RFR: 8305324: C2: Wrong execution of vectorizing Interger.reverseBytes

[Daohan Qu]

On Tue, 11 Apr 2023 02:14:11 GMT, Pengfei Li <pli at openjdk.org> wrote:

>> This patch should fix [JDK-8305324](https://bugs.openjdk.org/browse/JDK-8305324).
>> 
>> `SuperWord::compute_vector_element_type()` implemented in `jdk/src/hotspot/share/opto/superword.cpp` propagates backward a narrower integer type when the upper bits of the value are not needed. However, `Integer.reverseBytes()` depends on higher-order bits of an integer and should be prevented from being narrowed and vectorized. Instead, it needs to be treated like `Math.abs()` (which is represented by `Op_AbsI` in the following code).
>> 
>> https://github.com/openjdk/jdk/blob/0243da2e4adc1b7ab6fcd5b10778532101158dce/src/hotspot/share/opto/superword.cpp#L3935-L3945
>> 
>> I have tested this patch for tier 1-3 on x86-64.
>
> src/hotspot/share/opto/superword.cpp line 3941:
> 
>> 3939:             // RShiftI or AbsI operations, the compiler has to know the precise
>> 3940:             // signedness info of the 1st operand. These operations shouldn't be
>> 3941:             // vectorized if the signedness info is imprecise.
> 
> Could you update the comments I wrote before?

Sorry, I forget to update the comments. I'll do it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13406#discussion_r1162529920