RFR: 8259925: [Vector API] Unreasonable IndexOutOfBoundsException message when length < vlen

[Jie Fu]

On Thu, 21 Jan 2021 16:54:36 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>>> Unfortunately it is still problematic because you have replaced the intrinsic check (that performed by `Object.checksIndex`, or more specifically [here](https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/util/Preconditions.java#L261)).
>>> 
>>> Further, you have replaced the bounds check options, in place for experimentation. We are not ready yet to collapse our options, further analysis is required as bounds checks can be very sensitive in C2.
>>> 
>>> I would be open to you adding a third case, so that you can analyze the performance without perturbing the default behavior. I suspect the correct fix is to make intrinsic `Objects.checkFromIndexSize` in a similar manner to `Object.checksIndex`.
>> 
>> Hi @PaulSandoz ,
>> 
>> Thanks for your kind review and comments.
>> 
>> To be honest, I didn't get your first point.
>> As for the example above, the intrinsified Objects.checkIndex seems to generate more code than inlined if-statements.
>> So I'm confused why it's a problem to replace the intrinsic check.
>> Did you mean an intrinsic is always (or for most cases) better than non-intrinc or inlined if-statements?
>> And why?
>> 
>> Could you please make it more clearer to us?
>> Thanks.
>
> The intrinsic enables C2 to more reliably elide bounds checks. I don't know the exact details but at a high-level it transforms signed values into unsigned values (and therefore the signed comparisons become unsigned comparisons), which helps C2 remove checks when there is a dominating check of, for example, an upper loop bound.
> 
> You say "the intrinsified Objects.checkIndex seems to generate more code than inlined if-statements". Can you present some examples of Java code and the corresponding C2 generated assembler where this happens?

Hi @PaulSandoz ,

I will show you the assembly code next week since it is already Friday night now.

Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/2127