This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: Add an alternative vector loop iv mechanism
Richard Biener <richard.guenther@gmail.com> writes:
> On Thu, Oct 19, 2017 at 12:28 AM, Richard Sandiford
> <richard.sandiford@linaro.org> wrote:
>> Richard Biener <richard.guenther@gmail.com> writes:
>>> On Fri, Oct 13, 2017 at 4:10 PM, Richard Sandiford
>>> <richard.sandiford@linaro.org> wrote:
>>>> Normally we adjust the vector loop so that it iterates:
>>>>
>>>> (original number of scalar iterations - number of peels) / VF
>>>>
>>>> times, enforcing this using an IV that starts at zero and increments
>>>> by one each iteration. However, dividing by VF would be expensive
>>>> for variable VF, so this patch adds an alternative in which the IV
>>>> increments by VF each iteration instead. We then need to take care
>>>> to handle possible overflow in the IV.
>>>
>>> Hmm, why do you need to handle possible overflow? Doesn't the
>>> original loop have a natural IV that evolves like this? After all we
>>> can compute an expression for niters of the scalar loop.
>>
>> The problem comes with loops like:
>>
>> unsigned char i = 0;
>> do
>> {
>> ...
>> i--;
>> }
>> while (i != 0);
>>
>> The loop statements execute 256 times and the latch executes 255 times.
>> LOOP_VINFO_NITERSM1 is then 255 but LOOP_VINFO_NITERS (stored as an
>> unsigned char) is 0.
>
> Yes, that's an existing issue and the reason why I introduced
> NITERSM1. All remaining uses of NITERS should really go away
> because of this corner-case. So you are introducing a new user?
It's not really an NITERSM1 vs. NITERS thing. We'd get the same
result/have the same problem with NITERSM1 - (STEP - 1) instead
of NITERS - STEP, namely:
- the new IV uses the same type as NITERS
- we only want the loop to iterate if there are at least STEP scalar
iterations to go
- this means that the natural limit is "IV <= NITERS - STEP"
or "IV <= NITERSM1 - (STEP - 1)" (both equivalent)
- the loop is only guaranteed to terminate if the IV can hit
a value STEP times higher than that, i.e. "IV == NITERS - STEP"
must be followed by an iteration in which the branch-back
condition is false
- but if NITERS can't represent the actual number of iterations,
then there is no value STEP times higher than that
- we cope with this by starting the IV at -1 and using a limit
of "IV < NITERS - STEP" i.e. "IV <= NITERSM1 - STEP".
So you could see this as using a limit based on NITERSM1 with a
start of -1, although the "< NITERS - STEP" avoids the need to
subtract 1 at runtime.
But it seems better to use a 0-based IV when we can, since that
leads to more natural ivopts opportunities. That's why the loop
tests for the overflow case and only uses the -1 based IV when
necessary.
Thanks,
Richard
>
> Richard.
>
>> This leads to things like:
>>
>> /* Constant case. */
>> if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
>> {
>> tree cst_niters = LOOP_VINFO_NITERS (loop_vinfo);
>> tree cst_nitersm1 = LOOP_VINFO_NITERSM1 (loop_vinfo);
>>
>> gcc_assert (TREE_CODE (cst_niters) == INTEGER_CST);
>> gcc_assert (TREE_CODE (cst_nitersm1) == INTEGER_CST);
>> if (wi::to_widest (cst_nitersm1) < wi::to_widest (cst_niters))
>> return true;
>> }
>>
>> in loop_niters_no_overflow.
>>
>>>> The new mechanism isn't used yet; a later patch replaces the
>>>> "if (1)" with a check for variable VF. If the patch is OK, I'll
>>>> hold off applying it until the follow-on is ready to go in.
>>>
>>> I indeed don't like code that isn't exercised. Otherwise looks reasonable.
>>
>> Thanks.
>>
>> Richard
>>
>>> Thanks,
>>> Richard.
>>>
>>>> Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64-linux-gnu.
>>>> OK to install when the time comes?
>>>>
>>>> Richard
>>>>
>>>>
>>>> 2017-10-13 Richard Sandiford <richard.sandiford@linaro.org>
>>>>
>>>> gcc/
>>>> * tree-vect-loop-manip.c: Include gimple-fold.h.
>>>> (slpeel_make_loop_iterate_ntimes): Add step, final_iv and
>>>> niters_maybe_zero parameters. Handle other cases besides a step of
> 1.
>>>> (vect_gen_vector_loop_niters): Add a step_vector_ptr parameter.
>>>> Add a path that uses a step of VF instead of 1, but disable it
>>>> for now.
>>>> (vect_do_peeling): Add step_vector, niters_vector_mult_vf_var
>>>> and niters_no_overflow parameters. Update calls to
>>>> slpeel_make_loop_iterate_ntimes and vect_gen_vector_loop_niters.
>>>> Create a new SSA name if the latter choses to use a ste other
>>>> than zero, and return it via niters_vector_mult_vf_var.
>>>> * tree-vect-loop.c (vect_transform_loop): Update calls to
>>>> vect_do_peeling, vect_gen_vector_loop_niters and
>>>> slpeel_make_loop_iterate_ntimes.
>>>> * tree-vectorizer.h (slpeel_make_loop_iterate_ntimes,
> vect_do_peeling)
>>>> (vect_gen_vector_loop_niters): Update declarations after above
>>> changes.
>>>>
>>>> Index: gcc/tree-vect-loop-manip.c
>>>> ===================================================================
>>>> --- gcc/tree-vect-loop-manip.c 2017-10-13 15:01:40.144777367 +0100
>>>> +++ gcc/tree-vect-loop-manip.c 2017-10-13 15:01:40.296014347 +0100
>>>> @@ -41,6 +41,7 @@ Software Foundation; either version 3, o
>>>> #include "tree-scalar-evolution.h"
>>>> #include "tree-vectorizer.h"
>>>> #include "tree-ssa-loop-ivopts.h"
>>>> +#include "gimple-fold.h"
>>>>
>>>> /*************************************************************************
>>>> Simple Loop Peeling Utilities
>>>> @@ -247,30 +248,115 @@ adjust_phi_and_debug_stmts (gimple *upda
>>>> gimple_bb (update_phi));
>>>> }
>>>>
>>>> -/* Make the LOOP iterate NITERS times. This is done by adding a new IV
>>>> - that starts at zero, increases by one and its limit is NITERS.
>>>> +/* Make LOOP iterate N == (NITERS - STEP) / STEP + 1 times,
>>>> + where NITERS is known to be outside the range [1, STEP - 1].
>>>> + This is equivalent to making the loop execute NITERS / STEP
>>>> + times when NITERS is nonzero and (1 << M) / STEP times otherwise,
>>>> + where M is the precision of NITERS.
>>>> +
>>>> + NITERS_MAYBE_ZERO is true if NITERS can be zero, false it is known
>>>> + to be >= STEP. In the latter case N is always NITERS / STEP.
>>>> +
>>>> + If FINAL_IV is nonnull, it is an SSA name that should be set to
>>>> + N * STEP on exit from the loop.
>>>>
>>>> Assumption: the exit-condition of LOOP is the last stmt in the loop. */
>>>>
>>>> void
>>>> -slpeel_make_loop_iterate_ntimes (struct loop *loop, tree niters)
>>>> +slpeel_make_loop_iterate_ntimes (struct loop *loop, tree niters, tree step,
>>>> + tree final_iv, bool niters_maybe_zero)
>>>> {
>>>> tree indx_before_incr, indx_after_incr;
>>>> gcond *cond_stmt;
>>>> gcond *orig_cond;
>>>> + edge pe = loop_preheader_edge (loop);
>>>> edge exit_edge = single_exit (loop);
>>>> gimple_stmt_iterator loop_cond_gsi;
>>>> gimple_stmt_iterator incr_gsi;
>>>> bool insert_after;
>>>> - tree init = build_int_cst (TREE_TYPE (niters), 0);
>>>> - tree step = build_int_cst (TREE_TYPE (niters), 1);
>>>> source_location loop_loc;
>>>> enum tree_code code;
>>>> + tree niters_type = TREE_TYPE (niters);
>>>>
>>>> orig_cond = get_loop_exit_condition (loop);
>>>> gcc_assert (orig_cond);
>>>> loop_cond_gsi = gsi_for_stmt (orig_cond);
>>>>
>>>> + tree init, limit;
>>>> + if (!niters_maybe_zero && integer_onep (step))
>>>> + {
>>>> + /* In this case we can use a simple 0-based IV:
>>>> +
>>>> + A:
>>>> + x = 0;
>>>> + do
>>>> + {
>>>> + ...
>>>> + x += 1;
>>>> + }
>>>> + while (x < NITERS); */
>>>> + code = (exit_edge->flags & EDGE_TRUE_VALUE) ? GE_EXPR : LT_EXPR;
>>>> + init = build_zero_cst (niters_type);
>>>> + limit = niters;
>>>> + }
>>>> + else
>>>> + {
>>>> + /* The following works for all values of NITERS except 0:
>>>> +
>>>> + B:
>>>> + x = 0;
>>>> + do
>>>> + {
>>>> + ...
>>>> + x += STEP;
>>>> + }
>>>> + while (x <= NITERS - STEP);
>>>> +
>>>> + so that the loop continues to iterate if x + STEP - 1 < NITERS
>>>> + but stops if x + STEP - 1 >= NITERS.
>>>> +
>>>> + However, if NITERS is zero, x never hits a value above NITERS -
> STEP
>>>> + before wrapping around. There are two obvious ways of dealing with
>>>> + this:
>>>> +
>>>> + - start at STEP - 1 and compare x before incrementing it
>>>> + - start at -1 and compare x after incrementing it
>>>> +
>>>> + The latter is simpler and is what we use. The loop in this case
>>>> + looks like:
>>>> +
>>>> + C:
>>>> + x = -1;
>>>> + do
>>>> + {
>>>> + ...
>>>> + x += STEP;
>>>> + }
>>>> + while (x < NITERS - STEP);
>>>> +
>>>> + In both cases the loop limit is NITERS - STEP. */
>>>> + gimple_seq seq = NULL;
>>>> + limit = force_gimple_operand (niters, &seq, true, NULL_TREE);
>>>> + limit = gimple_build (&seq, MINUS_EXPR, TREE_TYPE (limit), limit,
>>> step);
>>>> + if (seq)
>>>> + {
>>>> + basic_block new_bb = gsi_insert_seq_on_edge_immediate (pe, seq);
>>>> + gcc_assert (!new_bb);
>>>> + }
>>>> + if (niters_maybe_zero)
>>>> + {
>>>> + /* Case C. */
>>>> + code = (exit_edge->flags & EDGE_TRUE_VALUE) ? GE_EXPR : LT_EXPR;
>>>> + init = build_all_ones_cst (niters_type);
>>>> + }
>>>> + else
>>>> + {
>>>> + /* Case B. */
>>>> + code = (exit_edge->flags & EDGE_TRUE_VALUE) ? GT_EXPR : LE_EXPR;
>>>> + init = build_zero_cst (niters_type);
>>>> + }
>>>> + }
>>>> +
>>>> standard_iv_increment_position (loop, &incr_gsi, &insert_after);
>>>> create_iv (init, step, NULL_TREE, loop,
>>>> &incr_gsi, insert_after, &indx_before_incr, &indx_after_incr);
>>>> @@ -278,11 +364,10 @@ slpeel_make_loop_iterate_ntimes (struct
>>>> indx_after_incr = force_gimple_operand_gsi (&loop_cond_gsi,
>>> indx_after_incr,
>>>> true, NULL_TREE, true,
>>>> GSI_SAME_STMT);
>>>> - niters = force_gimple_operand_gsi (&loop_cond_gsi, niters, true,
> NULL_TREE,
>>>> + limit = force_gimple_operand_gsi (&loop_cond_gsi, limit, true, NULL_TREE,
>>>> true, GSI_SAME_STMT);
>>>>
>>>> - code = (exit_edge->flags & EDGE_TRUE_VALUE) ? GE_EXPR : LT_EXPR;
>>>> - cond_stmt = gimple_build_cond (code, indx_after_incr, niters, NULL_TREE,
>>>> + cond_stmt = gimple_build_cond (code, indx_after_incr, limit, NULL_TREE,
>>>> NULL_TREE);
>>>>
>>>> gsi_insert_before (&loop_cond_gsi, cond_stmt, GSI_SAME_STMT);
>>>> @@ -301,8 +386,23 @@ slpeel_make_loop_iterate_ntimes (struct
>>>> }
>>>>
>>>> /* Record the number of latch iterations. */
>>>> - loop->nb_iterations = fold_build2 (MINUS_EXPR, TREE_TYPE (niters),
> niters,
>>>> - build_int_cst (TREE_TYPE (niters), 1));
>>>> + if (limit == niters)
>>>> + /* Case A: the loop iterates NITERS times. Subtract one to get the
>>>> + latch count. */
>>>> + loop->nb_iterations = fold_build2 (MINUS_EXPR, niters_type, niters,
>>>> + build_int_cst (niters_type, 1));
>>>> + else
>>>> + /* Case B or C: the loop iterates (NITERS - STEP) / STEP + 1 times.
>>>> + Subtract one from this to get the latch count. */
>>>> + loop->nb_iterations = fold_build2 (TRUNC_DIV_EXPR, niters_type,
>>>> + limit, step);
>>>> +
>>>> + if (final_iv)
>>>> + {
>>>> + gassign *assign = gimple_build_assign (final_iv, MINUS_EXPR,
>>>> + indx_after_incr, init);
>>>> + gsi_insert_on_edge_immediate (single_exit (loop), assign);
>>>> + }
>>>> }
>>>>
>>>> /* Helper routine of slpeel_tree_duplicate_loop_to_edge_cfg.
>>>> @@ -1170,23 +1270,32 @@ vect_gen_scalar_loop_niters (tree niters
>>>> return niters;
>>>> }
>>>>
>>>> -/* This function generates the following statements:
>>>> +/* NITERS is the number of times that the original scalar loop executes
>>>> + after peeling. Work out the maximum number of iterations N that can
>>>> + be handled by the vectorized form of the loop and then either:
>>>> +
>>>> + a) set *STEP_VECTOR_PTR to the vectorization factor and generate:
>>>> +
>>>> + niters_vector = N
>>>> +
>>>> + b) set *STEP_VECTOR_PTR to one and generate:
>>>>
>>>> - niters = number of iterations loop executes (after peeling)
>>>> - niters_vector = niters / vf
>>>> + niters_vector = N / vf
>>>>
>>>> - and places them on the loop preheader edge. NITERS_NO_OVERFLOW is
>>>> - true if NITERS doesn't overflow. */
>>>> + In both cases, store niters_vector in *NITERS_VECTOR_PTR and add
>>>> + any new statements on the loop preheader edge. NITERS_NO_OVERFLOW
>>>> + is true if NITERS doesn't overflow (i.e. if NITERS is always
> nonzero). */
>>>>
>>>> void
>>>> vect_gen_vector_loop_niters (loop_vec_info loop_vinfo, tree niters,
>>>> - tree *niters_vector_ptr, bool niters_no_overflow)
>>>> + tree *niters_vector_ptr, tree *step_vector_ptr,
>>>> + bool niters_no_overflow)
>>>> {
>>>> tree ni_minus_gap, var;
>>>> - tree niters_vector, type = TREE_TYPE (niters);
>>>> + tree niters_vector, step_vector, type = TREE_TYPE (niters);
>>>> int vf = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
>>>> edge pe = loop_preheader_edge (LOOP_VINFO_LOOP (loop_vinfo));
>>>> - tree log_vf = build_int_cst (type, exact_log2 (vf));
>>>> + tree log_vf = NULL_TREE;
>>>>
>>>> /* If epilogue loop is required because of data accesses with gaps, we
>>>> subtract one iteration from the total number of iterations here for
>>>> @@ -1207,21 +1316,32 @@ vect_gen_vector_loop_niters (loop_vec_in
>>>> else
>>>> ni_minus_gap = niters;
>>>>
>>>> - /* Create: niters >> log2(vf) */
>>>> - /* If it's known that niters == number of latch executions + 1 doesn't
>>>> - overflow, we can generate niters >> log2(vf); otherwise we generate
>>>> - (niters - vf) >> log2(vf) + 1 by using the fact that we know ratio
>>>> - will be at least one. */
>>>> - if (niters_no_overflow)
>>>> - niters_vector = fold_build2 (RSHIFT_EXPR, type, ni_minus_gap, log_vf);
>>>> + if (1)
>>>> + {
>>>> + /* Create: niters >> log2(vf) */
>>>> + /* If it's known that niters == number of latch executions + 1
> doesn't
>>>> + overflow, we can generate niters >> log2(vf); otherwise we generate
>>>> + (niters - vf) >> log2(vf) + 1 by using the fact that we know ratio
>>>> + will be at least one. */
>>>> + log_vf = build_int_cst (type, exact_log2 (vf));
>>>> + if (niters_no_overflow)
>>>> + niters_vector = fold_build2 (RSHIFT_EXPR, type, ni_minus_gap,
> log_vf);
>>>> + else
>>>> + niters_vector
>>>> + = fold_build2 (PLUS_EXPR, type,
>>>> + fold_build2 (RSHIFT_EXPR, type,
>>>> + fold_build2 (MINUS_EXPR, type,
>>>> + ni_minus_gap,
>>>> + build_int_cst (type, vf)),
>>>> + log_vf),
>>>> + build_int_cst (type, 1));
>>>> + step_vector = build_one_cst (type);
>>>> + }
>>>> else
>>>> - niters_vector
>>>> - = fold_build2 (PLUS_EXPR, type,
>>>> - fold_build2 (RSHIFT_EXPR, type,
>>>> - fold_build2 (MINUS_EXPR, type, ni_minus_gap,
>>>> - build_int_cst (type, vf)),
>>>> - log_vf),
>>>> - build_int_cst (type, 1));
>>>> + {
>>>> + niters_vector = ni_minus_gap;
>>>> + step_vector = build_int_cst (type, vf);
>>>> + }
>>>>
>>>> if (!is_gimple_val (niters_vector))
>>>> {
>>>> @@ -1231,7 +1351,7 @@ vect_gen_vector_loop_niters (loop_vec_in
>>>> gsi_insert_seq_on_edge_immediate (pe, stmts);
>>>> /* Peeling algorithm guarantees that vector loop bound is at least
> ONE,
>>>> we set range information to make niters analyzer's life easier. */
>>>> - if (stmts != NULL)
>>>> + if (stmts != NULL && log_vf)
>>>> set_range_info (niters_vector, VR_RANGE,
>>>> wi::to_wide (build_int_cst (type, 1)),
>>>> wi::to_wide (fold_build2 (RSHIFT_EXPR, type,
>>>> @@ -1239,6 +1359,7 @@ vect_gen_vector_loop_niters (loop_vec_in
>>>> log_vf)));
>>>> }
>>>> *niters_vector_ptr = niters_vector;
>>>> + *step_vector_ptr = step_vector;
>>>>
>>>> return;
>>>> }
>>>> @@ -1600,7 +1721,12 @@ slpeel_update_phi_nodes_for_lcssa (struc
>>>> - TH, CHECK_PROFITABILITY: Threshold of niters to vectorize loop if
>>>> CHECK_PROFITABILITY is true.
>>>> Output:
>>>> - - NITERS_VECTOR: The number of iterations of loop after vectorization.
>>>> + - *NITERS_VECTOR and *STEP_VECTOR describe how the main loop should
>>>> + iterate after vectorization; see slpeel_make_loop_iterate_ntimes
>>>> + for details.
>>>> + - *NITERS_VECTOR_MULT_VF_VAR is either null or an SSA name that
>>>> + should be set to the number of scalar iterations handled by the
>>>> + vector loop. The SSA name is only used on exit from the loop.
>>>>
>>>> This function peels prolog and epilog from the loop, adds guards
> skipping
>>>> PROLOG and EPILOG for various conditions. As a result, the changed CFG
>>>> @@ -1657,8 +1783,9 @@ slpeel_update_phi_nodes_for_lcssa (struc
>>>>
>>>> struct loop *
>>>> vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
>>>> - tree *niters_vector, int th, bool check_profitability,
>>>> - bool niters_no_overflow)
>>>> + tree *niters_vector, tree *step_vector,
>>>> + tree *niters_vector_mult_vf_var, int th,
>>>> + bool check_profitability, bool niters_no_overflow)
>>>> {
>>>> edge e, guard_e;
>>>> tree type = TREE_TYPE (niters), guard_cond;
>>>> @@ -1754,7 +1881,9 @@ vect_do_peeling (loop_vec_info loop_vinf
>>>> /* Generate and update the number of iterations for prolog loop. */
>>>> niters_prolog = vect_gen_prolog_loop_niters (loop_vinfo, anchor,
>>>> &bound_prolog);
>>>> - slpeel_make_loop_iterate_ntimes (prolog, niters_prolog);
>>>> + tree step_prolog = build_one_cst (TREE_TYPE (niters_prolog));
>>>> + slpeel_make_loop_iterate_ntimes (prolog, niters_prolog, step_prolog,
>>>> + NULL_TREE, false);
>>>>
>>>> /* Skip the prolog loop. */
>>>> if (skip_prolog)
>>>> @@ -1867,9 +1996,20 @@ vect_do_peeling (loop_vec_info loop_vinf
>>>> overflows. */
>>>> niters_no_overflow |= (prolog_peeling > 0);
>>>> vect_gen_vector_loop_niters (loop_vinfo, niters,
>>>> - niters_vector, niters_no_overflow);
>>>> - vect_gen_vector_loop_niters_mult_vf (loop_vinfo, *niters_vector,
>>>> - &niters_vector_mult_vf);
>>>> + niters_vector, step_vector,
>>>> + niters_no_overflow);
>>>> + if (!integer_onep (*step_vector))
>>>> + {
>>>> + /* On exit from the loop we will have an easy way of calcalating
>>>> + NITERS_VECTOR / STEP * STEP. Install a dummy definition
>>>> + until then. */
>>>> + niters_vector_mult_vf = make_ssa_name (TREE_TYPE
> (*niters_vector));
>>>> + SSA_NAME_DEF_STMT (niters_vector_mult_vf) = gimple_build_nop ();
>>>> + *niters_vector_mult_vf_var = niters_vector_mult_vf;
>>>> + }
>>>> + else
>>>> + vect_gen_vector_loop_niters_mult_vf (loop_vinfo, *niters_vector,
>>>> + &niters_vector_mult_vf);
>>>> /* Update IVs of original loop as if they were advanced by
>>>> niters_vector_mult_vf steps. */
>>>> gcc_checking_assert (vect_can_advance_ivs_p (loop_vinfo));
>>>> Index: gcc/tree-vect-loop.c
>>>> ===================================================================
>>>> --- gcc/tree-vect-loop.c 2017-10-13 15:01:40.144777367 +0100
>>>> +++ gcc/tree-vect-loop.c 2017-10-13 15:01:40.296014347 +0100
>>>> @@ -7273,7 +7273,9 @@ vect_transform_loop (loop_vec_info loop_
>>>> basic_block *bbs = LOOP_VINFO_BBS (loop_vinfo);
>>>> int nbbs = loop->num_nodes;
>>>> int i;
>>>> - tree niters_vector = NULL;
>>>> + tree niters_vector = NULL_TREE;
>>>> + tree step_vector = NULL_TREE;
>>>> + tree niters_vector_mult_vf = NULL_TREE;
>>>> int vf = LOOP_VINFO_VECT_FACTOR (loop_vinfo);
>>>> bool grouped_store;
>>>> bool slp_scheduled = false;
>>>> @@ -7342,17 +7344,21 @@ vect_transform_loop (loop_vec_info loop_
>>>> LOOP_VINFO_NITERS_UNCHANGED (loop_vinfo) = niters;
>>>> tree nitersm1 = unshare_expr (LOOP_VINFO_NITERSM1 (loop_vinfo));
>>>> bool niters_no_overflow = loop_niters_no_overflow (loop_vinfo);
>>>> - epilogue = vect_do_peeling (loop_vinfo, niters, nitersm1,
>>> &niters_vector, th,
>>>> + epilogue = vect_do_peeling (loop_vinfo, niters, nitersm1, &niters_vector,
>>>> + &step_vector, &niters_vector_mult_vf, th,
>>>> check_profitability, niters_no_overflow);
>>>> if (niters_vector == NULL_TREE)
>>>> {
>>>> if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
>>>> - niters_vector
>>>> - = build_int_cst (TREE_TYPE (LOOP_VINFO_NITERS (loop_vinfo)),
>>>> - LOOP_VINFO_INT_NITERS (loop_vinfo) / vf);
>>>> + {
>>>> + niters_vector
>>>> + = build_int_cst (TREE_TYPE (LOOP_VINFO_NITERS (loop_vinfo)),
>>>> + LOOP_VINFO_INT_NITERS (loop_vinfo) / vf);
>>>> + step_vector = build_one_cst (TREE_TYPE (niters));
>>>> + }
>>>> else
>>>> vect_gen_vector_loop_niters (loop_vinfo, niters, &niters_vector,
>>>> - niters_no_overflow);
>>>> + &step_vector, niters_no_overflow);
>>>> }
>>>>
>>>> /* 1) Make sure the loop header has exactly two entries
>>>> @@ -7603,7 +7609,13 @@ vect_transform_loop (loop_vec_info loop_
>>>> } /* stmts in BB */
>>>> } /* BBs in loop */
>>>>
>>>> - slpeel_make_loop_iterate_ntimes (loop, niters_vector);
>>>> + /* The vectorization factor is always > 1, so if we use an IV
>>> increment of 1.
>>>> + a zero NITERS becomes a nonzero NITERS_VECTOR. */
>>>> + if (integer_onep (step_vector))
>>>> + niters_no_overflow = true;
>>>> + slpeel_make_loop_iterate_ntimes (loop, niters_vector, step_vector,
>>>> + niters_vector_mult_vf,
>>>> + !niters_no_overflow);
>>>>
>>>> scale_profile_for_vect_loop (loop, vf);
>>>>
>>>> Index: gcc/tree-vectorizer.h
>>>> ===================================================================
>>>> --- gcc/tree-vectorizer.h 2017-10-13 15:01:40.144777367 +0100
>>>> +++ gcc/tree-vectorizer.h 2017-10-13 15:01:40.296014347 +0100
>>>> @@ -1138,13 +1138,14 @@ vect_get_scalar_dr_size (struct data_ref
>>>>
>>>> /* Simple loop peeling and versioning utilities for vectorizer's purposes -
>>>> in tree-vect-loop-manip.c. */
>>>> -extern void slpeel_make_loop_iterate_ntimes (struct loop *, tree);
>>>> +extern void slpeel_make_loop_iterate_ntimes (struct loop *, tree, tree,
>>>> + tree, bool);
>>>> extern bool slpeel_can_duplicate_loop_p (const struct loop *, const_edge);
>>>> struct loop *slpeel_tree_duplicate_loop_to_edge_cfg (struct loop *,
>>>> struct loop *, edge);
>>>> extern void vect_loop_versioning (loop_vec_info, unsigned int, bool);
>>>> extern struct loop *vect_do_peeling (loop_vec_info, tree, tree,
>>>> - tree *, int, bool, bool);
>>>> + tree *, tree *, tree *, int, bool, bool);
>>>> extern source_location find_loop_location (struct loop *);
>>>> extern bool vect_can_advance_ivs_p (loop_vec_info);
>>>>
>>>> @@ -1258,7 +1259,8 @@ extern gimple *vect_force_simple_reducti
>>>> /* Drive for loop analysis stage. */
>>>> extern loop_vec_info vect_analyze_loop (struct loop *, loop_vec_info);
>>>> extern tree vect_build_loop_niters (loop_vec_info, bool * = NULL);
>>>> -extern void vect_gen_vector_loop_niters (loop_vec_info, tree, tree
> *, bool);
>>>> +extern void vect_gen_vector_loop_niters (loop_vec_info, tree, tree *,
>>>> + tree *, bool);
>>>> /* Drive for loop transformation stage. */
>>>> extern struct loop *vect_transform_loop (loop_vec_info);
>>>> extern loop_vec_info vect_analyze_loop_form (struct loop *);