JEP 370 - text example leads to exception

Tue Feb 18 03:52:00 UTC 2020

Maurizio, thank you for your suggestion! One of my coworkers suggested also
using padding instead of keeping a compact/packed memory layout.
The reason my example worked was because Java char is 2 bytes and int is
4... So using 16-bit alignment for "creditScore" the was the common
denominator.

Thank you for all the feedback!
  Cristi

On Mon, Feb 17, 2020 at 10:43 AM Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> Hi Chris,
> thanks for the links - took a look at your first test and, more
> specifically at example03SSNToCreditScore:
>
> MemoryLayout ssnAndCreditStruct = MemoryLayout.ofStruct(
>         MemoryLayout.ofSequence(9,
> MemoryLayout.ofValueBits(Character.SIZE, order)).withName("ssn"),
>         MemoryLayout.ofValueBits(Integer.SIZE,
> order).withName("creditScore").withBitAlignment(16));
>
> I guess this is what you referred to when you spoke about alignment. Here
> the root problem is that you have a sequence of 9 chars - hence 9 bytes,
> which is not a multiple of 4 (the size of an int). So the "creditScore"
> field will start at offset 9 (in bytes) - meaning that the int will not be
> aligned. You have two options here:
>
> 1) You do what a C compiler would have done - e.g. you add a padding
> layout between the sequence layout and the credit score layout:
>
> MemoryLayout ssnAndCreditStruct = MemoryLayout.ofStruct(
>         MemoryLayout.ofSequence(9,
> MemoryLayout.ofValueBits(Character.SIZE, order)).withName("ssn"),
> *MemoryLayout.ofPaddingBits(16)*,
>         MemoryLayout.ofValueBits(Integer.SIZE,
> order).withName("creditScore"));
>
> 2) relax the alignment constraints of the credit score field - e.g. from
> being 4-byte aligned to be 1-byte aligned (your example relaxes it to
> 2-byte aligned, not 100% if that is correct?)
>
> You do (1) in normal cases, where you want fast, aligned access - (2)
> should be done in cases where you want packed layouts - but it is possible
> that (2) might still not work (not all platform supports unaligned access
> primitives, and not in all possible access modes - e.g. atomic).
>
> Maurizio
>
> This should take care of the issue
>
>
> On 17/02/2020 04:58, Chris T wrote:
>
> Maurizio, thanks for pointing the bug out - however I don't think I was
> impacted by it.
>
> I agree with you that a more complex examples might distract the audience
> from the main presentation points.
>
> As mentioned in one of my previous email, I finalized some examples myself:
> 1. One example for memory layouts creates a structure where we associate
> the SSN (social security number) to a credit score. The main point here is
> to make a mix between char arrays and integers.
> Code is here:
> https://github.com/knowledge-base-and-tutorials/java14-features/blob/master/src/main/java/com/github/kbnt/java14/fma/ForeignMemoryAccessExamples.java
> <https://urldefense.com/v3/__https://github.com/knowledge-base-and-tutorials/java14-features/blob/master/src/main/java/com/github/kbnt/java14/fma/ForeignMemoryAccessExamples.java__;!!GqivPVa7Brio!Pf9Nt42jivBKFcxTzsgBVXVMil8JZN4IRFDQyoAO0Z-g4vnp-zkHGM8CgPO6aZD458bQ8Ps$>
> (method called example03SSNToCreditScore)
> Presentation of the case is here:
> https://www.youtube.com/watch?v=NwXzT8T6mb8&list=PLGDP1Irs2PmWNwAwMPdyOxCqkFqB6gtp9&index=7&t=1287s
> <https://urldefense.com/v3/__https://www.youtube.com/watch?v=NwXzT8T6mb8&list=PLGDP1Irs2PmWNwAwMPdyOxCqkFqB6gtp9&index=7&t=1287s__;!!GqivPVa7Brio!Pf9Nt42jivBKFcxTzsgBVXVMil8JZN4IRFDQyoAO0Z-g4vnp-zkHGM8CgPO6aZD4Sn1yqXM$>
> 2. A more complex example (in memory off-heap analytics and memory mapped
> files) is the sleep analytics:
> Code is here:
> https://github.com/knowledge-base-and-tutorials/java14-features/blob/master/src/main/java/com/github/kbnt/java14/fma/SleepAnalytics.java
> <https://urldefense.com/v3/__https://github.com/knowledge-base-and-tutorials/java14-features/blob/master/src/main/java/com/github/kbnt/java14/fma/SleepAnalytics.java__;!!GqivPVa7Brio!Pf9Nt42jivBKFcxTzsgBVXVMil8JZN4IRFDQyoAO0Z-g4vnp-zkHGM8CgPO6aZD4obcpcxw$>
> (the class' javadoc should describe it)
> Presentation of the case is here:
> https://www.youtube.com/watch?v=NwXzT8T6mb8&list=PLGDP1Irs2PmWNwAwMPdyOxCqkFqB6gtp9&index=7&t=1734s
> <https://urldefense.com/v3/__https://www.youtube.com/watch?v=NwXzT8T6mb8&list=PLGDP1Irs2PmWNwAwMPdyOxCqkFqB6gtp9&index=7&t=1734s__;!!GqivPVa7Brio!Pf9Nt42jivBKFcxTzsgBVXVMil8JZN4IRFDQyoAO0Z-g4vnp-zkHGM8CgPO6aZD47XZakCQ$>
>
> If you find anything useful in those and want to use but the license
> (Apache 2.0 for the code and CC-BY-SA for the videos) is in the way, let me
> know and I can change them to something friendlier (where e.g. no
> attribution is needed).
>
> Cheers!
>   Chris T
>
>
> On Fri, Feb 14, 2020 at 6:26 PM Maurizio Cimadamore <
> maurizio.cimadamore at oracle.com> wrote:
>
>>
>> On 13/02/2020 03:39, Chris T wrote:
>>
>> No problem! Nice talk at FOSDEM, Maurizio ;-)!
>>
>> One suggestion, for future talks - when it comes to memory layouts please
>> construct an example that is a little bit more complex (by end of the
>> upcoming weekend I will publish one that can be used). I had trouble with
>> bit alignment when working on mine, but I will come back with the details
>> (no bug or anything but more clarity would be beneficial in the docs). In
>> your example (the Point one) the alignment is 32 but that is now always the
>> case... The reason I mention this as an issue is that the Java development
>> community is more "high-level". Believe it or not, bit alignment is not
>> anymore "a thing" with most of us ;-)...
>>
>> Hey Chris - on alignment there was an issue that was uncovered on
>> panama-dev - I think this fix should probably be ported to mainline:
>>
>> https://bugs.openjdk.java.net/browse/JDK-8238320
>>
>> This might make working with layout with padding a bit more tedious than
>> intended, although this is probably not what you ran into.
>>
>> Re: talk suggestion - yes, more realistic examples would probably be
>> better - but it's always hard to strike the right balance; if the example
>> is more realistic it can be harder to follow, which might not be good when
>> you are showing new concepts. But I'll keep that in mind for the future -
>> after all this is a new API, and, as it's always the case, the more you
>> think about how to explain these concepts and the more you do it, the more
>> ways you find to get the message across in an optimal way.
>>
>> Thanks!
>> Maurizio
>>
>>
>>
>> I will also think about some API enhancements I would like to see as a
>> developer...
>>
>> Thanks!
>>   Chris T
>>
>> On Tue, Feb 11, 2020 at 4:07 AM Maurizio Cimadamore <
>> maurizio.cimadamore at oracle.com> wrote:
>>
>>> Thanks Paul!
>>>
>>> Maurizio
>>>
>>> On 10/02/2020 17:58, Paul Sandoz wrote:
>>> > I modified the JEP with updated code snippets that compile against the
>>> latest API in JDK 14 [*].
>>>
>>