RFR: 8279508: Auto-vectorize Math.round API [v9]

[Quan Anh Mai]

On Sat, 26 Feb 2022 03:02:51 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

>> src/hotspot/cpu/x86/x86.ad line 7263:
>> 
>>> 7261:     __ vector_round_float_avx($dst$$XMMRegister, $src$$XMMRegister, $xtmp1$$XMMRegister,
>>> 7262:                               $xtmp2$$XMMRegister, $xtmp3$$XMMRegister, $xtmp4$$XMMRegister,
>>> 7263:                               ExternalAddress(vector_float_signflip()), new_mxcsr, $scratch$$Register, vlen_enc);
>> 
>> The vector_float_signflip() here should be replaced by vector_all_bits_set().
>> cvtps2dq description:
>> If a converted result cannot be represented in the destination
>> format, the floating-point invalid exception is raised, and if this exception is masked, the indefinite integer value
>> (2w-1, where w represents the number of bits in the destination format) is returned.
>
> Clarification, the number in my comments above is (2^w  - 1). This is from Intel SDM (https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html).
> Also you will need to take care when the valid unoverflowed result is -1 i.e. 0xFFFFFFFF (2^32 - 1).

I believe the indefinite value should be 2^(w - 1) (a.k.a 0x80000000) and the documentation is typoed. If you look at `cvtss2si`, the indefinite value is also written as 2^w - 1 but yet in `MacroAssembler::convert_f2i` we compare it with 0x80000000. In addition, choosing -1 as an indefinite value is weird enough and to complicate it as 2^w - 1 is really unusual.

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