vect_recog_divmod_pattern currently bails out if the target has native support for integer division, but I think in practice it's always going to be better to open-code it anyway, just as we usually open-code scalar divisions by constants.
I think the only currently affected target is MIPS MSA, where for: void foo (int *x) { for (int i = 0; i < 100; ++i) x[i] /= 2; } we previously preferred to use division for powers of 2: .set noreorder bnz.w $w1,1f div_s.w $w0,$w0,$w1 break 7 .set reorder 1: (or just the div_s.w for -mno-check-zero-division), but after the patch we open-code them using shifts: clt_s.w $w1,$w0,$w2 subv.w $w0,$w0,$w1 srai.w $w0,$w0,1 I assume that's better. Matthew, is that right? MSA doesn't define a high-part pattern, so it still uses a division instruction for the non-power-of-2 case. Richard B pointed out that this would disable SLP of division by different amounts, but I think in practice that's a price worth paying, since the current cost model can't really tell whether using a general vector division is better than using open-coded scalar divisions. The fix would be either to support SLP of mixed open-coded divisions or to improve the cost model and try SLP again without the patterns. The patch adds an XFAILed test for this. Tested on aarch64-linux-gnu (with and without SVE), aarch64_be-elf and x86_64-linux-gnu. OK to install? Richard 2018-05-24 Richard Sandiford <richard.sandif...@linaro.org> gcc/ * tree-vect-patterns.c (vect_recog_divmod_pattern): Remove check for division support. gcc/testsuite/ * gcc.dg/vect/bb-slp-div-1.c: New XFAILed test. Index: gcc/tree-vect-patterns.c =================================================================== --- gcc/tree-vect-patterns.c 2018-05-16 12:48:59.115202362 +0100 +++ gcc/tree-vect-patterns.c 2018-05-24 09:18:10.445466941 +0100 @@ -2639,7 +2639,6 @@ vect_recog_divmod_pattern (vec<gimple *> enum tree_code rhs_code; stmt_vec_info stmt_vinfo = vinfo_for_stmt (last_stmt); vec_info *vinfo = stmt_vinfo->vinfo; - optab optab; tree q; int dummy_int, prec; stmt_vec_info def_stmt_vinfo; @@ -2674,17 +2673,6 @@ vect_recog_divmod_pattern (vec<gimple *> if (vectype == NULL_TREE) return NULL; - /* If the target can handle vectorized division or modulo natively, - don't attempt to optimize this. */ - optab = optab_for_tree_code (rhs_code, vectype, optab_default); - if (optab != unknown_optab) - { - machine_mode vec_mode = TYPE_MODE (vectype); - int icode = (int) optab_handler (optab, vec_mode); - if (icode != CODE_FOR_nothing) - return NULL; - } - prec = TYPE_PRECISION (itype); if (integer_pow2p (oprnd1)) { Index: gcc/testsuite/gcc.dg/vect/bb-slp-div-1.c =================================================================== --- /dev/null 2018-04-20 16:19:46.369131350 +0100 +++ gcc/testsuite/gcc.dg/vect/bb-slp-div-1.c 2018-05-24 09:18:10.444466986 +0100 @@ -0,0 +1,19 @@ +/* { dg-do compile } */ +/* { dg-additional-options "-msve-vector-bits=256" { target aarch64_sve } } */ + +int x[8]; + +void +f (void) +{ + x[0] /= 2; + x[1] /= 3; + x[2] /= 4; + x[3] /= 5; + x[4] /= 6; + x[5] /= 7; + x[6] /= 8; + x[7] /= 9; +} + +/* { dg-final { scan-tree-dump "basic block vectorized" "slp2" { xfail *-*-* } } } */