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The Neon multiply/multiply-accumulate/multiply-subtract instructions can select the top or bottom half of the operand registers. This selection does not change the cost of the underlying instruction and this should be reflected by the RTL cost function. This patch adds RTL tree traversal in the Neon multiply cost function to match vec_select high-half of its operands. This traversal prevents the cost of the vec_select from being added into the cost of the multiply - meaning that these instructions can now be emitted in the combine pass as they are no longer deemed prohibitively expensive. gcc/ChangeLog: 2021-07-19 Jonathan Wright <jonathan.wright@arm.com> * config/aarch64/aarch64.c (aarch64_strip_extend_vec_half): Define. (aarch64_rtx_mult_cost): Traverse RTL tree to prevent cost of vec_select high-half from being added into Neon multiply cost. * rtlanal.c (vec_series_highpart_p): Define. * rtlanal.h (vec_series_highpart_p): Declare. gcc/testsuite/ChangeLog: * gcc.target/aarch64/vmul_high_cost.c: New test.
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