RFR: 8292898: [vectorapi] Unify vector mask cast operation [v2]

[Xiaohong Gong]

> The current implementation of the vector mask cast operation is
> complex that the compiler generates different patterns for different
> scenarios. For architectures that do not support the predicate
> feature, vector mask is represented the same as the normal vector.
> So the vector mask cast is implemented by `VectorCast `node. But this
> is not always needed. When two masks have the same element size (e.g.
> int vs. float), their bits layout are the same. So casting between
> them does not need to emit any instructions.
> 
> Currently the compiler generates different patterns based on the
> vector type of the input/output and the platforms. Normally the
> "`VectorMaskCast`" op is only used for cases that doesn't emit any
> instructions, and "`VectorCast`" op is used to implement the necessary
> expand/narrow operations. This can avoid adding some duplicate rules
> in the backend. However, this also has the drawbacks:
> 
>  1) The codes are complex, especially when the compiler needs to
>     check whether the hardware supports the necessary IRs for the
>     vector mask cast. It needs to check different patterns for
>     different cases.
>  2) The vector mask cast operation could be implemented with cheaper
>     instructions than the vector casting on some architectures.
> 
> Instead of generating `VectorCast `or `VectorMaskCast `nodes for different
> cases of vector mask cast operations, this patch unifies the vector
> mask cast implementation with "`VectorMaskCast`" node for all vector types
> and platforms. The missing backend rules are also added for it.
> 
> This patch also simplies the vector mask conversion happened in
> "`VectorUnbox::Ideal()`". Normally "`VectorUnbox (VectorBox vmask)`" can
> be optimized to "`vmask`" if the unboxing type matches with the boxed
> "`vmask`" type. Otherwise, it needs the type conversion. Currently the
> "`VectorUnbox`" will be transformed to two different patterns to implement
> the conversion:
> 
>  1) If the element size is not changed, it is transformed to:
> 
>     "VectorMaskCast vmask"
> 
>  2) Otherwise, it is transformed to:
> 
>     "VectorLoadMask (VectorStoreMask vmask)"
> 
> It firstly converts the "`vmask`" to a boolean vector with "`VectorStoreMask`",
> and then uses "`VectorLoadMask`" to convert the boolean vector to the
> dst mask vector. Since this patch makes "`VectorMaskCast`" op supported
> for all types on all platforms, it doesn't need the "`VectorLoadMask`" and
> "`VectorStoreMask`" to do the conversion. The existing transformation:
> 
>   VectorUnbox (VectorBox vmask) => VectorLoadMask (VectorStoreMask vmask)
> 
> can be simplified to:
> 
>   VectorUnbox (VectorBox vmask) => VectorMaskCast vmask

Xiaohong Gong has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:

  8292898: [vectorapi] Unify vector mask cast operation

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

Changes: https://git.openjdk.org/jdk/pull/10192/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10192&range=01
  Stats: 360 lines in 8 files changed: 278 ins; 62 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/10192.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10192/head:pull/10192

PR: https://git.openjdk.org/jdk/pull/10192