RFR: 8308994: C2: Re-implement experimental post loop vectorization [v2]

[Pengfei Li]

On Fri, 23 Jun 2023 14:56:25 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Pengfei Li has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Address part of comments from Emanuel
>
> src/hotspot/share/opto/vmaskloop.cpp line 363:
> 
>> 361:           // Otherwise, use signed subword type or the statement's bottom type
>> 362:           if (subword_stmt) {
>> 363:             set_elem_bt(node, get_signed_subword_bt(stmt_bottom_type));
> 
> Why are you taking only the signed subword type, and not unsigned (eg for char you take short)?

Current SuperWord also does in this way (see `SuperWord::container_type()`). A main reason is that some matching rules on some backends (like x86) only matches signed subword type. AFAICR, it's good to removing this for AArch64.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1250564508