RFR: 8324751: C2 SuperWord: Aliasing Analysis runtime check [v10]

[Emanuel Peter]

On Thu, 14 Aug 2025 14:10:55 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> This is a big patch, but about 3.5k lines are tests. And a large part of the VM changes is comments / proofs.
>> 
>> I am adding a dynamic (runtime) aliasing check to the auto-vectorizer (SuperWord). We use the infrastructure from https://github.com/openjdk/jdk/pull/22016:
>> - Use the auto-vectorization `predicate` when available: we speculate that there is no aliasing, else we trap and re-compile without the predicate.
>> - If the predicate is not available, we use `multiversioning`, i.e. we have a `fast_loop` where there is no aliasing, and hence vectorization. And a `slow_loop` if the check fails, with no vectorization.
>> 
>> --------------------------
>> 
>> **Where to start reviewing**
>> 
>> - `src/hotspot/share/opto/mempointer.hpp`:
>>   - Read the class comment for `MemPointerRawSummand`.
>>   - Familiarize yourself with the `MemPointer Linearity Corrolary`. We need it for the proofs of the aliasing runtime checks.
>> 
>> - `src/hotspot/share/opto/vectorization.cpp`:
>>   - Read the explanations and proofs above `VPointer::can_make_speculative_aliasing_check_with`. It explains how the aliasing runtime check works.
>> 
>> - `src/hotspot/share/opto/vtransform.hpp`:
>>   - Understand the difference between weak and strong edges.
>> 
>> If you need to see some examples, then look at the tests:
>> - `test/hotspot/jtreg/compiler/loopopts/superword/TestAliasing.java`: simple array cases. IR rules that check for vectors and in somecases if we used multiversioning.
>> - `test/micro/org/openjdk/bench/vm/compiler/VectorAliasing.java`: the miro-benchmarks I show below. Simple array cases.
>> - `test/hotspot/jtreg/compiler/loopopts/superword/TestMemorySegmentAliasing.java`: a bit advanced, but similar cases.
>> - `test/hotspot/jtreg/compiler/loopopts/superword/TestAliasingFuzzer.java`: very large and rather compliex. Generates random loops, some with and some without aliasing at runtime. IR verification, but mostly currently only for array cases, MemorySegment cases have some issues (see comments).
>> --------------------------
>> 
>> **Details**
>> 
>> Most fundamentally:
>> - I had to refactor / extend `MemPointer` so that we have access to `MemPointerRawSummand`s.
>> - These raw summands us to reconstruct the `VPointer` at any `iv` value with `VPointer::make_pointer_expression(Node* iv_value)`.
>>    - With the raw summands, a pointer may look like this: `p = base + ConvI2L(x + 2) + ConvI2L(y + 2)`
>>   - With "regular" summands, this gets simplified to `p = base + 4L +ConvI2L(x) + Conv...
>
> Emanuel Peter has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Update src/hotspot/share/opto/c2_globals.hpp
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - improve predicates.hpp documentation

@chhagedorn Thanks for the drive-by comments about the Predicate documentation. Are you now satisfied? Maybe @rwestrel should have a look at it too, since I completed the missing documentation from the Short-Running-Long-Loop-Predicates as well.

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PR Comment: https://git.openjdk.org/jdk/pull/24278#issuecomment-3188677355