RFR: 8343685: C2 SuperWord: refactor VPointer with MemPointer [v19]

[Quan Anh Mai]

On Sat, 18 Jan 2025 16:23:04 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> **This is a required step towards adding runtime-checks for Aliasing Analysis, especially Important for FFM / MemorySegments.**
>> See: https://eme64.github.io/blog/2025/01/01/AutoVectorization-Status.html
>> 
>> I know this one is large, but it consists of a lot of renamings, and new tests. On the whole, the new `VPointer` code is less than the old!
>> 
>> **Goal**
>> 
>> Replace old `VPointer` with a new version that relies on `MemPointer` - which then is a shared utility for both `MergeStores` and `SuperWord / AutoVectorization`. `MemPointer` generally parses pointers, and `VPointer` specializes this facility for the use in loops (`VLoop`).
>> 
>> The old `VPointer` implementation with its recursive pattern matching was quite complicated and difficult to reason about for correctness. The approach in `MemPointer` is much simpler: iteratively decomposing sub-expressions. Further: the new implementation is more powerful at detecting equivalent invariants.
>> 
>> **Future**:  with the `MemPointer` implementation of `VPointer`, it should be easier to implement speculative runtime-checks for Aliasing-Analysis [JDK-8324751](https://bugs.openjdk.org/browse/JDK-8324751). The pressing need for this has come from the FFM / MemorySegment folks, like @mcimadamore and @minborg .
>> 
>> **Details**
>> 
>> This looks like a rather big patch, so let me explain the parts.
>> - Refactor of `MemPointer` in `mepointer.hpp/cpp`:
>>   - Added concept of `Base` to `MemPointer`. This is required for the aliasing computation in `VPointer`.
>>   - `sub_expression_has_native_base_candidate`: add special case to parse through `CastX2P` if we find a native memory base `MemorySegment.address()`, i.e. `jdk.internal.foreign.NativeMemorySegmentImpl.min`. This helps some native memory segment cases to vectorize that did not before.
>>   - So far `MemPointer` could only answer adjacency queries. But VPointer also needs overlap queries, see the old `VPointer::not_equal` (i.e. can we prove that the two `VPointer` never overlap?). So I had to add a new case to aliasing computation: `NotOrAtDistance`. It is useful to answer the new and better named `MemPointer::never_overlaps_with`.
>>   - Collapsed together `MemPointerDecomposedForm` and `MemPointer`. It was an unnecessary and unhelpful split.
>> - Re-write of `VPointer` based on `MemPointer`:
>>   - Old pattern:
>>     - `VPointer[mem:  847     StoreI, base:   37, adr:   37,  base[  37] + offset(  16) + invar(   0) + scale(   4) * iv]`
>>     - `VPointer[mem: 31...
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix guard

src/hotspot/share/opto/superwordVTransformBuilder.cpp line 144:

> 142:     const VPointer& scalar_p = _vloop_analyzer.vpointers().vpointer(p0->as_Load());
> 143:     const VPointer vector_p(scalar_p.make_with_size(scalar_p.size() * pack_size));
> 144:     vtn = new (_vtransform.arena()) VTransformLoadVectorNode(_vtransform, pack_size, vector_p);

You should use `::new` here

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PR Review Comment: https://git.openjdk.org/jdk/pull/21926#discussion_r1921917693