On Sun, 13 Feb 2022 13:12:35 GMT, Jatin Bhateja <jbhat...@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
