[PATCH] D12334: [ARM] Do not use vtrn for vectorshuffle if the order is reversed

[Ahmed Bougacha via llvm-commits]

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This revision is now accepted and ready to land.

LGTM with the additional suggested testcase (if it does make sense), thanks!

I'm surprised we didn't see this earlier, I guess it was exposed by r240118?


================
Comment at: lib/Target/ARM/ARMISelLowering.cpp:5059
@@ -5050,3 +5058,3 @@
   for (unsigned i = 0; i < M.size(); i += NumElts) {
     WhichResult = M[i] == 0 ? 0 : 1;
     for (unsigned j = 0; j < NumElts; j += 2) {
----------------
Looking at this again, I realize: isn't this a tad too conservative? What happens when you have:


```
<-1, 4, 2, 6, 1, 5, 3, 7>
```

More importantly: could this break, say with:

```
<-1, 5, 3, 7, 1, 5, 3, 7>
```

which isn't a vtrn, but looks like it will match?  After this patch the '>=' should catch this, I think.  If so, could you add a testcase? 


http://reviews.llvm.org/D12334