RFR: 8279508: Auto-vectorize Math.round API [v3]

[Andrew Haley]

On Sun, 13 Feb 2022 13:12:35 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>>> Hi, IIRC for evex encoding you can embed the RC control bit directly in the evex prefix, removing the need to rely on global MXCSR register. Thanks.
>> 
>> Hi @merykitty ,  You are correct, we can embed RC mode in instruction encoding of round instruction (towards -inf,+inf, zero). But to match the semantics of Math.round API one needs to add 0.5[f] to input value and then perform rounding over resultant value, which is why @sviswa7 suggested to use a global rounding mode driven by MXCSR.RC so that intermediate floating inexact values are resolved as desired, but OOO execution may misplace LDMXCSR and hence may have undesired side effects.
>
>> What does this do? Comment, even pseudo code, would be nice.
> 
> Thanks @theRealAph , I shall append the comments over the routine.
> BTW, entire rounding algorithm can also be implemented using  Vector API which can perform if-conversion using masked operations.
> 
> class roundf {
>    public static VectorSpecies ISPECIES = IntVector.SPECIES_512;
>    public static VectorSpecies SPECIES = FloatVector.SPECIES_512;
> 
>    public static int round_vector(float[] a, int[] r, int ctr) {
>       IntVector shiftVBC = (IntVector) ISPECIES.broadcast(24 - 2 + 127);
>       for (int i = 0; i < a.length; i += SPECIES.length()) {
>          FloatVector fv = FloatVector.fromArray(SPECIES, a, i);
>          IntVector iv = fv.reinterpretAsInts();
>          IntVector biasedExpV = iv.lanewise(VectorOperators.AND, 0x7F800000);
>          biasedExpV = biasedExpV.lanewise(VectorOperators.ASHR, 23);
>          IntVector shiftV = shiftVBC.lanewise(VectorOperators.SUB, biasedExpV);
>          VectorMask cond = shiftV.lanewise(VectorOperators.AND, -32)
>                .compare(VectorOperators.EQ, 0);
>          IntVector res = iv.lanewise(VectorOperators.AND, 0x007FFFFF)
>                .lanewise(VectorOperators.OR, 0x007FFFFF + 1);
>          VectorMask cond1 = iv.compare(VectorOperators.LT, 0);
>          VectorMask cond2 = cond1.and(cond);
>          res = res.lanewise(VectorOperators.NEG, cond2);
>          res = res.lanewise(VectorOperators.ASHR, shiftV)
>                .lanewise(VectorOperators.ADD, 1)
>                .lanewise(VectorOperators.ASHR, 1);
>          res = fv.convert(VectorOperators.F2I, 0)
>                .reinterpretAsInts()
>                .blend(res, cond);
>          res.intoArray(r, i);
>       }
>       return r[ctr];
>    }

That pseudocode would make a very useful comment too. This whole patch is very thinly commented.

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PR: https://git.openjdk.java.net/jdk/pull/7094