What is the purpose of loop predication lower bounds check

[Cao, Joshua]

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.

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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?
> 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.
>
>
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