RFR: 8325520: Vector loads with offsets incorrectly compiled

[Tobias Hartmann]

On Mon, 18 Mar 2024 12:20:34 GMT, Damon Fenacci <dfenacci at openjdk.org> wrote:

> # Issue
> When loading multiple vectors using offsets or masks (e.g. `LongVector::fromArray(LongVector.SPECIES_256, storage, 0, offsets, 0` or `LongVector::fromArray(LongVector.SPECIES_256, storage, 0, longMask)`) there is an error in the C2 compiled code that makes different vectors be treated as equal even though they are not.
> 
> # Causes
> On vector-capable platforms, vector loads with masks and offsets (for Long, Integer, Float and Double) create specific nodes in the ideal graph (i.e. `LoadVectorGather`, `LoadVectorMasked`, `LoadVectorGatherMasked`). Vector loads without mask or offsets are mapped as `LoadVector` nodes instead.
> While running GVN loops, we can get to the situation in which there are multiple loads from the same address. If successive loads are deemed to be identical to the first one, they might get folded, which is what happens in the problematic examples of this issue. The check for identity happens in
> https://github.com/openjdk/jdk/blob/481c866df87c693a90a1da20e131e5654b084ddd/src/hotspot/share/opto/memnode.cpp#L1253
> This version of `Identity` is defined in `LoadNode` but it is also the one used by the subclasses `LoadVectorGather`, `LoadVectorMasked` and `LoadVectorGatherMasked`. Although this definition of `Identity` is enough for `LoadVector` nodes it is not sufficient for `LoadVectorGather`, `LoadVectorMasked` and `LoadVectorGatherMasked` ones, as the value being loaded also depends on the offsets and mask (different offsets and/or masks load completely different values). This is the reason why these nodes get folded even if they shouldn't.
> 
> # Solution
> `LoadVectorGather`, `LoadVectorMasked` and `LoadVectorGatherMasked` need their own version of the `Identity` method, which specialize `LoadVector::Identity` by restricting the results to nodes that also have the same offsets and masks.
> 
> The same issue exists for _StoreVector_ nodes (i.e. `StoreVectorScatter`, `StoreVectorMasked` and `StoreVectorScatterMasked`). So, `Identity` has to be redefined there as well.
> 
> Regression tests for all versions of `Load/StoreVectorGather/Masked` have been added too.

test/hotspot/jtreg/compiler/vectorapi/VectorGatherMaskFoldingTest.java line 39:

> 37:  * @modules jdk.incubator.vector
> 38:  *
> 39:  * @run main compiler.vectorapi.VectorGatherMaskFoldingTest

Suggestion:

 * @run driver compiler.vectorapi.VectorGatherMaskFoldingTest

test/hotspot/jtreg/compiler/vectorapi/VectorGatherMaskFoldingTest.java line 151:

> 149: 
> 150:     @Test
> 151:     @Warmup(10000)

Since all tests use the same warmup, I would suggest to set it once via `testFrameworkobject.setDefaultWarmup(10000)`, see https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/compiler/lib/ir_framework/README.md

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PR Review Comment: https://git.openjdk.org/jdk/pull/18347#discussion_r1574432877
PR Review Comment: https://git.openjdk.org/jdk/pull/18347#discussion_r1574435811