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/* Loop unroll-and-jam.
Copyright (C) 2017-2018 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3, or (at your option) any
later version.
GCC is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
for more details.
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3. If not see
<>. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "params.h"
#include "tree-pass.h"
#include "backend.h"
#include "tree.h"
#include "gimple.h"
#include "ssa.h"
#include "fold-const.h"
#include "tree-cfg.h"
#include "tree-ssa.h"
#include "tree-ssa-loop-niter.h"
#include "tree-ssa-loop.h"
#include "tree-ssa-loop-manip.h"
#include "cfgloop.h"
#include "tree-scalar-evolution.h"
#include "gimple-iterator.h"
#include "cfghooks.h"
#include "tree-data-ref.h"
#include "tree-ssa-loop-ivopts.h"
#include "tree-vectorizer.h"
/* Unroll and Jam transformation
This is a combination of two transformations, where the second
is not always valid. It's applicable if a loop nest has redundancies
over the iterations of an outer loop while not having that with
an inner loop.
Given this nest:
for (i) {
for (j) {
first unroll:
for (i by 2) {
for (j) {
for (j) {
then fuse the two adjacent inner loops resulting from that:
for (i by 2) {
for (j) {
As the order of evaluations of the body B changes this is valid
only in certain situations: all distance vectors need to be forward.
Additionally if there are multiple induction variables than just
a counting control IV (j above) we can also deal with some situations.
The validity is checked by unroll_jam_possible_p, and the data-dep
testing below.
A trivial example where the fusion is wrong would be when
B(i,j) == x[j-1] = x[j];
for (i by 2) {
for (j) {
x[j-1] = x[j];
for (j) {
x[j-1] = x[j];
} effect: move content to front by two elements
for (i by 2) {
for (j) {
x[j-1] = x[j];
x[j-1] = x[j];
} effect: move content to front by one element
/* Modify the loop tree for the fact that all code once belonging
to the OLD loop or the outer loop of OLD now is inside LOOP. */
static void
merge_loop_tree (struct loop *loop, struct loop *old)
basic_block *bbs;
int i, n;
struct loop *subloop;
edge e;
edge_iterator ei;
/* Find its nodes. */
bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (cfun));
n = get_loop_body_with_size (loop, bbs, n_basic_blocks_for_fn (cfun));
for (i = 0; i < n; i++)
/* If the block was direct child of OLD loop it's now part
of LOOP. If it was outside OLD, then it moved into LOOP
as well. This avoids changing the loop father for BBs
in inner loops of OLD. */
if (bbs[i]->loop_father == old
|| loop_depth (bbs[i]->loop_father) < loop_depth (old))
remove_bb_from_loops (bbs[i]);
add_bb_to_loop (bbs[i], loop);
/* If we find a direct subloop of OLD, move it to LOOP. */
subloop = bbs[i]->loop_father;
if (loop_outer (subloop) == old && subloop->header == bbs[i])
flow_loop_tree_node_remove (subloop);
flow_loop_tree_node_add (loop, subloop);
/* Update the information about loop exit edges. */
for (i = 0; i < n; i++)
FOR_EACH_EDGE (e, ei, bbs[i]->succs)
rescan_loop_exit (e, false, false);
loop->num_nodes = n;
free (bbs);
/* BB is part of the outer loop of an unroll-and-jam situation.
Check if any statements therein would prevent the transformation. */
static bool
bb_prevents_fusion_p (basic_block bb)
gimple_stmt_iterator gsi;
/* BB is duplicated by outer unrolling and then all N-1 first copies
move into the body of the fused inner loop. If BB exits the outer loop
the last copy still doess so, and the first N-1 copies are cancelled
by loop unrolling, so also after fusion it's the exit block.
But there might be other reasons that prevent fusion:
* stores or unknown side-effects prevent fusion
* loads don't
* computations into SSA names: these aren't problematic. Their
result will be unused on the exit edges of the first N-1 copies
(those aren't taken after unrolling). If they are used on the
other edge (the one leading to the outer latch block) they are
loop-carried (on the outer loop) and the Nth copy of BB will
compute them again (i.e. the first N-1 copies will be dead). */
for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
gimple *g = gsi_stmt (gsi);
if (gimple_vdef (g) || gimple_has_side_effects (g))
return true;
return false;
/* Given an inner loop LOOP (of some OUTER loop) determine if
we can safely fuse copies of it (generated by outer unrolling).
If so return true, otherwise return false. */
static bool
unroll_jam_possible_p (struct loop *outer, struct loop *loop)
basic_block *bbs;
int i, n;
struct tree_niter_desc niter;
/* When fusing the loops we skip the latch block
of the first one, so it mustn't have any effects to
preserve. */
if (!empty_block_p (loop->latch))
return false;
if (!single_exit (loop))
return false;
/* We need a perfect nest. Quick check for adjacent inner loops. */
if (outer->inner != loop || loop->next)
return false;
/* Prevent head-controlled inner loops, that we usually have.
The guard block would need to be accepted
(invariant condition either entering or skipping the loop),
without also accepting arbitrary control flow. When unswitching
ran before us (as with -O3) this won't be a problem because its
outer loop unswitching will have moved out the invariant condition.
If we do that we need to extend fuse_loops() to cope with this
by threading through the (still invariant) copied condition
between the two loop copies. */
if (!dominated_by_p (CDI_DOMINATORS, outer->latch, loop->header))
return false;
/* The number of iterations of the inner loop must be loop invariant
with respect to the outer loop. */
if (!number_of_iterations_exit (loop, single_exit (loop), &niter,
false, true)
|| niter.cmp == ERROR_MARK
|| !integer_zerop (niter.may_be_zero)
|| !expr_invariant_in_loop_p (outer, niter.niter))
return false;
/* And check blocks belonging to just outer loop. */
bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (cfun));
n = get_loop_body_with_size (outer, bbs, n_basic_blocks_for_fn (cfun));
for (i = 0; i < n; i++)
if (bbs[i]->loop_father == outer && bb_prevents_fusion_p (bbs[i]))
free (bbs);
if (i != n)
return false;
/* For now we can safely fuse copies of LOOP only if all
loop carried variables are inductions (or the virtual op).
We could handle reductions as well (the initial value in the second
body would be the after-iter value of the first body) if it's over
an associative and commutative operation. We wouldn't
be able to handle unknown cycles. */
gphi_iterator psi;
for (psi = gsi_start_phis (loop->header); !gsi_end_p (psi); gsi_next (&psi))
affine_iv iv;
tree op = gimple_phi_result (psi.phi ());
if (virtual_operand_p (op))
if (!simple_iv (loop, loop, op, &iv, true))
return false;
/* The inductions must be regular, loop invariant step and initial
value. */
if (!expr_invariant_in_loop_p (outer, iv.step)
|| !expr_invariant_in_loop_p (outer, iv.base))
return false;
/* XXX With more effort we could also be able to deal with inductions
where the initial value is loop variant but a simple IV in the
outer loop. The initial value for the second body would be
the original initial value plus iv.base.step. The next value
for the fused loop would be the original next value of the first
copy, _not_ the next value of the second body. */
return true;
/* Fuse LOOP with all further neighbors. The loops are expected to
be in appropriate form. */
static void
fuse_loops (struct loop *loop)
struct loop *next = loop->next;
while (next)
edge e;
remove_branch (single_pred_edge (loop->latch));
/* Make delete_basic_block not fiddle with the loop structure. */
basic_block oldlatch = loop->latch;
loop->latch = NULL;
delete_basic_block (oldlatch);
e = redirect_edge_and_branch (loop_latch_edge (next),
loop->latch = e->src;
flush_pending_stmts (e);
gcc_assert (EDGE_COUNT (next->header->preds) == 1);
/* The PHI nodes of the second body (single-argument now)
need adjustments to use the right values: either directly
the value of the corresponding PHI in the first copy or
the one leaving the first body which unrolling did for us.
See also unroll_jam_possible_p() for further possibilities. */
gphi_iterator psi_first, psi_second;
e = single_pred_edge (next->header);
for (psi_first = gsi_start_phis (loop->header),
psi_second = gsi_start_phis (next->header);
!gsi_end_p (psi_first);
gsi_next (&psi_first), gsi_next (&psi_second))
gphi *phi_first = psi_first.phi ();
gphi *phi_second = psi_second.phi ();
tree firstop = gimple_phi_result (phi_first);
/* The virtual operand is correct already as it's
always live at exit, hence has a LCSSA node and outer
loop unrolling updated SSA form. */
if (virtual_operand_p (firstop))
/* Due to unroll_jam_possible_p() we know that this is
an induction. The second body goes over the same
iteration space. */
add_phi_arg (phi_second, firstop, e,
gimple_location (phi_first));
gcc_assert (gsi_end_p (psi_second));
merge_loop_tree (loop, next);
gcc_assert (!next->num_nodes);
struct loop *ln = next->next;
delete_loop (next);
next = ln;
rewrite_into_loop_closed_ssa_1 (NULL, 0, SSA_OP_USE, loop);
/* Returns true if the distance in DDR can be determined and adjusts
the unroll factor in *UNROLL to make unrolling valid for that distance.
Otherwise return false.
If this data dep can lead to a removed memory reference, increment
*REMOVED and adjust *PROFIT_UNROLL to be the necessary unroll factor
for this to happen. */
static bool
adjust_unroll_factor (struct data_dependence_relation *ddr,
unsigned *unroll, unsigned *profit_unroll,
unsigned *removed)
bool ret = false;
if (DDR_ARE_DEPENDENT (ddr) != chrec_known)
if (DDR_NUM_DIST_VECTS (ddr) == 0)
return false;
unsigned i;
lambda_vector dist_v;
FOR_EACH_VEC_ELT (DDR_DIST_VECTS (ddr), i, dist_v)
/* A distance (a,b) is at worst transformed into (a/N,b) by the
unrolling (factor N), so the transformation is valid if
a >= N, or b > 0, or b is zero and a > 0. Otherwise the unroll
factor needs to be limited so that the first condition holds.
That may limit the factor down to zero in the worst case. */
int dist = dist_v[0];
if (dist < 0)
gcc_unreachable ();
else if ((unsigned)dist >= *unroll)
else if (lambda_vector_lexico_pos (dist_v + 1, DDR_NB_LOOPS (ddr) - 1)
|| (lambda_vector_zerop (dist_v + 1, DDR_NB_LOOPS (ddr) - 1)
&& dist > 0))
*unroll = dist;
/* With a distance (a,0) it's always profitable to unroll-and-jam
(by a+1), because one memory reference will go away. With
(a,b) and b != 0 that's less clear. We will increase the
number of streams without lowering the number of mem refs.
So for now only handle the first situation. */
if (lambda_vector_zerop (dist_v + 1, DDR_NB_LOOPS (ddr) - 1))
*profit_unroll = MAX (*profit_unroll, (unsigned)dist + 1);
ret = true;
return ret;
/* Main entry point for the unroll-and-jam transformation
described above. */
static unsigned int
tree_loop_unroll_and_jam (void)
struct loop *loop;
bool changed = false;
gcc_assert (scev_initialized_p ());
/* Go through all innermost loops. */
struct loop *outer = loop_outer (loop);
if (loop_depth (loop) < 2
|| optimize_loop_nest_for_size_p (outer))
if (!unroll_jam_possible_p (outer, loop))
vec<data_reference_p> datarefs;
vec<ddr_p> dependences;
unsigned unroll_factor, profit_unroll, removed;
struct tree_niter_desc desc;
bool unroll = false;
auto_vec<loop_p, 3> loop_nest;
dependences.create (10);
datarefs.create (10);
if (!compute_data_dependences_for_loop (outer, true, &loop_nest,
&datarefs, &dependences))
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "Cannot analyze data dependencies\n");
free_data_refs (datarefs);
free_dependence_relations (dependences);
return false;
if (!datarefs.length ())
if (dump_file && (dump_flags & TDF_DETAILS))
dump_data_dependence_relations (dump_file, dependences);
unroll_factor = (unsigned)-1;
profit_unroll = 1;
removed = 0;
/* Check all dependencies. */
unsigned i;
struct data_dependence_relation *ddr;
FOR_EACH_VEC_ELT (dependences, i, ddr)
struct data_reference *dra, *drb;
/* If the refs are independend there's nothing to do. */
if (DDR_ARE_DEPENDENT (ddr) == chrec_known)
dra = DDR_A (ddr);
drb = DDR_B (ddr);
/* Nothing interesting for the self dependencies. */
if (dra == drb)
/* Now check the distance vector, for determining a sensible
outer unroll factor, and for validity of merging the inner
loop copies. */
if (!adjust_unroll_factor (ddr, &unroll_factor, &profit_unroll,
/* Couldn't get the distance vector. For two reads that's
harmless (we assume we should unroll). For at least
one write this means we can't check the dependence direction
and hence can't determine safety. */
if (DR_IS_WRITE (dra) || DR_IS_WRITE (drb))
unroll_factor = 0;
/* We regard a user-specified minimum percentage of zero as a request
to ignore all profitability concerns and apply the transformation
always. */
profit_unroll = 2;
else if (removed * 100 / datarefs.length ()
profit_unroll = 1;
if (unroll_factor > profit_unroll)
unroll_factor = profit_unroll;
if (unroll_factor > (unsigned)PARAM_VALUE (PARAM_UNROLL_JAM_MAX_UNROLL))
unroll = (unroll_factor > 1
&& can_unroll_loop_p (outer, unroll_factor, &desc));
if (unroll)
if (dump_enabled_p ())
find_loop_location (outer),
"applying unroll and jam with factor %d\n",
initialize_original_copy_tables ();
tree_unroll_loop (outer, unroll_factor, single_dom_exit (outer),
free_original_copy_tables ();
fuse_loops (outer->inner);
changed = true;
loop_nest.release ();
free_dependence_relations (dependences);
free_data_refs (datarefs);
if (changed)
scev_reset ();
free_dominance_info (CDI_DOMINATORS);
return TODO_cleanup_cfg;
return 0;
/* Pass boilerplate */
namespace {
const pass_data pass_data_loop_jam =
GIMPLE_PASS, /* type */
"unrolljam", /* name */
OPTGROUP_LOOP, /* optinfo_flags */
TV_LOOP_JAM, /* tv_id */
PROP_cfg, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
0, /* todo_flags_finish */
class pass_loop_jam : public gimple_opt_pass
pass_loop_jam (gcc::context *ctxt)
: gimple_opt_pass (pass_data_loop_jam, ctxt)
/* opt_pass methods: */
virtual bool gate (function *) { return flag_unroll_jam != 0; }
virtual unsigned int execute (function *);
unsigned int
pass_loop_jam::execute (function *fun)
if (number_of_loops (fun) <= 1)
return 0;
return tree_loop_unroll_and_jam ();
gimple_opt_pass *
make_pass_loop_jam (gcc::context *ctxt)
return new pass_loop_jam (ctxt);