This patch makes shift_amt_for_vec_perm_mask use series_p to check for the simple case of a natural linear series before falling back to testing each element individually. The series_p test works with variable-length vectors but testing every individual element doesn't.
2017-12-09 Richard Sandiford <richard.sandif...@linaro.org> gcc/ * optabs.c (shift_amt_for_vec_perm_mask): Try using series_p before testing each element individually. Index: gcc/optabs.c =================================================================== --- gcc/optabs.c 2017-12-09 22:48:52.266015836 +0000 +++ gcc/optabs.c 2017-12-09 22:48:56.257154317 +0000 @@ -5375,20 +5375,20 @@ vector_compare_rtx (machine_mode cmp_mod static rtx shift_amt_for_vec_perm_mask (machine_mode mode, const vec_perm_indices &sel) { - unsigned int i, first, nelt = GET_MODE_NUNITS (mode); + unsigned int nelt = GET_MODE_NUNITS (mode); unsigned int bitsize = GET_MODE_UNIT_BITSIZE (mode); - - first = sel[0]; + unsigned int first = sel[0]; if (first >= nelt) return NULL_RTX; - for (i = 1; i < nelt; i++) - { - int idx = sel[i]; - unsigned int expected = i + first; - /* Indices into the second vector are all equivalent. */ - if (idx < 0 || (MIN (nelt, (unsigned) idx) != MIN (nelt, expected))) - return NULL_RTX; - } + + if (!sel.series_p (0, 1, first, 1)) + for (unsigned int i = 1; i < nelt; i++) + { + unsigned int expected = i + first; + /* Indices into the second vector are all equivalent. */ + if (MIN (nelt, sel[i]) != MIN (nelt, expected)) + return NULL_RTX; + } return GEN_INT (first * bitsize); }