[External] : Re: What is the purpose of loop predication lower bounds check

[Vladimir Kozlov]

If array is external it could be 0 sized.
With locally allocated array the check should collapse unless we missing 
such optimization.

On 2/1/24 11:52 AM, Cao, Joshua wrote:
> If I use the same example but with negative stride:
> 
> 
> ```
> 
> for (int i = iters - 1; i >= 0; --i) {
> 
>    blackhole(data[i]);
> 
> }
> ```
> 
> We still generate these two checks:
> ```
> 
> if (data.length <= 0) { trap() }
> if (data.length <= iters - 1) { trap() }
> 
> ```
> 
> 
> So even with negative stride, the upper bounds check is sufficient by 
> itself.
> 
> 
> I am playing around with stride/scale/offset and I cannot come up with an
> 
> example where the lower bounds check is important.
> 
> ------------------------------------------------------------------------
> *From:* hotspot-compiler-dev <hotspot-compiler-dev-retn at openjdk.org> on 
> behalf of Vladimir Kozlov <vladimir.kozlov at oracle.com>
> *Sent:* Thursday, February 1, 2024 11:44:29 AM
> *To:* hotspot-compiler-dev at openjdk.org
> *Subject:* RE: [EXTERNAL] What is the purpose of loop predication lower 
> bounds check
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> 
> 
> Initial index value is variable.
> 
> Reverse (negative stride) processing of array?
> 
> On 2/1/24 11:31 AM, Cao, Joshua wrote:
>> Can I get some help with https://bugs.openjdk.org/browse/JDK-8325146? 
> <https://bugs.openjdk.org/browse/JDK-8325146?>
>> What is
>> the purpose of the lower bounds check for loop predication? It seems like an
>> unnecessary if-check for any example I can come up with.
>>
>> Whatever the reason might be, I think there are cases in which we can ellide
>> the lower bounds check, like in the example in the JBS issue.
>>
>>