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As Jakub pointed out, if we narrow a plus, minus or mult operation based on the number of bits that consumers need, we have to convert a signed operation to an unsigned one in order to avoid new undefined behaviour. This patch does that and generalises vect_convert_input and vect_recog_over_widening_pattern to cope with the extra casts. (The changes to both functions are covered by existing tests.) 2018-12-03 Richard Sandiford <richard.sandiford@arm.com> gcc/ PR tree-optimization/88064 * tree-vect-patterns.c (vect_convert_input): Convert the result of an existing cast if it has the right width but the wrong sign. Do not test the signedness of the required result when considering whether to split an existing cast; instead split to a type with the same signedness as the source of the cast, then convert it to the opposite signedness where necessary. (vect_recog_over_widening_pattern): Handle sign changes between the final PLUS_EXPR and the RSHIFT_EXPR. (vect_recog_average_pattern): Use an unsigned operation when truncating an addition, subtraction or multiplication. Cast the result back to the "real" signedness before promoting. gcc/testsuite/ PR tree-optimization/88064 * gcc.dg/vect/vect-over-widen-23.c: New test. From-SVN: r266829
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