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

[Vladimir Ivanov]

On Fri, 18 Oct 2024 05:39:08 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> `vpmuludq` does a long multiplication but throws away the upper bits of the operands, effectively does a `(x & max_juint) * (y & max_juint)`
>
> You can see its pseudocode here https://www.felixcloutier.com/x86/pmuludq
> 
>     VPMULUDQ (VEX.256 Encoded Version)[ ¶](https://www.felixcloutier.com/x86/pmuludq#vpmuludq--vex-256-encoded-version-)
>     DEST[63:0] := SRC1[31:0] * SRC2[31:0]
>     DEST[127:64] := SRC1[95:64] * SRC2[95:64]
>     DEST[191:128] := SRC1[159:128] * SRC2[159:128]
>     DEST[255:192] := SRC1[223:192] * SRC2[223:192]
>     DEST[MAXVL-1:256] := 0

Got it. Now it makes perfect sense. Thanks for the clarifications!

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PR Review Comment: https://git.openjdk.org/jdk/pull/21244#discussion_r1805894106