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The following adds the ability to discover a reduction chain on a series of statements that invoke undefined behavior on integer overflow. This inhibits the reassoc pass from associating stmts in the way naturally leading to a reduction chain. The common mistake on the source side is to rely on the += operator to sum multiple inputs. After the refactoring of how we handle reduction chains we can easily use vect_slp_linearize_chain to do this our selves and rely on the vectorizer punning operations to unsigned given reduction vectorization always associates. PR tree-optimization/120687 * tree-vect-slp.cc (vect_analyze_slp_reduc_chain): When there's no natural reduction chain see if vect_slp_linearize_chain can recover one and built the SLP instance manually in that case. (vect_schedule_slp): Deal with NULL lanes when looking for stores to remove. * tree-vect-loop.cc (vect_transform_cycle_phi): Dump when we are successfully transforming a reduction chain. * gcc.dg/vect/vect-reduc-chain-4.c: New testcase.
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