RFR: 8341137: Optimize long vector multiplication using x86 VPMUL[U]DQ instruction

[Jatin Bhateja]

On Fri, 18 Oct 2024 02:41:47 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

> The issues I have with this patch are that:
> 
> * It convolutes the graph with machine-dependent nodes early in the compiling process.

MulVL is a machine independent IR, we create a machine dependent IR post matching.

> * It overloads `MulVL` with alternative behaviours, it is fine now as we do not perform much analysis on this node but it would be problematic later. I think it is more preferable to have a separate IR node for this like `MulVLowIToLNode`, or have this transformation be done only just before matching, or both.

I see this is as a twostep optimization, in the first step we do analysis and annotate additional information on existing IR, which is later used by instruction selector.  I plan to subsume first stage with enhanced dataflow analysis going forward.

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PR Comment: https://git.openjdk.org/jdk/pull/21244#issuecomment-2421300738