[LLVMdev] Confusion between GetElementPtr and C++ API

[David Blaikie]

On Sun, Apr 12, 2015 at 6:19 PM, charles quarra
<charllsnotieneningunputocorreo at gmail.com> wrote:
> Looking at the documentation of `GetElementPtr`:
>
> http://llvm.org/docs/GetElementPtr.html
>
>
> The examples rely on multiple indexes: the 1st for the struct member
> and the 2nd for the element in the array. This supposedly returns an
> offset from the base pointer

Sort of the other way around. Imagine that GEP assumes that your
pointer is a pointer to the first element of an array. (eg: you have a
char* pointing to multiple characters) So the first index indexes into
that implicit array ((gep %x, 1) gives you a char* that points to the
second character rather than the first). The second index then indexes
into the structure your pointer points to, and so on.

> I'm trying to figure out what's the correct way to create a given
> `GetElementPtr` instruction with the C++ API. Unfortunately, there are
> several varieties of the `CreateXXXGEP` instruction, with a parameter
> "val" that I presume is one of the indices.

Val would be the pointer you want to index into.

> No version of it seems to
> use two indices as in the documentation:
> http://llvm.org/docs/doxygen/html/classllvm_1_1IRBuilder.html

Several variants take exactly two indices (CreateConstGEP.*2_*) which
would be fine for your needs (you just want a (gep %x, 0, 0 - which
should give you an i8*)).

> Even the `CreateStructGEP` uses a single idx parameter!

It has an implicit 0 first index (if you follow the doxygen links you
can see that it just forwards to CreateConstInboundsGEP2_32 (
http://llvm.org/docs/doxygen/html/IRBuilder_8h_source.html#l01157 )) -
as the name (perhaps poorly) implies, this is for indexing into a
single structure, so the first index is just 0 (you're not pointing to
the first element of an array, so there are no other elements to
access along that axis).

- David

>
>
> I want to do a very simple thing; I want to take a char buffer:
>
>     Value* vB = llvm::ConstantDataArray::GetString(...)
>
> and use the pointer to the array to pass it to another function that
> expects `i8*`