RFR: 8264104: Eliminate unnecessary vector mask conversion during VectorUnbox for floating point VectorMask [v2]

[Xiaohong Gong]

On Wed, 7 Apr 2021 16:06:44 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> @iwanowww should confirm correctness of such optimization.
>> Regarding changes - they seem fine to me. I notice that VectorNode and its subclasses do not check for TOP inputs. Since Vector API introduce vectors in graph before SuperWord transformation their input could become dead. How such cases handled? And why we did not hit them yet? is_vect() should hit assert.
>
> I'm not fond of the proposed approach. 
> 
> It hard-codes some implicit assumptions about vector mask representation.  
> Also, vector type mismatch can trigger asserts since some vector-related code expects that types of vector inputs match precisely).
> 
> I suggest to introduce artificial cast nodes (`VectorLoadMask (VectorStoreMask vmask) ==> VectorMaskCast (vmask)`) which are then lowered into no-ops on backend side.

> @iwanowww should confirm correctness of such optimization.
> Regarding changes - they seem fine to me. I notice that VectorNode and its subclasses do not check for TOP inputs. Since Vector API introduce vectors in graph before SuperWord transformation their input could become dead. How such cases handled? And why we did not hit them yet? is_vect() should hit assert.

Thanks for looking at this PR @vnkozlov . To be honest, I'v no idea about the TOP checking issue to the inputs of the VectorNode. Hope @iwanowww  could explain more. Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/3238