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This adds a gather vectorization capability to the vectorizer without target support by decomposing the offset vector, doing sclar loads and then building a vector from the result. This is aimed mainly at cases where vectorizing the rest of the loop offsets the cost of vectorizing the gather. Note it's difficult to avoid vectorizing the offset load, but in some cases later passes can turn the vector load + extract into scalar loads, see the followup patch. On SPEC CPU 2017 510.parest_r this improves runtime from 250s to 219s on a Zen2 CPU which has its native gather instructions disabled (using those the runtime instead increases to 254s) using -Ofast -march=znver2 [-flto]. It turns out the critical loops in this benchmark all perform gather operations. 2021-07-30 Richard Biener <rguenther@suse.de> * tree-vect-data-refs.c (vect_check_gather_scatter): Include widening conversions only when the result is still handed by native gather or the current offset size not already matches the data size. Also succeed analysis in case there's no native support, noted by a IFN_LAST ifn and a NULL decl. (vect_analyze_data_refs): Always consider gathers. * tree-vect-patterns.c (vect_recog_gather_scatter_pattern): Test for no IFN gather rather than decl gather. * tree-vect-stmts.c (vect_model_load_cost): Pass in the gather-scatter info and cost emulated gathers accordingly. (vect_truncate_gather_scatter_offset): Properly test for no IFN gather. (vect_use_strided_gather_scatters_p): Likewise. (get_load_store_type): Handle emulated gathers and its restrictions. (vectorizable_load): Likewise. Emulate them by extracting scalar offsets, doing scalar loads and a vector construct. * gcc.target/i386/vect-gather-1.c: New testcase. * gfortran.dg/vect/vect-8.f90: Adjust.
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