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

[Jatin Bhateja]

On Mon, 14 Feb 2022 09:12:54 GMT, Andrew Haley <aph at openjdk.org> wrote:

>>> 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.

> > 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.

**Just want to correct above statement, LDMXCSR will not be re-ordered/re-scheduled early OOO backend.**

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