RFR: 8351623: VectorAPI: Add SVE implementation of subword gather load operation [v3]
Xiaohong Gong
xgong at openjdk.org
Fri Jul 25 03:26:36 UTC 2025
> This is a follow-up patch of [1], which aims at implementing the subword gather load APIs for AArch64 SVE platform.
>
> ### Background
> Vector gather load APIs load values from memory addresses calculated by adding a base pointer to integer indices. SVE provides native gather load instructions for `byte`/`short` types using `int` vectors for indices. The vector size for a gather-load instruction is determined by the index vector (i.e. `int` elements). Hence, the total size is `32 * elem_num` bits, where `elem_num` is the number of loaded elements in the vector register.
>
> ### Implementation
>
> #### Challenges
> Due to size differences between `int` indices (32-bit) and `byte`/`short` data (8/16-bit), operations must be split across multiple vector registers based on the target SVE vector register size constraints.
>
> For a 512-bit SVE machine, loading a `byte` vector with different vector species require different approaches:
> - SPECIES_64: Single operation with mask (8 elements, 256-bit)
> - SPECIES_128: Single operation, full register (16 elements, 512-bit)
> - SPECIES_256: Two operations + merge (32 elements, 1024-bit)
> - SPECIES_512/MAX: Four operations + merge (64 elements, 2048-bit)
>
> Use `ByteVector.SPECIES_512` as an example:
> - It contains 64 elements. So the index vector size should be `64 * 32` bits, which is 4 times of the SVE vector register size.
> - It requires 4 times of vector gather-loads to finish the whole operation.
>
>
> byte[] arr = [a, a, a, a, ..., a, b, b, b, b, ..., b, c, c, c, c, ..., c, d, d, d, d, ..., d, ...]
> int[] idx = [0, 1, 2, 3, ..., 63, ...]
>
> 4 gather-load:
> idx_v1 = [15 14 13 ... 1 0] gather_v1 = [... 0000 0000 0000 0000 aaaa aaaa aaaa aaaa]
> idx_v2 = [31 30 29 ... 17 16] gather_v2 = [... 0000 0000 0000 0000 bbbb bbbb bbbb bbbb]
> idx_v3 = [47 46 45 ... 33 32] gather_v3 = [... 0000 0000 0000 0000 cccc cccc cccc cccc]
> idx_v4 = [63 62 61 ... 49 48] gather_v4 = [... 0000 0000 0000 0000 dddd dddd dddd dddd]
> merge: v = [dddd dddd dddd dddd cccc cccc cccc cccc bbbb bbbb bbbb bbbb aaaa aaaa aaaa aaaa]
>
>
> #### Solution
> The implementation simplifies backend complexity by defining each gather load IR to handle one vector gather-load operation, with multiple IRs generated in the compiler mid-end.
>
> Here is the main changes:
> - Enhanced IR generation with architecture-specific patterns based on `gather_scatter_needs_vector_index()` matcher.
> - Added `VectorSliceNode` for result merging.
> - Added `VectorMaskWidenNode` for mask spliting and type conversion fo...
Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
Refine IR pattern and clean backend rules
-------------
Changes:
- all: https://git.openjdk.org/jdk/pull/26236/files
- new: https://git.openjdk.org/jdk/pull/26236/files/c39dade2..be63ade6
Webrevs:
- full: https://webrevs.openjdk.org/?repo=jdk&pr=26236&range=02
- incr: https://webrevs.openjdk.org/?repo=jdk&pr=26236&range=01-02
Stats: 854 lines in 17 files changed: 308 ins; 275 del; 271 mod
Patch: https://git.openjdk.org/jdk/pull/26236.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/26236/head:pull/26236
PR: https://git.openjdk.org/jdk/pull/26236
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