[vectorIntrinsics] RFR: 8283413: Add C2 mid-end and x86 back-end implementation for bit REVERSE and REVERSE_BYTES operation [v2]

[Jatin Bhateja]

On Tue, 22 Mar 2022 10:02:28 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

>> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   8283413: Adding Ideal transform for (ReverseV (ReverseV VEC)) => VEC and (ReverseV (ReverseV VEC MASK) MASK)) => VEC
>
> src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 4397:
> 
>> 4395: #endif
>> 4396: 
>> 4397: void C2_MacroAssembler::vector_reverse_bit_avx(BasicType bt, XMMRegister dst, XMMRegister src, XMMRegister xtmp1,
> 
> You can do a this bit reverse using lookup table on each nibble and oring the results, the pseudocode would look something like this
> 
>     lut = broadcasti128(0b0000, 0b1000, 0b0100, 0b1100, 0b0010, 0b1010, 0b0110, 0b1110, 0b0001, ...)
>     mask = pbroadcastd(0x0f0f0f0f)
>     hi = pand(src, mask)
>     hi = pshufb(lut, hi)
>     hi = pslld(hi, 4)
>     lo = psrld(src, 4)
>     lo = pand(lo, mask)
>     lo = pshufb(lut, lo)
>     dst = por(lo, hi)

@merykitty , your comments addressed.

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PR: https://git.openjdk.java.net/panama-vector/pull/182