Account for the cost of generating loop masks

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.

2019-11-13  Richard Sandiford  <richard.sandiford@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_2.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_2_run.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3_run.c: Likewise.

From-SVN: r278125

1 files changed, 26 insertions, 0 deletions

diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index 83fb848..005fa30 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -3291,6 +3291,32 @@ vect_estimate_min_profitable_iters (loop_vec_info loop_vinfo,
 				  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)
     {