RFR: 8351623: VectorAPI: Add SVE implementation of subword gather load operation

[Xiaohong Gong]

On Mon, 14 Jul 2025 13:23:33 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

>> 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 mer...
>
> src/hotspot/cpu/aarch64/aarch64_vector.ad line 5990:
> 
>> 5988: %}
>> 5989: 
>> 5990: instruct vmaskwiden_hi_sve(pReg dst, pReg src) %{
> 
> can both the hi and lo widen rules be combined into a single one as the arguments are the same? or would it make it less understandable?

The main problem is that we cannot get the flag of `__is_lo`  easily from the relative machnode as far as I know.

> src/hotspot/cpu/aarch64/aarch64_vector_ad.m4 line 352:
> 
>> 350:   // SVE requires vector indices for gather-load/scatter-store operations
>> 351:   // on all data types.
>> 352:   bool Matcher::gather_scatter_needs_vector_index(BasicType bt) {
> 
> There's already a function that tests for `UseSVE > 0` here - https://github.com/openjdk/jdk/blob/bcd86d575fe0682a234228c18b0c2e817d3816da/src/hotspot/cpu/aarch64/matcher_aarch64.hpp#L36
> 
> Can it be reused?

Do you mean directly using `supports_scalable_vector` instead of the new added method in mid-end? I'm afraid we cannot use it. Because on X86, the indexes for subword types are passed with address of the index array, while it's a vector for other types even on AVX-512.

But yes, we can call `supports_scalable_vector()` in the new added method for AArch64.

> src/hotspot/cpu/aarch64/aarch64_vector_ad.m4 line 3430:
> 
>> 3428: 
>> 3429: instruct vslice_neon(vReg dst, vReg src1, vReg src2, immI index) %{
>> 3430:    predicate(VM_Version::use_neon_for_vector(Matcher::vector_length_in_bytes(n)));
> 
> nit: indentation. I think there're 3 spaces here.. Same with the SVE version below.

Good catch! I will update it. Thanks a lot!

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2206092888
PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2206096909
PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2206097514