Re: [Qemu-devel] [PATCH v1 33/33] s390x/tcg: Implement VECTOR UNPACK *

[Richard Henderson <richard.henderson@linaro.org>]

On 2/28/19 2:54 AM, David Hildenbrand wrote:
> Hmm, as v2 and v3 are handled concatenated it is not that easy. I am not
> sure if we can handle this without a temporary vector.
> 
> I thought about packing them first interleaved
> 
> v2 = [v2e0, v2e1]
> v3 = [v3e0, ve31]
> v1 = [v2e0_packed, v3e0_packed, v2e1_packed, v3e1_packed]
> 
> And then restoring the right order
> 
> v1 = [v2e0_packed, v2e1_packed, v3e0_packed, v3e1_packed]
> 
> But than the second operation seems to be the problem. That shuffling
> would have to be hard coded as far as I can see. (shuffling with MO_8 is
> nasty -> 14 element shave to be exchanged, in my opinion needing
> eventually 14 temporary variables)

I suppose you could do it in registers.

  load_element_i64(t1, v2, 0);
  for (i = 1; i < N; i++) {
    load_element_i64(t3, v2, i, es);
    tcg_gen_deposit_i64(t1, t1, t3, i << es, 1 << es);
  }
  // repeat for v3 into t2
  // store t1,t2 into v1.

Now you have only 3 temporaries, which is manageable.

The only question, when it comes to MO_8, is whether the code expansion of this
is reasonable (16 byte loads, 15 deposits, 2 stores -- minimum 33 insns,
probably 48 for x86_64 host), or whether a helper function would be better in
the end.  But then the same is true for all of the other merge & unpack
operations wrt MO_8.


r~