RFR: 8318650: Optimized subword gather for x86 targets.
Xiaohong Gong
xgong at openjdk.org
Thu Oct 26 07:42:35 UTC 2023
On Wed, 25 Oct 2023 04:34:59 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:
> Hi All,
>
> This patch optimizes sub-word gather operation for x86 targets with AVX2 and AVX512 features.
>
> Following is the summary of changes:-
>
> 1) Intrinsify sub-word gather with high performance backend implementation based on hybrid algorithm which initially partially unrolls scalar loop to accumulates values from gather indices into a quadword(64bit) slice followed by vector permutation to place the slice into appropriate vector lanes, it prevents code bloating and generates compact
> JIT sequence. This coupled with savings from expansive array allocation in existing java implementation translates into significant performance of 1.3-5x gains with included micro.
>
>
> ![image](https://github.com/openjdk/jdk/assets/59989778/e25ba4ad-6a61-42fa-9566-452f741a9c6d)
>
>
> 2) Patch was also compared against modified java fallback implementation by replacing temporary array allocation with zero initialized vector and a scalar loops which inserts gathered values into vector. But, vector insert operation in higher vector lanes is a three step process which first extracts the upper vector 128 bit lane, updates it with gather subword value and then inserts the lane back to its original position. This makes inserts into higher order lanes costly w.r.t to proposed solution. In addition generated JIT code for modified fallback implementation was very bulky. This may impact in-lining decisions into caller contexts.
>
> 3) Some minor adjustments in existing gather instruction pattens for double/quad words.
>
>
> Kindly review and share your feedback.
>
>
> Best Regards,
> Jatin
src/hotspot/share/opto/vectorIntrinsics.cpp line 1486:
> 1484: // Check whether the predicated gather/scatter node is supported by architecture.
> 1485: VectorMaskUseType mask = (is_scatter || !is_subword_type(elem_bt)) ? (VectorMaskUseType) (VecMaskUseLoad | VecMaskUsePred) : VecMaskUseLoad;
> 1486: if (!arch_supports_vector(is_scatter ? Op_StoreVectorScatterMasked : Op_LoadVectorGatherMasked, num_elem, elem_bt, mask)) {
What is the difference between subword-type load gather and others? It seems only check `VecMaskUseLoad` is enough for all kinds of masked gather/scatter. Only checking `is_match_rule_supported_vector` for these ops are ok. WDYT?
src/hotspot/share/opto/vectorIntrinsics.cpp line 1599:
> 1597: if (is_subword_type(elem_bt)) {
> 1598: Node* index_arr_base = array_element_address(argument(12), argument(13), T_INT);
> 1599: vload = gvn().transform(new LoadVectorGatherMaskedNode(control(), memory(addr), addr, addr_type, vector_type, index_arr_base, mask, argument(11)));
It's better to define the meaningful variables for `argument(12), argument(13), argument(11)`.
src/hotspot/share/opto/vectornode.hpp line 1001:
> 999: assert(req() == MemNode::ValueIn + 2, "match_edge expects that last input is in MemNode::ValueIn+1");
> 1000: if (is_subword_type(vect_type()->is_vect()->element_basic_type())) {
> 1001: add_req(offset);
`vect_type()->is_vect()` -> `vt` ?
src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ByteVector.java line 3062:
> 3060: // Constant folding should sweep out following conditonal logic.
> 3061: if (isp.length() > IntVector.SPECIES_PREFERRED.length()) {
> 3062: lsp = IntVector.SPECIES_PREFERRED;
What happens if the needed index vector length is larger that the max vector length of int type?
For example, byte vectors with `SPECIES_128`, may needs an index vector with `IntVector.SPECIES_512` species? And on some architectures like NEON, or SVE with 128-bit max vector size, the preferred species is only `SPECIES_128`.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1372703788
PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1372705631
PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1372710351
PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1372720580
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