[LLVMdev] Pointer sizes, GetElementPtr, and offset sizes

[Marc de Kruijf]

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
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