dec 8, 2011 at 6:07 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
> Eli,
>
>    Ok, thanks, this is a big help. So how can I use the TargetData (or get
> the TargetData) without having a DAG?
If you're writing a transformation pass, just write
"getAnalysisIfAvailable<TargetData>()" to get a TargetData*.

-Eli

For the gep:

%idx1 = getelementptr i32* %MyVar, i32 0

i32* is the type that MyVar is pointing to and i32 is the type of the
offset value, or what? If it's the type of offset value, then
the size of the pointer shouldn't be less than i32, correct?

The index is 0, so in this example, the address computation is idx1 =
&MyVar+0.

What I want to know is the size in bits of the values above, it looks
like 0 is 32 bits in size, which would make the pointer size also 32?





On Thu, Dec 8, 2011 at 6:45 PM, Eli Friedman <eli.friedman at gmail.com>
wrote:
> On Thu, Dec 8, 2011 at 6:07 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
> > Eli,
> >
> >    Ok, thanks, this is a big help. So how can I use the TargetData (or
> get
> > the TargetData) without having a DAG?
>
> If you're writing a transformation pass, just write
> "getAnalysisIfAvailable<TargetData>()" to get a
TargetData*.
>
> -Eli
>
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