On Fri, 18 Oct 2024 05:37:16 GMT, Quan Anh Mai <qa...@openjdk.org> wrote:
>> src/hotspot/share/opto/vectornode.cpp line 2122: >> >>> 2120: // MulL (URShift SRC1 , 32) (URShift SRC2, 32) >>> 2121: // MulL (URShift SRC1 , 32) ( And SRC2, 0xFFFFFFFF) >>> 2122: // MulL ( And SRC1, 0xFFFFFFFF) (URShift SRC2 , 32) >> >> I don't understand how it works... According to the documentation, >> `VPMULDQ`/`VPMULUDQ` consume vectors of double words and produce a vector of >> quadwords. But it looks like `SRC1`/`SRC2` are always vectors of longs >> (quadwords). And `vmuludq_reg` in `x86.ad` just takes the immedate operands >> and pass them into `vpmuludq` which doesn't look right... > > `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)[ ΒΆ](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 ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21244#discussion_r1805888984
