RFR: 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast [v5]

[Jie Fu]

On Wed, 31 Aug 2022 02:09:54 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> Recently we found the performance of "`FIRST_NONZERO`" for double type is largely worse than the other types on x86 when `UseAVX=2`. The main reason is the "`VectorCastL2X`" op is not supported by the backend when the dst element type is `T_DOUBLE`. This makes the check of `VectorCast` op fail before intrinsifying "`VectorMask.cast()`" which is used in the
>> "`FIRST_NONZERO`" java implementation (see [1]). However, the compiler will not generate the `VectorCast `op for `VectorMask.cast()` if:
>> 
>>  1) the current platform supports the predicated feature
>>  2) the element size (in bytes) of the src and dst type is the same
>> 
>> So the check of "`VectorCast`" op is needless for such cases. To fix it, this patch:
>> 
>>  1) limits the specified vector cast op check to vectors
>>  2) adds the relative mask cast op check for VectorMask.cast()
>>  3) cleans up the unnecessary codes
>> 
>> Here is the performance of "`FIRST_NONZERO`" benchmark [2] on a x86 machine with `UseAVX=2`:
>> 
>> Benchmark                          (size) Mode Cnt Before  After   Units
>> DoubleMaxVector.FIRST_NONZERO       1024  thrpt 15 49.266 2460.886 ops/ms
>> DoubleMaxVector.FIRST_NONZEROMasked 1024  thrpt 15 49.554 1892.223 ops/ms
>> 
>> [1] https://github.com/openjdk/jdk/blob/master/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/DoubleVector.java#L770
>> [2] https://github.com/openjdk/panama-vector/blob/vectorIntrinsics/test/micro/org/openjdk/bench/jdk/incubator/vector/operation/DoubleMaxVector.java#L246
>
> Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add vector cast op check for vector mask for some cases

src/hotspot/share/opto/vectorIntrinsics.cpp line 2498:

> 2496:     }
> 2497:     if (is_mask && is_floating_point_type(elem_bt_to)) {
> 2498:       new_elem_bt_to = elem_bt_to == T_FLOAT ? T_INT : T_LONG;

Why we need this change?

src/hotspot/share/opto/vectorIntrinsics.cpp line 2503:

> 2501: 
> 2502:     // Make sure that vector cast is implemented to particular type/size combination.
> 2503:     bool no_vec_cast_check = is_mask && (src_type->isa_vectmask() ||

To make it safer, let's also check `dst_type->isa_vectmask()` like the following.

      if (is_mask) {
        if ((dst_type->isa_vectmask() && src_type->isa_vectmask()) ||
            (type2aelembytes(elem_bt_from) == type2aelembytes(elem_bt_to))) {
          op = gvn().transform(new VectorMaskCastNode(op, dst_type));
        } else {
          op = VectorMaskCastNode::makeCastNode(&gvn(), op, dst_type);
        }
      }

-------------

PR: https://git.openjdk.org/jdk/pull/9737