llvm dev - Jul 2016 - missing opt in DAGCombiner ?

I've been surprised by how "over the top" the set of opts in
DAGCombiner are,
But here's one that might be missing

                Array[ I + (pointer-difference) ]

Generates  ...   (shl  (sra (sub  pointer1,  pointer2),  2),  2)   ...

Which gets Combined into

                (and   (sub  pointer1,  pointer2),  -4)

But IIUC the C/C++ languages require pointer1 and pointer2 to be addresses of
elements in the same array,
So their difference must be (0 mod element-size),
(even if the array itself is somehow misaligned, the difference between elements
will be aligned)
so the "SRA" is only shifting out zeros,
So the "AND" by  "-4"  is superfluous

Please let me know if I am misunderstanding something ?!

--Peter Lawrence.
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On 5 July 2016 at 15:42, Lawrence, Peter via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> But IIUC the C/C++ languages require pointer1 and pointer2 to be addresses
> of elements in the same array, So their difference must be (0 mod
element-size),
LLVM IR is much less strict about type punning (overlapping objects
wouldn't be a problem, for example) so the DAG combiner is rather
limited in what it can do.

But Clang ought to be emitting "exact" modifiers on the relevant
instructions so that LLVM's mid-end can do the right thing. When I
compile

    unsigned long foo(int *idx1, int *idx2) {
      return 4 * (idx1 - idx2);
    }

I find the shifts get eliminated based on reasoning very similar to yours.

Cheers.

Tim.