[LLVMdev] Pointer sizes, GetElementPtr, and offset sizes

[Andrew Lenharth]

On Fri, May 2, 2008 at 1:22 PM, Marc de Kruijf <dekruijf at cs.wisc.edu> wrote:
> The LLVA and LLVM papers motivate the GetElementPtr instruction by arguing
> that it abstracts implementation details, in particular pointer size, from
> the compiler.  While it does this fine for pointer addresses, it does not
> manage it for address offsets.  Consider the following code:
>
> $ cat test.c
> int main() {
>     int *x[2];
>     int **y = &x[1];
>     return (y - x);
> }

Idefine i32 @main() nounwind  {
entry:
        %x = alloca [2 x i32*]          ; <[2 x i32*]*> [#uses=2]
        %tmp1 = getelementptr [2 x i32*]* %x, i32 0, i32 1
 ; <i32**> [#uses=1]
        %tmp23 = ptrtoint i32** %tmp1 to i32            ; <i32> [#uses=1]
        %x45 = ptrtoint [2 x i32*]* %x to i32           ; <i32> [#uses=1]
        %tmp6 = sub i32 %tmp23, %x45            ; <i32> [#uses=1]
        %size = getelementptr i32** null, i32 1         ; <i32**> [#uses=1]
        %sizeI = ptrtoint i32** %size to i32            ; <i32> [#uses=1]
        %tmp7 = ashr i32 %tmp6, %sizeI          ; <i32> [#uses=1]
        ret i32 %tmp7
}

There, pointer size independent.  The problem you see is you are using
a frontend targeting a specific platform, so pointersize is known (see
the target datalayout line).

Andrew