RFR: 8292898: [vectorapi] Unify vector mask cast operation [v4]

[Jie Fu]

On Mon, 19 Sep 2022 03:04:57 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> 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 incrementally with one additional commit since the last revision:
> 
>   Add assertion to the elem num for mast cast

It sounds good to unify some vector mask operations.

I'm not familiar with vector mask. 
Let's wait for the real experts.
Thanks.

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

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