[vectorIntrinsics] RFR: 8266720: Wrong implementation in LibraryCallKit::inline_vector_shuffle_iota

[Wang Huang]

On Fri, 14 May 2021 01:37:57 GMT, Wang Huang <whuang at openjdk.org> wrote:

>> src/hotspot/share/opto/vectorIntrinsics.cpp line 456:
>> 
>>> 454:     res = gvn().transform(VectorNode::make(Op_AndI, res, bcast_mod, num_elem, elem_bt));
>>> 455:   } else {
>>> 456:     ConINode* pred_node = (ConINode*)gvn().makecon(TypeInt::make(BoolTest::ugt));
>> 
>> Unsigned comparison adds overhead and is not supported on all architectures.
>
> After changing notes with @XiaohongGong , I think we can also fix like this: 
> 
>     ConINode* pred_node = (ConINode*)gvn().makecon(TypeInt::make(BoolTest::ge));
>     Node * lane_cnt_tmp  = gvn().makecon(TypeInt::make(num_elem - 1));
>     Node * bcast_lane_cnt = gvn().transform(VectorNode::scalar2vector(lane_cnt_tmp, num_elem, type_bt));
>     Node* mask = gvn().transform(new VectorMaskCmpNode(BoolTest::ge, bcast_lane_cnt, res, pred_node, vt));
> 
>     // Make the indices greater than lane count as -ve values. This matches the java side implementation.
>     res = gvn().transform(VectorNode::make(Op_AndI, res, bcast_mod, num_elem, elem_bt));
>     Node * lane_cnt  = gvn().makecon(TypeInt::make(num_elem)); // Add a mov & bcast here
>     Node * bcast_lane_cnt = gvn().transform(VectorNode::scalar2vector(lane_cnt, num_elem, type_bt));
>     Node * biased_val = gvn().transform(VectorNode::make(Op_SubI, res, bcast_lane_cnt, num_elem, elem_bt));
>     res = gvn().transform(new VectorBlendNode(biased_val, res, mask));

> Unsigned comparison adds overhead and is not supported on all architectures.

However, if we don't use ugt ,we will encounter problem if  length > 1024  in future.  Changing `< num_elem` to `<= 128` is just a solution to `1024` itself. If `num_elem > 128`, it will be invalid.

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PR: https://git.openjdk.java.net/panama-vector/pull/81