[LLVMdev] Pointer sizes, GetElementPtr, and offset sizes

[Marc de Kruijf]

I didn't realize this before, but perhaps the fact that llvm-gcc was unable
to optimize out the offset calculation at -O3 is sufficient evidence for
supporting such an instruction. :)

Marc

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);
> }
>
> $ llvm-gcc -O3 -c test.c -emit-llvm -o - | llvm-dis
> ; ModuleID = '<stdin>'
> target datalayout =
> "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
> target triple = "i686-pc-linux-gnu"
>
> define 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]
>         %tmp7 = ashr i32 %tmp6, 2               ; <i32> [#uses=1]
>         ret i32 %tmp7
> }
>
>
> The return value is 1.  The ashr exposes the pointer size by shifting the
> 4 byte distance over by 2.
> For the analysis that I am doing, it would be nice to have an instruction
> that explicitly performs this distance calculation in a type-safe manner,
> irrespective of pointer size.  Something like this:
>
> define 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]
>         %tmp2 = getdistance i32** %x, %tmp1            ; <i32> [#uses=1]
>         ret i32 %tmp2
> }
>
> I'm not really a compiler person, so I'm wondering if a need for such an
> instruction ever arises in more compiler-oriented situations such as
> optimization or wrt. portability?  Does the fact that pointer size is never
> completely hidden ever cause problems?  Is GetElementPtr generally "good
> enough"?  It would be nice to have a complete solution though, wouldn't it?
> Thoughts?
>
> Marc
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20080502/601dad71/attachment.html>