RFR (XXL): 8223347: Integration of Vector API (Incubator): General HotSpot changes

Tue Aug 4 19:52:29 UTC 2020

Hi Vladimir,

Looks good. I have only few small questions.

compile.cpp: what is next comment about?

+  // FIXME for_igvn() is corrupted from here: new_worklist which is set_for_ignv() was allocated on stack.

print_method(): NodeClassNames[] should be available in product. Node::Name() method is not, but we can move it to 
product. But I am fine to do that later.

Why VectorSupport.java does not have copyright header?

Thanks,
Vladimir K

On 7/28/20 3:29 PM, Vladimir Ivanov wrote:
> Hi,
> 
> Thanks for the feedback on webrev.00, Remi, Coleen, Vladimir K., and Ekaterina!
> 
> Here are the latest changes for Vector API support in HotSpot shared code:
> 
> http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.01
> 
> Incremental changes (diff against webrev.00):
> 
> http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.01_00
> 
> I decided to post it here and not initiate a new round of reviews because the changes are mostly limited to minor 
> cleanups / simple bug fixes.
> 
> Detailed summary:
>    - rebased to jdk/jdk tip;
>    - got rid of NotV, VLShiftV, VRShiftV, VURShiftV nodes;
>    - restore lazy cleanup logic during incremental inlining (see needs_cleanup in compile.cpp);
>    - got rid of x86-specific changes in shared code;
>    - fix for 8244867 [1];
>    - fix Graal test failure: enumerate VectorSupport intrinsics in CheckGraalIntrinsics
>    - numerous minor cleanups
> 
> Best regards,
> Vladimir Ivanov
> 
> [1] http://hg.openjdk.java.net/panama/dev/rev/dcfc7b6e8977
>      http://jbs.oracle.com/browse/JDK-8244867
>      8244867: 2 vector api tests crash with assert(is_reference_type(basic_type())) failed: wrong type
> Summary: Adding safety checks to prevent intrinsification if class arguments of non-primitive types are uninitialized.
> 
> On 04.04.2020 02:12, Vladimir Ivanov wrote:
>> Hi,
>>
>> Following up on review requests of API [0] and Java implementation [1] for Vector API (JEP 338 [2]), here's a request 
>> for review of general HotSpot changes (in shared code) required for supporting the API:
>>
>>
>> http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.00/all.00-03/
>>
>> (First of all, to set proper expectations: since the JEP is still in Candidate state, the intention is to initiate 
>> preliminary round(s) of review to inform the community and gather feedback before sending out final/official RFRs once 
>> the JEP is Targeted to a release.)
>>
>> Vector API (being developed in Project Panama [3]) relies on JVM support to utilize optimal vector hardware 
>> instructions at runtime. It interacts with JVM through intrinsics (declared in jdk.internal.vm.vector.VectorSupport 
>> [4]) which expose vector operations support in C2 JIT-compiler.
>>
>> As Paul wrote earlier: "A vector intrinsic is an internal low-level vector operation. The last argument to the 
>> intrinsic is fall back behavior in Java, implementing the scalar operation over the number of elements held by the 
>> vector.  Thus, If the intrinsic is not supported in C2 for the other arguments then the Java implementation is 
>> executed (the Java implementation is always executed when running in the interpreter or for C1)."
>>
>> The rest of JVM support is about aggressively optimizing vector boxes to minimize (ideally eliminate) the overhead of 
>> boxing for vector values.
>> It's a stop-the-gap solution for vector box elimination problem until inline classes arrive. Vector classes are 
>> value-based and in the longer term will be migrated to inline classes once the support becomes available.
>>
>> Vector API talk from JVMLS'18 [5] contains brief overview of JVM implementation and some details.
>>
>> Complete implementation resides in vector-unstable branch of panama/dev repository [6].
>>
>> Now to gory details (the patch is split in multiple "sub-webrevs"):
>>
>> ===========================================================
>>
>> (1) http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.00/00.backend.shared/
>>
>> Ideal vector nodes for new operations introduced by Vector API.
>>
>> (Platform-specific back end support will be posted for review separately).
>>
>> ===========================================================
>>
>> (2) http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.00/01.intrinsics/
>>
>> JVM Java interface (VectorSupport) and intrinsic support in C2.
>>
>> Vector instances are initially represented as VectorBox macro nodes and "unboxing" is represented by VectorUnbox node. 
>> It simplifies vector box elimination analysis and the nodes are expanded later right before EA pass.
>>
>> Vectors have 2-level on-heap representation: for the vector value primitive array is used as a backing storage and it 
>> is encapsulated in a typed wrapper (e.g., Int256Vector - vector of 8 ints - contains a int[8] instance which is used 
>> to store vector value).
>>
>> Unless VectorBox node goes away, it needs to be expanded into an allocation eventually, but it is a pure node and 
>> doesn't have any JVM state associated with it. The problem is solved by keeping JVM state separately in a 
>> VectorBoxAllocate node associated with VectorBox node and use it during expansion.
>>
>> Also, to simplify vector box elimination, inlining of vector reboxing calls (VectorSupport::maybeRebox) is delayed 
>> until the analysis is over.
>>
>> ===========================================================
>>
>> (3) http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.00/02.vbox_elimination/
>>
>> Vector box elimination analysis implementation. (Brief overview: slides #36-42 [5].)
>>
>> The main part is devoted to scalarization across safepoints and rematerialization support during deoptimization. In 
>> C2-generated code vector operations work with raw vector values which live in registers or spilled on the stack and it 
>> allows to avoid boxing/unboxing when a vector value is alive across a safepoint. As with other values, there's just a 
>> location of the vector value at the safepoint and vector type information recorded in the relevant nmethod metadata 
>> and all the heavy-lifting happens only when rematerialization takes place.
>>
>> The analysis preserves object identity invariants except during aggressive reboxing (guarded by 
>> -XX:+EnableAggressiveReboxing).
>>
>> (Aggressive reboxing is crucial for cases when vectors "escape": it allocates a fresh instance at every escape point 
>> thus enabling original instance to go away.)
>>
>> ===========================================================
>>
>> (4) http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.00/03.module.hotspot/
>>
>> HotSpot changes for jdk.incubator.vector module. Vector support is makred experimental and turned off by default. JEP 
>> 338 proposes the API to be released as an incubator module, so a user has to specify "--add-module 
>> jdk.incubator.vector" on the command line to be able to use it.
>> When user does that, JVM automatically enables Vector API support.
>> It improves usability (user doesn't need to separately "open" the API and enable JVM support) while minimizing risks 
>> of destabilitzation from new code when the API is not used.
>>
>>
>> That's it! Will be happy to answer any questions.
>>
>> And thanks in advance for any feedback!
>>
>> Best regards,
>> Vladimir Ivanov
>>
>> [0] https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-March/065345.html
>>
>> [1] https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-April/041228.html
>>
>> [2] https://openjdk.java.net/jeps/338
>>
>> [3] https://openjdk.java.net/projects/panama/
>>
>> [4] 
>> http://cr.openjdk.java.net/~vlivanov/panama/vector/jep338/hotspot.shared/webrev.00/01.intrinsics/src/java.base/share/classes/jdk/internal/vm/vector/VectorSupport.java.html 
>>
>>
>> [5] http://cr.openjdk.java.net/~vlivanov/talks/2018_JVMLS_VectorAPI.pdf
>>
>> [6] http://hg.openjdk.java.net/panama/dev/shortlog/92bbd44386e9
>>
>>      $ hg clone http://hg.openjdk.java.net/panama/dev/ -b vector-unstable