We didn't take the cost of generating loop masks into account, and so tended to underestimate the cost of loops that need multiple masks.
2019-11-05 Richard Sandiford <richard.sandif...@arm.com> gcc/ * tree-vect-loop.c (vect_estimate_min_profitable_iters): Include the cost of generating loop masks. gcc/testsuite/ * gcc.target/aarch64/sve/mask_struct_store_3.c: Add -fno-vect-cost-model. * gcc.target/aarch64/sve/mask_struct_store_3_run.c: Likewise. * gcc.target/aarch64/sve/peel_ind_3.c: Likewise. * gcc.target/aarch64/sve/peel_ind_3_run.c: Likewise. Index: gcc/tree-vect-loop.c =================================================================== --- gcc/tree-vect-loop.c 2019-11-05 14:19:58.781197820 +0000 +++ gcc/tree-vect-loop.c 2019-11-05 14:20:40.188909187 +0000 @@ -3435,6 +3435,32 @@ vect_estimate_min_profitable_iters (loop si->kind, si->stmt_info, si->misalign, vect_epilogue); } + + /* Calculate how many masks we need to generate. */ + unsigned int num_masks = 0; + rgroup_masks *rgm; + unsigned int num_vectors_m1; + FOR_EACH_VEC_ELT (LOOP_VINFO_MASKS (loop_vinfo), num_vectors_m1, rgm) + if (rgm->mask_type) + num_masks += num_vectors_m1 + 1; + gcc_assert (num_masks > 0); + + /* In the worst case, we need to generate each mask in the prologue + and in the loop body. One of the loop body mask instructions + replaces the comparison in the scalar loop, and since we don't + count the scalar comparison against the scalar body, we shouldn't + count that vector instruction against the vector body either. + + Sometimes we can use unpacks instead of generating prologue + masks and sometimes the prologue mask will fold to a constant, + so the actual prologue cost might be smaller. However, it's + simpler and safer to use the worst-case cost; if this ends up + being the tie-breaker between vectorizing or not, then it's + probably better not to vectorize. */ + (void) add_stmt_cost (target_cost_data, num_masks, vector_stmt, + NULL, 0, vect_prologue); + (void) add_stmt_cost (target_cost_data, num_masks - 1, vector_stmt, + NULL, 0, vect_body); } else if (npeel < 0) { Index: gcc/testsuite/gcc.target/aarch64/sve/mask_struct_store_3.c =================================================================== --- gcc/testsuite/gcc.target/aarch64/sve/mask_struct_store_3.c 2019-03-08 18:14:29.768994780 +0000 +++ gcc/testsuite/gcc.target/aarch64/sve/mask_struct_store_3.c 2019-11-05 14:20:40.184909216 +0000 @@ -1,5 +1,5 @@ /* { dg-do compile } */ -/* { dg-options "-O2 -ftree-vectorize -ffast-math" } */ +/* { dg-options "-O2 -ftree-vectorize -ffast-math -fno-vect-cost-model" } */ #include <stdint.h> Index: gcc/testsuite/gcc.target/aarch64/sve/mask_struct_store_3_run.c =================================================================== --- gcc/testsuite/gcc.target/aarch64/sve/mask_struct_store_3_run.c 2019-03-08 18:14:29.772994767 +0000 +++ gcc/testsuite/gcc.target/aarch64/sve/mask_struct_store_3_run.c 2019-11-05 14:20:40.184909216 +0000 @@ -1,5 +1,5 @@ /* { dg-do run { target aarch64_sve_hw } } */ -/* { dg-options "-O2 -ftree-vectorize -ffast-math" } */ +/* { dg-options "-O2 -ftree-vectorize -ffast-math -fno-vect-cost-model" } */ #include "mask_struct_store_3.c" Index: gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3.c =================================================================== --- gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3.c 2019-03-08 18:14:29.776994751 +0000 +++ gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3.c 2019-11-05 14:20:40.184909216 +0000 @@ -1,7 +1,7 @@ /* { dg-do compile } */ /* Pick an arbitrary target for which unaligned accesses are more expensive. */ -/* { dg-options "-O3 -msve-vector-bits=256 -mtune=thunderx" } */ +/* { dg-options "-O3 -msve-vector-bits=256 -mtune=thunderx -fno-vect-cost-model" } */ #define N 32 #define MAX_START 8 Index: gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3_run.c =================================================================== --- gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3_run.c 2019-03-08 18:14:29.784994721 +0000 +++ gcc/testsuite/gcc.target/aarch64/sve/peel_ind_3_run.c 2019-11-05 14:20:40.184909216 +0000 @@ -1,6 +1,6 @@ /* { dg-do run { target aarch64_sve_hw } } */ -/* { dg-options "-O3 -mtune=thunderx" } */ -/* { dg-options "-O3 -mtune=thunderx -msve-vector-bits=256" { target aarch64_sve256_hw } } */ +/* { dg-options "-O3 -mtune=thunderx -fno-vect-cost-model" } */ +/* { dg-options "-O3 -mtune=thunderx -msve-vector-bits=256 -fno-vect-cost-model" { target aarch64_sve256_hw } } */ #include "peel_ind_3.c"