[LLVMdev] GEP instructions: is it possible to reverse-engineer array accesses?

[Duncan Sands]

Hi Gabriel, I suggest you don't bother with testcases like this that are doing
undefined things.  For example, neither i nor k are initialized, so the result
of accessing the array is undefined.  Thus the frontend can (and apparently
does) produce anything strange thing it does.  What is more, the result aux is
unused, so there is no obligation to compute it correctly.  I think you will
get more understandable results with a more sensible testcase.

Ciao, Duncan.

> As of late I am having a hard time getting my head around how array accesses
> are translated by Clang into LLVM IR: the often misunderstood GEP instruction.
> I am trying to reverse-engineer array accesses to discover the number of dimensions
> and actual indexes of the original, and I am beginning to wonder whether this is
> possible at all. To illustrate (some of) my troubles, consider the following
> code and
> the LLVM IR for both 32 and 64 bit memory addresses:
>
> --
> original C:
>
> #define N 1000
>
> int main(int argc, char **argv)
> {
> int i, k;
> float aux, A[N][N];
>
> aux = A[k][i];
> }
>
> --
> 32-bit addresses LLVM IR (relevant part):
>
> %4 = load i32* %i, align 4
> %5 = load i32* %k, align 4
> %6 = getelementptr inbounds [1000 x [1000 x float]]* %A, i32 0, i32 %5
> %7 = getelementptr inbounds [1000 x float]* %6, i32 0, i32 %4
> %8 = load float* %7
> store float %8, float* %aux, align 4
>
> --
> 64-bit addresses LLVM IR (relevant part):
>
> %4 = load i32* %i, align 4
> %5 = load i32* %k, align 4
> %6 = getelementptr inbounds [1000 x [1000 x float]]* %A, i32 0, i32 0
> %7 = sext i32 %5 to i64
> %8 = getelementptr inbounds [1000 x float]* %6, i64 %7
> %9 = load float* %8
> store float %9, float* %aux, align 4
>
> --
>
>
> Why does the 64-bit addresses version use two leading 0s instead of one? I have
> tried reading
> http://llvm.org/docs/GetElementPtr.html and I don't think the explanation
> provided is accurate, or
> at least I can't see how to apply it to this particular case.
>
> Besides, there is an incredible diversity of variations in how arrays can be
> represented and accessed
> in C codes, leading to my final question: is it really possible to
> reverse-engineer array accesses? If so,
> any insights?
>
>
> Thanks in advance, and best regards,
> Gabriel
>
>
>
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