RFR: 8370454: C2 SuperWord: unsigned comparison information is lost for VectorMaskCmp

[Hamlin Li]

On Thu, 23 Oct 2025 05:36:53 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Hi,
>> Can you help to review the patch? @eme64 
>> 
>> Currently, in SLP if we support transformation from (Bool + CmpU + CMove) to (VectorMaskCmp + VectorBlend), the unsigned comparison information is lost, it's in CmpU, but current code only check Bool for the information. For details please check code at `SuperWordVTransformBuilder::make_vector_vtnode_for_pack` and `PackSet::get_bool_test`.
>> 
>> This loss of unsigned comparison information blocks the optimization proposed in https://github.com/openjdk/jdk/pull/25336 and https://github.com/openjdk/jdk/pull/25341.
>> 
>> Currently, `BoolTest` does not support an unsigned construction (`BoolTest( mask btm ) : _test(btm) { assert((btm & unsigned_compare) == 0, "unsupported");}`), seems to me a feasible solution would be get the unsigned information from CmpU (which could be an input of Bool) and pass it to VectorMaskCmp.
>> 
>> Thanks
>
> src/hotspot/share/opto/superword.cpp line 1704:
> 
>> 1702:                      cmp0->Opcode() == Op_CmpUL ||
>> 1703:                      cmp0->Opcode() == Op_CmpU3 ||
>> 1704:                      cmp0->Opcode() == Op_CmpUL3;
> 
> Maybe it is time to create a `switch` statement below ;)

Yes, will fix! :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/27942#discussion_r2454598742