This patch adds a mode in which the vectoriser tries each available
base vector mode and picks the one with the lowest cost. For now
the behaviour is behind a default-off --param, but a later patch
enables it by default for SVE.
The patch keeps the current behaviour of preferring a VF of
loop->simdlen over any larger or smaller VF, regardless of costs
or target preferences.
2019-11-05 Richard Sandiford <richard.sandif...@arm.com>
gcc/
* params.def (vect-compare-loop-costs): New param.
* doc/invoke.texi: Document it.
* tree-vectorizer.h (_loop_vec_info::vec_outside_cost)
(_loop_vec_info::vec_inside_cost): New member variables.
* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Initialize them.
(vect_better_loop_vinfo_p, vect_joust_loop_vinfos): New functions.
(vect_analyze_loop): When the new parameter allows, try vectorizing
the loop with each available vector mode and picking the one with
the lowest cost.
(vect_estimate_min_profitable_iters): Record the computed costs
in the loop_vec_info.
Index: gcc/params.def
===================================================================
--- gcc/params.def 2019-10-31 17:15:25.470517368 +0000
+++ gcc/params.def 2019-11-05 14:19:58.781197820 +0000
@@ -661,6 +661,13 @@ DEFPARAM(PARAM_VECT_MAX_PEELING_FOR_ALIG
"Maximum number of loop peels to enhance alignment of data references
in a loop.",
-1, -1, 64)
+DEFPARAM(PARAM_VECT_COMPARE_LOOP_COSTS,
+ "vect-compare-loop-costs",
+ "Whether to try vectorizing a loop using each supported"
+ " combination of vector types and picking the version with the"
+ " lowest cost.",
+ 0, 0, 1)
+
DEFPARAM(PARAM_MAX_CSELIB_MEMORY_LOCATIONS,
"max-cselib-memory-locations",
"The maximum memory locations recorded by cselib.",
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi 2019-11-04 21:13:57.611756365 +0000
+++ gcc/doc/invoke.texi 2019-11-05 14:19:58.777197850 +0000
@@ -11563,6 +11563,12 @@ doing loop versioning for alias in the v
The maximum number of loop peels to enhance access alignment
for vectorizer. Value -1 means no limit.
+@item vect-compare-loop-costs
+Whether to try vectorizing a loop using each supported combination of
+vector types and picking the version with the lowest cost. This parameter
+has no effect when @option{-fno-vect-cost-model} or
+@option{-fvect-cost-model=unlimited} are used.
+
@item max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm
for analysis of the number of iterations of the loop tries to evaluate.
Index: gcc/tree-vectorizer.h
===================================================================
--- gcc/tree-vectorizer.h 2019-11-05 14:19:33.829371745 +0000
+++ gcc/tree-vectorizer.h 2019-11-05 14:19:58.781197820 +0000
@@ -601,6 +601,13 @@ typedef class _loop_vec_info : public ve
/* Cost of a single scalar iteration. */
int single_scalar_iteration_cost;
+ /* The cost of the vector prologue and epilogue, including peeled
+ iterations and set-up code. */
+ int vec_outside_cost;
+
+ /* The cost of the vector loop body. */
+ int vec_inside_cost;
+
/* Is the loop vectorizable? */
bool vectorizable;
Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c 2019-11-05 14:19:33.829371745 +0000
+++ gcc/tree-vect-loop.c 2019-11-05 14:19:58.781197820 +0000
@@ -830,6 +830,8 @@ _loop_vec_info::_loop_vec_info (class lo
scan_map (NULL),
slp_unrolling_factor (1),
single_scalar_iteration_cost (0),
+ vec_outside_cost (0),
+ vec_inside_cost (0),
vectorizable (false),
can_fully_mask_p (true),
fully_masked_p (false),
@@ -2373,6 +2375,80 @@ vect_analyze_loop_2 (loop_vec_info loop_
goto start_over;
}
+/* Return true if vectorizing a loop using NEW_LOOP_VINFO appears
+ to be better than vectorizing it using OLD_LOOP_VINFO. Assume that
+ OLD_LOOP_VINFO is better unless something specifically indicates
+ otherwise.
+
+ Note that this deliberately isn't a partial order. */
+
+static bool
+vect_better_loop_vinfo_p (loop_vec_info new_loop_vinfo,
+ loop_vec_info old_loop_vinfo)
+{
+ struct loop *loop = LOOP_VINFO_LOOP (new_loop_vinfo);
+ gcc_assert (LOOP_VINFO_LOOP (old_loop_vinfo) == loop);
+
+ poly_int64 new_vf = LOOP_VINFO_VECT_FACTOR (new_loop_vinfo);
+ poly_int64 old_vf = LOOP_VINFO_VECT_FACTOR (old_loop_vinfo);
+
+ /* Always prefer a VF of loop->simdlen over any other VF. */
+ if (loop->simdlen)
+ {
+ bool new_simdlen_p = known_eq (new_vf, loop->simdlen);
+ bool old_simdlen_p = known_eq (old_vf, loop->simdlen);
+ if (new_simdlen_p != old_simdlen_p)
+ return new_simdlen_p;
+ }
+
+ /* Limit the VFs to what is likely to be the maximum number of iterations,
+ to handle cases in which at least one loop_vinfo is fully-masked. */
+ HOST_WIDE_INT estimated_max_niter = likely_max_stmt_executions_int (loop);
+ if (estimated_max_niter != -1)
+ {
+ if (known_le (estimated_max_niter, new_vf))
+ new_vf = estimated_max_niter;
+ if (known_le (estimated_max_niter, old_vf))
+ old_vf = estimated_max_niter;
+ }
+
+ /* Check whether the (fractional) cost per scalar iteration is lower
+ or higher: new_inside_cost / new_vf vs. old_inside_cost / old_vf. */
+ poly_widest_int rel_new = (new_loop_vinfo->vec_inside_cost
+ * poly_widest_int (old_vf));
+ poly_widest_int rel_old = (old_loop_vinfo->vec_inside_cost
+ * poly_widest_int (new_vf));
+ if (maybe_lt (rel_old, rel_new))
+ return false;
+ if (known_lt (rel_new, rel_old))
+ return true;
+
+ /* If there's nothing to choose between the loop bodies, see whether
+ there's a difference in the prologue and epilogue costs. */
+ if (new_loop_vinfo->vec_outside_cost != old_loop_vinfo->vec_outside_cost)
+ return new_loop_vinfo->vec_outside_cost < old_loop_vinfo->vec_outside_cost;
+
+ return false;
+}
+
+/* Decide whether to replace OLD_LOOP_VINFO with NEW_LOOP_VINFO. Return
+ true if we should. */
+
+static bool
+vect_joust_loop_vinfos (loop_vec_info new_loop_vinfo,
+ loop_vec_info old_loop_vinfo)
+{
+ if (!vect_better_loop_vinfo_p (new_loop_vinfo, old_loop_vinfo))
+ return false;
+
+ if (dump_enabled_p ())
+ dump_printf_loc (MSG_NOTE, vect_location,
+ "***** Preferring vector mode %s to vector mode %s\n",
+ GET_MODE_NAME (new_loop_vinfo->vector_mode),
+ GET_MODE_NAME (old_loop_vinfo->vector_mode));
+ return true;
+}
+
/* Function vect_analyze_loop.
Apply a set of analyses on LOOP, and create a loop_vec_info struct
@@ -2408,6 +2484,8 @@ vect_analyze_loop (class loop *loop, vec
machine_mode next_vector_mode = VOIDmode;
poly_uint64 lowest_th = 0;
unsigned vectorized_loops = 0;
+ bool pick_lowest_cost_p = (PARAM_VALUE (PARAM_VECT_COMPARE_LOOP_COSTS)
+ && !unlimited_cost_model (loop));
bool vect_epilogues = false;
opt_result res = opt_result::success ();
@@ -2428,6 +2506,34 @@ vect_analyze_loop (class loop *loop, vec
bool fatal = false;
+ /* When pick_lowest_cost_p is true, we should in principle iterate
+ over all the loop_vec_infos that LOOP_VINFO could replace and
+ try to vectorize LOOP_VINFO under the same conditions.
+ E.g. when trying to replace an epilogue loop, we should vectorize
+ LOOP_VINFO as an epilogue loop with the same VF limit. When trying
+ to replace the main loop, we should vectorize LOOP_VINFO as a main
+ loop too.
+
+ However, autovectorize_vector_modes is usually sorted as follows:
+
+ - Modes that naturally produce lower VFs usually follow modes that
+ naturally produce higher VFs.
+
+ - When modes naturally produce the same VF, maskable modes
+ usually follow unmaskable ones, so that the maskable mode
+ can be used to vectorize the epilogue of the unmaskable mode.
+
+ This order is preferred because it leads to the maximum
+ epilogue vectorization opportunities. Targets should only use
+ a different order if they want to make wide modes available while
+ disparaging them relative to earlier, smaller modes. The assumption
+ in that case is that the wider modes are more expensive in some
+ way that isn't reflected directly in the costs.
+
+ There should therefore be few interesting cases in which
+ LOOP_VINFO fails when treated as an epilogue loop, succeeds when
+ treated as a standalone loop, and ends up being genuinely cheaper
+ than FIRST_LOOP_VINFO. */
if (vect_epilogues)
LOOP_VINFO_ORIG_LOOP_INFO (loop_vinfo) = first_loop_vinfo;
@@ -2475,13 +2581,34 @@ vect_analyze_loop (class loop *loop, vec
LOOP_VINFO_ORIG_LOOP_INFO (loop_vinfo) = NULL;
simdlen = 0;
}
+ else if (pick_lowest_cost_p && first_loop_vinfo)
+ {
+ /* Keep trying to roll back vectorization attempts while the
+ loop_vec_infos they produced were worse than this one. */
+ vec<loop_vec_info> &vinfos = first_loop_vinfo->epilogue_vinfos;
+ while (!vinfos.is_empty ()
+ && vect_joust_loop_vinfos (loop_vinfo, vinfos.last ()))
+ {
+ gcc_assert (vect_epilogues);
+ delete vinfos.pop ();
+ }
+ if (vinfos.is_empty ()
+ && vect_joust_loop_vinfos (loop_vinfo, first_loop_vinfo))
+ {
+ delete first_loop_vinfo;
+ first_loop_vinfo = opt_loop_vec_info::success (NULL);
+ LOOP_VINFO_ORIG_LOOP_INFO (loop_vinfo) = NULL;
+ }
+ }
if (first_loop_vinfo == NULL)
{
first_loop_vinfo = loop_vinfo;
lowest_th = LOOP_VINFO_VERSIONING_THRESHOLD (first_loop_vinfo);
}
- else if (vect_epilogues)
+ else if (vect_epilogues
+ /* For now only allow one epilogue loop. */
+ && first_loop_vinfo->epilogue_vinfos.is_empty ())
{
first_loop_vinfo->epilogue_vinfos.safe_push (loop_vinfo);
poly_uint64 th = LOOP_VINFO_VERSIONING_THRESHOLD (loop_vinfo);
@@ -2501,12 +2628,14 @@ vect_analyze_loop (class loop *loop, vec
&& loop->inner == NULL
&& PARAM_VALUE (PARAM_VECT_EPILOGUES_NOMASK)
&& LOOP_VINFO_PEELING_FOR_NITER (first_loop_vinfo)
- /* For now only allow one epilogue loop. */
- && first_loop_vinfo->epilogue_vinfos.is_empty ());
+ /* For now only allow one epilogue loop, but allow
+ pick_lowest_cost_p to replace it. */
+ && (first_loop_vinfo->epilogue_vinfos.is_empty ()
+ || pick_lowest_cost_p));
/* Commit to first_loop_vinfo if we have no reason to try
alternatives. */
- if (!simdlen && !vect_epilogues)
+ if (!simdlen && !vect_epilogues && !pick_lowest_cost_p)
break;
}
else
@@ -3454,7 +3583,11 @@ vect_estimate_min_profitable_iters (loop
&vec_inside_cost, &vec_epilogue_cost);
vec_outside_cost = (int)(vec_prologue_cost + vec_epilogue_cost);
-
+
+ /* Stash the costs so that we can compare two loop_vec_infos. */
+ loop_vinfo->vec_inside_cost = vec_inside_cost;
+ loop_vinfo->vec_outside_cost = vec_outside_cost;
+
if (dump_enabled_p ())
{
dump_printf_loc (MSG_NOTE, vect_location, "Cost model analysis: \n");
