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

Viswanathan, Sandhya sandhya.viswanathan at intel.com
Wed Jul 29 18:19:14 UTC 2020 

Hi,

Likewise, the corresponding x86 backend changes since first review are also only minor cleanups and simple bug fixes:

X86:
   Full: http://cr.openjdk.java.net/~sviswanathan/VAPI_RFR/x86_webrev/webrev.01/
   Incremental: http://cr.openjdk.java.net/~sviswanathan/VAPI_RFR/x86_webrev/webrev.00-webrev.01/

Summary:
   - rebased to jdk/jdk tip;
   - backend changes related to removal of NotV, VLShiftV, VRShiftV, VURShiftV nodes;
   - vector insert bug fix
   - some minor cleanups

Older webrev links for your reference:
   X86b backend: http://cr.openjdk.java.net/~sviswanathan/VAPI_RFR/x86_webrev/webrev.00/

Best Regards,
Sandhya

-----Original Message-----
From: Vladimir Ivanov <vladimir.x.ivanov at oracle.com> 
Sent: Tuesday, July 28, 2020 3:30 PM
To: hotspot-dev <hotspot-dev at openjdk.java.net>; hotspot compiler <hotspot-compiler-dev at openjdk.java.net>
Cc: Viswanathan, Sandhya <sandhya.viswanathan at intel.com>; panama-dev <panama-dev at openjdk.java.net>
Subject: Re: RFR (XXL): 8223347: Integration of Vector API (Incubator): General HotSpot changes

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.shar
> ed/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.shar
> ed/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.shar
> ed/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.shar
> ed/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.shar
> ed/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/06534
> 5.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.shar
> ed/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

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