JEP 370 - text example leads to exception

Tue Feb 18 16:18:47 UTC 2020

On 18/02/2020 03:52, Chris T wrote:
> 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.

Hah - been staring at C types too much lately - that explains it of 
course :-)

Thanks
Maurizio

>
> Thank you for all the feedback!
>   Cristi
>
>
> On Mon, Feb 17, 2020 at 10:43 AM Maurizio Cimadamore 
> <maurizio.cimadamore at oracle.com 
> <mailto: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
>>     <mailto: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
>>>         <mailto: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 [*].
>>>