8312332: C2: Refactor SWPointer out from SuperWord #15013

pfustc · 2023-07-25T08:51:38Z

As discussed in JDK-8308994, we should first do some refactoring work before proceeding with the new post loop vectorization. In this patch, we have done the following.

We have created new C2 source files vectorization.[cpp|hpp] for shared logics and utilities for C2's auto-vectorization. So far we have moved class SWPointer and VectorElementSizeStats here from superword.[cpp|hpp].
We have decoupled SWPointer from class SuperWord and renamed it to VPointer as it will be used by vectorizers other than SuperWord. The original class SWPointer and its inner class Tracer both have a _slp field initialized in their constructors. In this patch, we have replaced them by other fields and re-written the constructors for the same functionality. Original SWPointer::invariant() calls function SuperWord::find_pre_loop_end() for loop invariant checks. To help decoupling, we moved function find_pre_loop_end() to class CountedLoopNode. As function SWPointer::Tracer::invariant_1() is tightly coupled with SuperWord but only prints some debug messages, we temporarily removed it in this patch. We will consider adding it back after later refactoring of SuperWord so we added a TODO at its call site in this patch.
We have a lot of memory phi node checks in loop optimizations. So we added a utility function is_memory_phi() in node.hpp.

Tested tier1~3 on x86 and AArch64. Also manually verified that option VectorizeDebug in compiler directives still works well.

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JDK-8312332: C2: Refactor SWPointer out from SuperWord (Enhancement - P4)

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Emanuel Peter (@eme64 - Reviewer) ⚠️ Review applies to 0ca709ef
Vladimir Kozlov (@vnkozlov - Reviewer)

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As discussed in JDK-8308994, we should first do some refactoring work before proceeding with the new post loop vectorization. In this patch, we have done the following. 1) We have created new C2 source files `vectorization.[cpp|hpp]` for shared logics and utilities for C2's auto-vectorization. So far we have moved class `SWPointer` and `VectorElementSizeStats` here from `superword.[cpp|hpp]`. 2) We have decoupled `SWPointer` from class `SuperWord` and renamed it to `VPointer` as it will be used by vectorizers other than SuperWord. The original class `SWPointer` and its inner class `Tracer` both have a `_slp` field initialized in their constructors. In this patch, we have replaced them by other fields and re-written the constructors for the same functionality. Original `SWPointer::invariant()` calls function `SuperWord::find_pre_loop_end()` for loop invariant checks. To help decoupling, we moved function `find_pre_loop_end()` to class `CountedLoopNode`. As function `SWPointer::Tracer::invariant_1()` is tightly coupled with `SuperWord` but only prints some debug messages, we temporarily removed it in this patch. We will consider adding it back after later refactoring of `SuperWord` so we added a `TODO` at its call site in this patch. 3) We have a lot of memory phi node checks in loop optimizations. So we added a utility function `is_memory_phi()` in `node.hpp`. Tested tier1~3 on x86 and AArch64. Also manually verified that option `VectorizeDebug` in compiler directives still works well.

bridgekeeper · 2023-07-25T08:53:02Z

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openjdk · 2023-07-25T08:55:24Z

@pfustc The following label will be automatically applied to this pull request:

hotspot-compiler

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mlbridge · 2023-07-25T09:00:06Z

Webrevs

eme64

@pfustc Thanks a lot for moving this code, and especially for untangling it from SuperWord. This will be helpful for many future vectorization projects.

I left a few comments, but otherwise this looks straight-forward and good to me.

eme64 · 2023-08-10T11:50:12Z

src/hotspot/share/opto/vectorization.cpp

+
+bool VPointer::invariant(Node* n) const {
+  NOT_PRODUCT(Tracer::Depth dd;)
+  // TODO: Add more trace output for invariant check after later refactoring


We generally don't like TODOs in the code. Best is to just drop it in the code and file an RFE if you think it is really important.

When did this even trace anything?
_slp->_lpt->is_member(_slp->_phase->get_loop(n_c)) != (int)_slp->in_bb(n)

Do you think this tracing is relevant enough?

If it should never happen: can we add an assert somewhere instead?

I don't know when this can happen. This was added in jdk9 without a test case. If this can happen, it should be in some real corner cases. But definitely, the trace message is not important and it's safe to drop that. TODO is removed in my latest commit.

eme64 · 2023-08-10T11:58:28Z

src/hotspot/share/opto/vectorization.cpp

+      if (pre_loop_end != nullptr) {
+        Node* n_c = phase()->get_ctrl(n);
+        return phase()->is_dominator(n_c, pre_loop_end->loopnode());
+      }


Is pre_loop_end != nullptr possible here? Before your patch we always found _slp->pre_loop_head().
I'm just worried that if we do not find it, then we still return is_not_member, but n is still located in the space between pre and post loop.
What do you think about this?

And: would it make sense to cache the pre_loop_head in the VPointer?

Yes, I have found that pre_loop_end could be null when we construct VPointer in SuperWord::output() - see the code in superword.cpp (L2574, L2580). It's null because CountedLoopNode::is_canonical_loop_entry() returns null at this time. Before my patch, it cannot be null as we cached _pre_loop_end in the SuperWord class. To address your concern, my latest commit moves the cache of _pre_loop_end from SuperWord to CountedLoopNode. Some more asserts are also added to make sure it's used for main loops only. (Caching it in VPointer doesn't help).

Can you explain a bit more why CountedLoopNode::is_canonical_loop_entry() returned null at the time, and why you did move the caching?

I think the comment before function CountedLoopNode::is_canonical_loop_entry() explains that. I quote it below.

// Check the shape of the graph at the loop entry. In some cases,
// the shape of the graph does not match the shape outlined below.
// That is caused by the Opaque1 node "protecting" the shape of
// the graph being removed by, for example, the IGVN performed
// in PhaseIdealLoop::build_and_optimize().
//
// After the Opaque1 node has been removed, optimizations (e.g., split-if,
// loop unswitching, and IGVN, or a combination of them) can freely change
// the graph's shape. As a result, the graph shape outlined below cannot
// be guaranteed anymore.
Node* CountedLoopNode::is_canonical_loop_entry() {

Moving the caching is just to address your above concern that we still return is_not_member when pre_loop_end is null. Before my patch, it cannot be null as we cached _pre_loop_end in the SuperWord class. But after decoupling, we cannot access the cache in the SuperWord class anymore.

eme64 · 2023-08-10T12:06:05Z

src/hotspot/share/opto/vectorization.cpp

+#endif
+  _nstack(nstack), _analyze_only(analyze_only), _stack_idx(0)
+#ifndef PRODUCT
+  , _tracer((phase->C->directive()->VectorizeDebugOption & 2) > 0)


You should also refactor the accessors for VectorizeDebugOption. I would move it from SuperWord to vectorization.hpp/cpp somehow. We should only do the "masking" & 2 in one single place.

Right, but doing this requires moving more handles including phase to vectorization.hpp/cpp. Perhaps we need to create a new class there first. I have created another task (https://bugs.openjdk.org/browse/JDK-8315361) and would like to do more refactoring later.

Ok, we can do that refactoring later.

eme64 · 2023-08-10T12:17:42Z

src/hotspot/share/opto/vectorization.cpp

+int VPointer::Tracer::_depth = 0;
+#endif
+
+VPointer::VPointer(MemNode* mem, PhaseIdealLoop* phase, IdealLoopTree* lpt,


You could also call it LPointer or LoopPointer. VPointer sounds like VectorPointer - but it is not a pointer of a vector but a scalar memop. That could be confusing. But you could also argue it is a VectorizationPointer, and hence VPointer.

My idea is keeping the name as short as possible. I think it should be VectorizationPointer so VPointer. In my opinion, LoopPointer also sounds like a pointer to a loop but it isn't. I'd like to have more suggestions from other reviewers about the name.

BTW: VPointer is also used for vector memop in SuperWord::output().

No worries, I can live with VPointer.

eme64 · 2023-09-04T10:29:07Z

Hi @pfustc sorry for the delay. I am still out of the office, and I have not yet had time to look at it. It will be one of the first things I'll have a look at when I get back. Looking forward to this refactoring :)

FYI: @TobiHartmann just launched testing for the 3rd commit (web rev 02)
Update: no related test failures.

eme64

Ok, this now looks good. I only have some questions about the caching of _pre_loop_end.
Thanks for doing this work, and sorry for the delay with the review.

eme64 · 2023-09-11T06:51:03Z

src/hotspot/share/opto/loopnode.hpp

-  // The eventual count of vectorizable packs in slp
-  int _slp_vector_pack_count;
+  // Cached CountedLoopEndNode of pre loop for main loops
+  CountedLoopEndNode* _pre_loop_end;


Is it safe to cached this node? What happens if the node were ever to be replaced?

Ah, we need to access it via pre_loop_end() which has asserts that check the consistency. Ok.

We just have to make sure that this field is never accessed directly. What happens if people start using it elsewhere?

The field is already private. What else do you think we can do to further protect it?

eme64 · 2023-09-11T07:07:28Z

src/hotspot/share/opto/vectorization.cpp

+int VPointer::Tracer::_depth = 0;
+#endif
+
+VPointer::VPointer(MemNode* mem, PhaseIdealLoop* phase, IdealLoopTree* lpt,


No worries, I can live with VPointer.

eme64 · 2023-09-11T07:08:29Z

src/hotspot/share/opto/vectorization.cpp

+#endif
+  _nstack(nstack), _analyze_only(analyze_only), _stack_idx(0)
+#ifndef PRODUCT
+  , _tracer((phase->C->directive()->VectorizeDebugOption & 2) > 0)


Ok, we can do that refactoring later.

eme64 · 2023-09-11T07:17:17Z

src/hotspot/share/opto/vectorization.cpp

+      if (pre_loop_end != nullptr) {
+        Node* n_c = phase()->get_ctrl(n);
+        return phase()->is_dominator(n_c, pre_loop_end->loopnode());
+      }


Can you explain a bit more why CountedLoopNode::is_canonical_loop_entry() returned null at the time, and why you did move the caching?

eme64

Thanks for the responses. LGTM. Thanks for the nice refactoring work!

openjdk · 2023-09-11T13:50:48Z

@pfustc This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8312332: C2: Refactor SWPointer out from SuperWord

Reviewed-by: epeter, kvn

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 238 new commits pushed to the master branch:

5cea53d: 8315810: Reimplement sun.reflect.ReflectionFactory::newConstructorForSerialization with method handles
eb1f67b: 8315937: Enable parallelism in vmTestbase/nsk/stress/numeric tests
a57b9da: 8316242: Opensource SwingGraphics manual test
3abe798: 8316115: Parallel: Fix -Wconversion warnings around NUMA node ID
83dca62: 8313638: Add test for dump of resolved references
cfa8901: 8315338: RISC-V: Change flags for stable extensions to non-experimental
edd454b: 8315766: Parallelize gc/stress/TestStressIHOPMultiThread.java test
de9b971: 8315933: Serial: Remove empty Generation::ensure_parsability
b48dbf6: 8316181: Move the fast locking implementation out of the .ad files
8f4dfc4: 8316164: Opensource JMenuBar manual test
... and 228 more: https://git.openjdk.org/jdk/compare/12de9b0225363377e9a76729b11698221d4f29f2...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

pfustc · 2023-09-12T01:51:51Z

Thanks @eme64. May I have another review from someone else?

vnkozlov

In general it looks good. I just want that you add more description.

And I have question. Part of code is under "#ifndef PRODUCT". Did you build "optimized" VM to make sure it works?

vnkozlov · 2023-09-12T16:33:18Z

src/hotspot/share/opto/vectorization.hpp

+#include "opto/loopnode.hpp"
+
+// ----------------------------------VPointer----------------------------------
+// Information about an address for dependence checking and vector alignment


I am fine with using VPointer name but I think you need to add more description here to avoid confusion.

May be add a general description what is in this file: "for shared logics and utilities for C2's auto-vectorization".

vnkozlov · 2023-09-12T16:45:10Z

src/hotspot/share/opto/vectorization.hpp

+#include "opto/node.hpp"
+#include "opto/loopnode.hpp"
+
+// ----------------------------------VPointer----------------------------------


We stop using such lines - there was cleaning RFE to remove them.

pfustc · 2023-09-13T09:55:20Z

In general it looks good. I just want that you add more description.

And I have question. Part of code is under "#ifndef PRODUCT". Did you build "optimized" VM to make sure it works?

@vnkozlov, I just checked with CompilerDirectives and got the same tracing output between "fastdebug" build and "optimized" build, and no tracing output is seen for "release" build.

vnkozlov

Good. Thank you for checking optimized build.

pfustc · 2023-09-15T01:02:00Z

/integrate

openjdk · 2023-09-15T01:02:50Z

Going to push as commit 96781ba.
Since your change was applied there have been 247 commits pushed to the master branch:

b55e418: 8301991: Convert l10n properties resource bundles to UTF-8 native
d475f61: 8315731: Open source several Swing Text related tests
4415261: 8315683: Parallelize java/util/concurrent/tck/JSR166TestCase.java
b2e9106: 8313452: Improve Classfile API attributes handling safety
2a6fb9c: 8316147: Remove serviceability/sa/TestJhsdbJstackMixed.java from -Xcomp problem list
ca747f0: 8315678: Classfile API ConstantPool::entryCount and ConstantPool::entryByIndex methods are confusing
6d47fc6: 8313258: RuntimeInvisibleTypeAnnotationsAttribute.annotations() API Index out of Bound error
c7d306c: 8315541: Classfile API TypeAnnotation.TargetInfo factory methods accept null labels
c11f835: 8315824: Open source several Swing Text/HTML related tests
5cea53d: 8315810: Reimplement sun.reflect.ReflectionFactory::newConstructorForSerialization with method handles
... and 237 more: https://git.openjdk.org/jdk/compare/12de9b0225363377e9a76729b11698221d4f29f2...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-09-15T01:03:03Z

@pfustc Pushed as commit 96781ba.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

openjdk bot added the rfr Pull request is ready for review label Jul 25, 2023

openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Jul 25, 2023

eme64 reviewed Aug 10, 2023

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Pengfei Li added 2 commits August 29, 2023 01:29

Merge branch 'master' into swpointer

c6cbec3

Move the cache of _pre_loop_end to CountedLoopNode

0ca709e

pfustc requested a review from eme64 September 4, 2023 01:25

eme64 reviewed Sep 11, 2023

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vnkozlov reviewed Sep 12, 2023

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Update some comments

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pfustc deleted the swpointer branch September 15, 2023 01:29

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8312332: C2: Refactor SWPointer out from SuperWord #15013

8312332: C2: Refactor SWPointer out from SuperWord #15013

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